Tank Floor Pitting: API 653 Minimum Remaining Thickness (MRT) Explained

Short answer: for product-side pitting on a tank floor, API 653 has the inspector project the bottom forward to the next internal inspection. Take the lower of the minimum remaining thickness from soil-side and product-side corrosion after repairs. Subtract the planned interval multiplied by the unrepaired product-side and soil-side corrosion rates. The resulting minimum remaining thickness (MRT) must meet the bottom minimums of Table 4.4 and the critical-zone rule. If it does not, the bottom is repaired, lined or replaced, or the interval is shortened.

That is the method in one paragraph. The rest of this guide walks through how an in-service tank inspector applies it to a petroleum tank with localized product-side pitting: how the pits are found and measured, which numbers feed each term, when a rate can legitimately be taken as zero, what the critical zone changes, which repairs count toward the calculation, and what the report has to record. Everything here is stated in our own words from API Standard 653, Fifth Edition, through Addendum 3 (2023). API issued Addendum 4 in July 2025, and it revised the bottom-plate thickness measurement text. Check the current edition and addendum in your licensed copy before you rely on any clause reference, and treat this page as an explanation, not a substitute for the code.

Planning an internal inspection with a pitted floor? Ask for an MFL and UT floor examination quote, answered within 24 hours.

What API 653 asks the inspector to calculate

API 653 asks for one number for the whole bottom, or for each portion of it: the minimum remaining thickness the plate will have at the end of the next in-service period. The standard calls this MRT. It is a forward projection, not today's thickness, and it is checked against fixed minimums that depend on how the tank is built.

The method in clause 4.4.5.1 combines four inputs:

• RTip: the minimum remaining thickness left by internal (product-side) corrosion, measured after any repairs are made.
• RTbc: the minimum remaining thickness left by bottom-side (soil-side) corrosion, also after repairs.
• StPr: the maximum product-side corrosion rate in areas that are not repaired.
• UPr: the maximum soil-side corrosion rate.

The calculation takes the lower of RTip and RTbc, then subtracts the planned operating interval in years (the standard's term is Or) multiplied by the sum of StPr and UPr. The interval itself cannot exceed what Section 6 allows for subsequent internal inspections. With the corrosion-rate method, that ceiling is 20 years for a tank without a release prevention barrier (RPB) and 30 years with one. Section 6 also allows an owner to set intervals with a risk-based inspection assessment. That route is described later, as an engineering decision the owner commissions.

API 653 presents this as "an acceptable method". It also mentions a probabilistic approach, in which statistical analysis of sampled thickness data projects the remaining thickness of the whole bottom. Most in-service inspectors work with the deterministic method, and it is the one this page explains. If your programme uses statistical extrapolation from partial scanning, the owner's written procedure should say so and explain how coverage was chosen.

One point causes confusion in exam preparation and in the field. The pitting rules for shell plates are different. For shells, API 653 lets an inspector disregard widely scattered pits that meet defined limits, and it uses separate averaging rules. Those shell provisions do not carry over to the floor. On the bottom, the MRT method uses the deepest unrepaired pit, and the code does not allow thickness averaging in the critical zone.

The minimum thickness limits MRT must meet

A calculated MRT only means something when it is compared with the right limit. API 653 Table 4.4 sets the minimum bottom plate thickness allowed at the next inspection according to the bottom and foundation design:

Two further limits sit on top of Table 4.4:

• The critical zone. API 653 defines the critical zone as the part of the bottom or annular plate within 3 in. (75 mm) of the inside edge of the shell, measured radially inward. Unless a stress analysis is done, the minimum plate thickness there is the smaller of 0.118 in. (3 mm) and half of the lower shell course's calculated minimum thickness, but never less than 0.10 in. at the next inspection. Averaging is not permitted. The code adds that isolated pits and isolated corroded areas may not affect the zone's structural integrity, and it gives the storage tank engineer the job of deciding whether they do.
• Annular plates. Where the tank has an annular ring, its minimum thickness comes from separate tables tied to product specific gravity and shell stress. It is usually more than 0.10 in. The plate projection outside the shell also has its own minimums.

The practical consequence is simple. A pit in the middle of the floor and an identical pit 2 in. from the shell are judged against different limits. Your scan data has to show where every significant indication sits relative to the shell, not just how deep it is.

Finding and measuring product-side pits

MRT is only as good as the thickness data behind it. API 653 recognizes that bottom examination methods differ in how reliably they measure general corrosion and pitting. A combination of methods, with extrapolation and analysis, may be needed to establish the probable condition of the whole bottom. In practice a pitted floor is usually examined like this:

1. Cleaning and visual examination. Product-side pits are visible once the floor is cleaned. The inspector or a qualified examiner maps pit clusters, records their appearance (isolated, clustered, lake-type) and marks areas for measurement. The internal inspection checklist in API 653 includes measuring pit depth and describing pit appearance.
2. Pit depth measurement. Product-side pits are open to the surface, so their depth can be measured directly with a pit gauge or depth micrometer, referenced to nearby sound plate. Remaining thickness under the pit is then the local plate thickness minus pit depth. UT from the top surface is unreliable inside a rough-bottomed pit, so pit gauging is the normal tool for product-side loss.
3. Floor scanning for soil-side loss. Soil-side pits cannot be seen from inside. API 653 notes that magnetic flux leakage (MFL) tools are commonly used with UT thickness tools to examine bottoms. MFL screens the plate area and flags indications above a set threshold. UT is often used to confirm and size them. MFL responds to metal loss on both faces, so the crew must tell product-side indications (visible and gaugeable) from soil-side ones (not visible). Compare the scan map with the visual pit map.
4. UT prove-up. Indications without a visible top-side cause are proved up with UT, often with a scanning or mapping technique where loss is extensive, to measure the minimum remaining thickness.
5. Coverage of what the scanner cannot reach. Areas near the shell, around sumps, under supports and at lap welds are often outside the main scanner's reach. They are covered with smaller scanners, hand UT or other methods, and any area not examined must be reported as such.

Annex G of API 653 can be used to qualify bottom-scanning procedures and operators. It uses test plates with defined underside and product-side test pits. Whether Annex G is invoked is the owner's decision. When it is, ask for the qualification records. For UT thickness work, API 653 calls for examiners qualified to an approved procedure and certified to ASNT UT Level II (full or limited, for digital or A-scan thickness measurement) under SNT-TC-1A or an equivalent national standard recognized by the owner. Trainees may work under direct supervision of a Level II or III.

For a comparison of the two floor methods and their blind spots, see our MFL vs UT tank floor scanning guide.

Turning measurements into RTip, StPr and the soil-side terms

Each MRT term has a specific meaning. Most calculation errors come from feeding the wrong number into one of them.

RTip: the product-side remaining thickness after repairs

RTip is the thinnest remaining thickness left by internal corrosion after the planned repairs. If the deepest pits will be welded or covered by a welded patch, they drop out of RTip. The new governing value is the thinnest product-side location that will stay unrepaired. Where a welded-on patch is used, API 653 lets its thickness be added in the calculation for that area. Both the repaired area and its future corrosion rate then have to be evaluated.

StPr: the unrepaired product-side rate

StPr is the maximum rate of product-side corrosion in areas not repaired. Programmes usually derive it from the deepest unrepaired pit and the time over which it developed. That period is normally the age of the bottom, or the time since the last reliable measurement or since the coating failed, if that can be shown. Be conservative and consistent. A rate based on an assumed coating-failure date that cannot be documented will not survive review.

The code sets StPr to zero for coated areas of the bottom, but only if the expected life of the coating equals or exceeds the planned interval. A new internal lining is therefore one of the strongest levers on a pitted floor. Its expected life has to be supported, usually from the lining specification and the coating inspection records under API 652.

RTbc and UPr: the soil side

RTbc is the minimum remaining thickness from soil-side corrosion after repairs. UPr is the maximum soil-side corrosion rate. API 653 tells you to calculate it from the minimum remaining thickness after repairs, assuming a linear rate over the age of the tank. UPr is zero for areas with effective cathodic protection. That is why CP survey results under API 651 belong in the inspection file.

There is an important note for MFL-scanned floors without effective CP. The thickness used to calculate UPr must be the lesser of the MFL threshold and the minimum thickness of unrepaired corrosion. The MFL threshold is the minimum remaining thickness the scan is set up to detect, and the owner should set it in advance based on the interval they want. A coarse threshold therefore produces a high assumed soil-side rate and a shorter interval, even if no deep soil-side pits were actually found. Setting the threshold is a planning decision, not a technician's default.

A worked example, described step by step

Take a welded, cone-roof crude or products tank in petroleum service at its first internal inspection after a long first interval. The floor is uncoated. There is no RPB and no leak detection, so the Table 4.4 limit is 0.10 in. The soil side has a CP system that the latest survey shows is effective. Cleaning reveals clusters of product-side pitting near the water draw-off and in low spots where water and sediment collect.

1. Map and gauge the pits. The crew maps every pit cluster and gauges the deepest pits in each. The inspector notes which clusters lie within 3 in. of the shell.
2. Scan the floor. MFL covers the accessible plate area at the threshold the owner set. Indications are compared with the visual pit map. Soil-side indications are proved up with UT, which gives RTbc.
3. Calculate the unrepaired rates. Because CP is effective, UPr is zero for the protected areas. StPr comes from the deepest product-side pit that will be left unrepaired, divided by the period over which it formed.
4. Project the first option: no repairs. Using the as-found deepest pit for RTip and the target interval, the inspector projects MRT. If it falls below 0.10 in., or below the critical-zone minimum for pits near the shell, the target interval cannot be supported as the floor stands.
5. Project the second option: weld the deep pits. If the deepest pits are pit-welded or covered by welded-on lap patches, RTip rises to the thinnest unrepaired location. StPr now reflects that unrepaired area. The projection is repeated.
6. Project the third option: weld and line. If the floor is also lined with a coating whose expected life covers the interval, StPr becomes zero for the coated areas. With a reinforced lining over 0.05 in. applied per API 652, Table 4.4 allows 0.05 in. instead of 0.10 in.
7. Choose and record. The owner chooses between repair scope and interval length. The inspector records the basis, and the report states the recommended maximum interval and how it was calculated.

Three rules decide what is allowed in steps 5 and 6:

• Repairs made to extend the interval must be welded: pit welding, overlay welding or welded-on lap patches under the repair sections of API 653, followed by examination and testing.
• Coatings, caulking, epoxy fillers and bolted repairs cannot be used to increase RTip or RTbc in the calculation. A lining can make StPr zero when its life covers the interval. Filling a pit with epoxy does not restore the thickness used for RTip.
• In the critical zone, repair of corroded plate is limited to pit welding or overlay welding within stated limits on how much pitting can be repaired along the shell. There is also a minimum plate thickness for welding, below which an engineer experienced in storage tank design must review the repair. Welded-on patches in the critical zone have their own restrictions. More extensive critical-zone damage means replacing the bottom plate welded to the shell.

If the projected MRT still falls short, API 653 leaves four outcomes: line the bottom, repair it, replace it, or shorten the interval to the next internal inspection. The owner can also use an RBI assessment under the standard's RBI provisions to set minimum thickness and intervals. That is an engineering assessment the owner commissions, not an inspector's adjustment of the numbers.

Common mistakes on pitted floors

• Using today's minimum instead of the projection. A floor that measures above 0.10 in. today can still fail the MRT check once the unrepaired rate is projected over a long interval.
• Using an average rate. StPr and UPr are maximum rates. Averaging across the floor hides the pit that governs.
• Counting epoxy-filled pits as restored. Non-welded repairs do not change RTip. Only a sound lining whose life covers the interval changes StPr.
• Claiming zero UPr without CP evidence. "Effective" cathodic protection needs survey records. Without them, the soil side carries a rate.
• Ignoring the MFL threshold rule. On a floor without effective CP, the threshold feeds UPr whether or not deep soil-side pits were found.
• Not locating pits relative to the shell. A pit within 3 in. of the shell is judged by the critical-zone rule and has more restricted repair options.
• Unreported scan gaps. Areas not scanned near the shell, sumps or obstructions must be listed and dealt with. Otherwise the calculation assumes condition data that does not exist.
• Using an old edition. Many free copies online are earlier editions, and the critical-zone rule has changed over time. Check the current edition and Addendum 4.

What the report must record about bottom corrosion

API 653 lists what internal inspection reports must contain. Several items relate directly to pitting and MRT. In our words, the report must include:

• The inspection methods and tests used (visual, MFL, UT and others) and the results of each.
• The minimum measured remaining thickness of corroded bottom plate to be detected, recorded and repaired. In effect, this is the threshold the examination was designed around.
• Bottom corrosion rates, both as calculated from data measured before repairs and as projected after repairs and mitigation.
• The repairs and mitigation needed to support the next-interval calculation.
• For every pit and corroded area to be repaired: its location, the measured range of remaining thickness, and whether it is product-side or soil-side. If nothing needs repair, the maximum product-side pit depth and the minimum soil-side thickness found.
• The number and location of any through-holes.
• Recommendations, including the recommended maximum inspection interval and the basis for calculating it.

The authorized inspector prepares and signs the report. The owner/operator reviews the recommendations, sets the repair scope and timing, and documents the disposition of each recommendation, including the reasons if an action is delayed. An NDE contractor's floor report is an input to that report, not a replacement for it.

Regulatory overlay: SPCC, PSM and Canada

API 653 is an industry standard. It becomes binding through regulation, permits, contracts or the owner's own programme:

• EPA SPCC (40 CFR 112). Facilities subject to SPCC must test or inspect bulk storage containers for integrity on a regular schedule according to industry standards, and API 653 is commonly the standard used for field-erected tanks. Many plans name it directly.
• OSHA PSM (29 CFR 1910.119(j)). Where a tank is part of a covered process, inspection and testing must follow recognized and generally accepted good engineering practices. API 653 is widely used as that practice for atmospheric tanks.
• State rules. Some states add their own aboveground storage tank requirements or inspection reporting. Confirm with your state environmental agency.
• Canada. Petroleum storage tanks fall under provincial fire codes, environmental regulations and, for federally regulated facilities, the federal storage tank regulations. Many of these reference API 653 or CSA/ULC standards. Confirm the applicable instrument with your provincial regulator before setting intervals.

Whichever rule applies, the authorized inspector decides the interval under the code and the jurisdiction. The NDE contractor does not.

How Atlantis supports this

Atlantis NDT performs the floor examination your API 653 authorized inspector needs to calculate MRT. That includes MFL floor scanning, UT prove-up and corrosion mapping of indications, pit-depth gauging and mapping of product-side pitting, and coverage of the critical zone and other areas the main scanner cannot reach. Our technicians are ASNT-certified and work under ASNT Level III oversight. Each floor report gives indication locations relative to the shell, a remaining-thickness range for each area, product-side or soil-side classification, the scan threshold used, and a list of areas not examined. Your inspector can carry these straight into the calculation. The owner's authorized inspector stays inspector of record and sets the interval. Atlantis does not perform RBI or fitness-for-service assessments. See aboveground storage tank inspection, magnetic flux leakage testing and corrosion mapping. Over 1,500 inspection activities completed.

Request a tank floor examination quote, answered within 24 hours.

Frequently asked questions

An aboveground storage tank floor in petroleum service shows localized pitting on the product side. What is the standard approach for minimum thickness and remaining life?

Map and gauge the pits, scan the floor for soil-side loss, then calculate MRT. Use the lower of the product-side and soil-side remaining thickness after planned repairs, minus the planned interval multiplied by the unrepaired product-side and soil-side rates. Compare the result with Table 4.4 and the critical-zone minimum. Then repair, line, replace or shorten the interval as needed.

What is the minimum tank floor thickness under API 653?

In the 5th edition through Addendum 3, the Table 4.4 minimum at the next inspection is 0.10 in. without leak detection and containment. It is 0.05 in. with detection and containment, or with a reinforced lining over 0.05 in. applied per API 652. The critical zone has its own rule. Check the current addendum.

What does MRT mean in API 653?

Minimum remaining thickness: the thinnest bottom plate thickness projected at the end of the planned interval, after repairs. It is compared with the code minimums to decide whether the interval can be supported.

Can product-side pitting be ignored if it is widely scattered?

Not on the floor in the way it can on the shell. The widely-scattered-pit allowance belongs to shell evaluation. On the bottom, the MRT method uses the governing unrepaired pit, and averaging is not permitted in the critical zone.

Does coating the floor reduce the corrosion rate used in MRT?

Yes. For coated areas, the product-side rate (StPr) is zero if the coating's expected life equals or exceeds the planned interval. A reinforced lining over 0.05 in. per API 652 also lowers the Table 4.4 minimum to 0.05 in.

Can epoxy-filled pits be counted as repaired?

No. API 653 does not let non-welded repairs such as coatings, caulking, epoxies or bolting increase the remaining-thickness terms in MRT. Repairs that extend the interval must be welded.

What is the MFL threshold and why does it matter?

It is the minimum remaining thickness the MFL scan is set to detect. On floors without effective cathodic protection, the soil-side rate must be calculated from the lesser of that threshold and the minimum unrepaired thickness. The owner should therefore set the threshold to suit the interval they want.

What is the critical zone of a tank bottom?

The part of the bottom or annular plate within 3 in. of the inside edge of the shell. Its minimum thickness and repair options are more restricted than elsewhere on the floor.

Who calculates MRT and sets the next internal inspection date?

The owner's API 653 authorized inspector, with the storage tank engineer where the code requires engineering judgement. The owner approves repairs and timing. The NDE contractor supplies the thickness data.

How long can the next internal inspection interval be?

Using the corrosion-rate method, the interval comes from MRT. It is capped at 20 years without a release prevention barrier and 30 years with one. An owner may instead use an RBI assessment under the standard's RBI provisions.

Need floor data your inspector can calculate from? Talk to our Level III team about your next tank outage.

Related reading: API 653 inspection intervals explained, API 653 tank inspection guide, API 650 vs API 653, tank lining inspection under API 652, cathodic protection under API 651 and minimum required thickness.