Remaining life calculation formula
Remaining life (years) = (t_actual − t_required) / corrosion rate.
Use actual and required thickness in the same units, and corrosion rate in those thickness units per year. For a rate in mils/yr and thickness in inches, divide the rate by 1,000 first. With millimeters, use mm/yr. One mil is 0.001 in = 0.0254 mm.
Source: NASA, Reliability-Centered Maintenance Guide for Facilities and Collateral Equipment, p. 6-37. This linear model divides the thickness margin by an assumed future rate. It estimates time to the entered required thickness, not time to rupture and not a guarantee of safe service.
Worked example
For fictional readings of t_actual = 0.400 in, an independently established t_required = 0.250 in, and a selected rate of 5 mils/yr = 0.005 in/yr, the thickness margin is 0.150 in. Remaining life = 0.150 / 0.005 = 30 years.
Using an example rule of half the remaining life and an example maximum of 5 years, the proposed interval is min(0.5 × 30, 5) = 5 years. Starting on 1 January 2026, the calculator adds floor(5 × 365.25) = 1,826 calendar days, giving 1 January 2031. This example cap is not a prescribed code maximum.
Inspection interval: an editable rule, not a code determination
The common general planning approach is interval = min(fraction × remaining life, user-entered maximum), often starting with fraction = 0.5. You must confirm the fraction, cap, inspection type, exceptions and jurisdictional rules against your applicable code edition, approved procedure and responsible inspector/engineer. RBI, service changes and other damage mechanisms can change the appropriate interval.
The interval equation above simply implements your chosen rule. Dates use UTC calendar days and 365.25 days/year, rounded down to avoid extending the numeric interval. Confirm how your inspection program schedules calendar years and due dates.
API's public Body of Knowledge section pointers are API 510 §7.1, §7.2 and §6, and API 570 §7.1.2, §7.2 and §6.3.3. See API 510 and API 570. Section numbering can vary by edition. No code text, inspection-interval tables or ASME design equations are reproduced here.
To see which limit sets the date, half the remaining life or a maximum interval, use the inspection interval calculator, which applies the 10-year vessel maximum and lets you enter a piping class maximum.
To run many points from one survey and see which has the least life, paste them into the thickness survey analyzer.
Zero rate and thickness at or below the required value
At zero rate with a positive thickness margin, division gives no finite life estimate. The tool withholds a proposed interval and date: zero observed loss does not justify unlimited service life or removal of inspections.
At t_actual ≤ t_required, the tool flags at or below required thickness: engineering evaluation needed, displays zero calculated remaining life and calculates no future inspection date. A positive corrosion rate cannot restore a consumed thickness margin.
Negative rates are invalid for this projection. Apparent metal growth should be investigated as measurement error or a change in conditions before selecting a representative rate. The AMPP monitoring study below discusses how temperature and repeatability affect thickness comparisons.
Using the result in Excel
For a remaining life calculation in Excel, download the free .xlsx workbook, where the blue input cells feed live formulas for the corrosion rate, remaining life and proposed next inspection date.
The CSV is a snapshot of the current inputs and computed values, with units, references and calculation-aid notices. It contains no executable spreadsheet formulas. To check the worked example in your own worksheet, convert 5 mils/yr to 0.005 in/yr, then divide (0.400 − 0.250) by 0.005.
Calculate short-term and long-term corrosion rates → Use comparable readings from one CML and confirm the selected rate with the inspector. This tool does not evaluate localized damage, cracks, creep or interacting thickness effects.
For piping, the pipe minimum thickness calculator gives the required thickness of straight pipe and the margin left on a reading.
Remaining life calculation FAQ
How do you calculate remaining life for piping or a pressure vessel?
Subtract the required thickness from the actual thickness and divide by the corrosion rate, all in the same units. With 0.400 in actual, 0.250 in required and 5 mils/yr (0.005 in/yr), the margin is 0.150 in, so the remaining life is 0.150 ÷ 0.005 = 30 years. The same arithmetic serves an API 510 vessel and an API 570 piping circuit.
Where is remaining life in API 510 and API 570?
API's public Body of Knowledge documents point to §7.2 for remaining life in both API 510 and API 570, with corrosion rates under API 510 §7.1 and API 570 §7.1.2. Section numbering can vary by edition, so read them in your own copy.
How do I get the next inspection date from remaining life?
Apply an interval rule, often half the remaining life with a maximum you enter, and add it to the inspection date. In the worked example, min(0.5 × 30, 5) = 5 years, and 1 January 2026 plus 1,826 days is 1 January 2031. The fraction and the cap are yours to confirm against your code edition and procedure; the 5-year cap is an example, not a code maximum.
Can I do a remaining life calculation in Excel?
Yes. Convert the rate to thickness units per year, then divide the margin by it: =(0.400-0.250)/0.005 returns 30. The calculator's free CSV saves your inputs, results, interval assumptions and warnings as values, without formulas, for Excel, Google Sheets or Numbers.