Unit converter for API 653 inspectors
Thickness (in / mils / mm), corrosion rate (mpy / mm per yr), length (ft / m), and pressure (psi / MPa).
For estimating and checking. The inspector of record is responsible for every number that goes into a report. Verify against the current edition of API 653 — this page was checked against API 653, 5th Edition, Addendum 2 (May 2020). Not a substitute for the standard or for engineering judgment.
A quick converter for the units that come up most on an API 653 inspection: shell and bottom thickness (inches, mils, and millimeters), corrosion rate (mils per year and millimeters per year), tank geometry (feet and meters), and allowable stress (psi and megapascals). Pick "from" and "to" units of the same kind and enter a value.
- 1 in = 25.4 mm
The formula
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
in / mils / mm | Thickness | in, mils | Exact SI unit definition |
mpy / mm/yr | Corrosion rate (thickness per year) | mpy | Exact SI unit definition |
ft / m | Length | ft | Exact SI unit definition |
psi / MPa | Pressure / stress | psi | Exact SI unit definition |
Worked example
Using the pre-filled example above (US customary units):
- 1 in = 25.4 mm
- 1 in: 25.4 mm
Where the inputs come from
- The value and its starting unit come from whatever reading, drawing dimension, or stress table you are already looking at.
- Thickness, rate, length, and pressure are kept as separate groups — the calculator will not convert between them (a thickness cannot become a pressure).
Built by TankForge — API 653 inspection software
TankForge stores and reports in the unit system you choose per tank, converting automatically wherever a number crosses between US customary and metric.
Related
Frequently asked
What is a mil, and how is it different from a millimeter?
A mil is one-thousandth of an inch — a common unit for reporting thin corrosion allowances and corrosion rates in US customary field work. It is unrelated to a millimeter beyond both being small length units; do not confuse the two when reading a UT gauge or a report table.
Why do you list mpy separately from mils?
Mpy (mils per year) is a rate, not a thickness — it describes how fast a thickness is being lost, not a thickness itself. Converting a rate to metric uses the same underlying factor as converting a thickness, since a rate is just a thickness divided by time.
Are these conversion factors exact?
Yes. The inch, foot, and pound-force are all internationally defined to an exact number of SI units, so the conversions here (25.4 mm per inch, 0.3048 m per foot, and the psi-to-pascal factor derived from them) are exact, not rounded field approximations.
Can I convert a thickness directly to a pressure?
No — they are different physical quantities. The calculator groups units by kind (thickness, rate, length, pressure) and will not convert across groups; pick two units from the same group.
Where to find labeled drawings
- API 650 Annex H (internal floating roofs) and Annex C (external floating roofs): figures of the roof and its fittings.
- API 653 Annex C checklists: the inspection names for most of these parts (C.1.4 roofs, C.1.5 roof accessories, C.2.8–C.2.9 floating roofs and seals).
- EPA AP-42, Chapter 7.1 (Organic Liquid Storage Tanks): cutaway diagrams of guide poles, deck fittings, rim seals and vents, each part named. EPA AP-42 Chapter 7
- Vendor catalogs for close-up photos: Enardo, Protectoseal, Groth, Varec, Shand & Jurs (vents, hatches, gauges); HMT, Matrix (floating roofs, seals).
Don’t see what you need?
Ask for a guide, calculator or tank part we haven’t covered. Email is optional — anonymous requests are welcome.
tankforge.us/guide/calc/units — TankForge