Shell minimum thickness (t-min) calculator
One-foot-method t-min per shell course — and its inverse, maximum fill height from a measured thickness.
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.
The one-foot method finds the minimum thickness a shell course needs to hold its own hydrostatic load, measured one foot above the course's own bottom. It's the formula almost every field UT reading against a t-min table traces back to. Enter the course geometry below, or flip to "maximum fill height" to run the same formula backward from a measured thickness.
- K · D · (H − 1) · G = 2.6 × 120 × (48 − 1) × 1 = 14664.00
- S · E = 23600 psi × 0.7 = 16520.00
- t-min = 14664.00 / 16520.00 = 0.8877 in (floor 0.1)
The formula
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
D | Tank diameter | ft | Tank record / As-built |
H | Fill height above this course | ft | Tank record — height from course bottom to max liquid level |
G | Specific gravity of the stored product | Product data sheet / MSDS | |
S | Allowable product stress | psi | Table 4.1 |
E | Joint efficiency | Table 4.2 | |
t | Measured (or as-built) course thickness | in | UT reading / mill cert |
Worked example
Using the pre-filled example above (US customary units):
- K · D · (H − 1) · G = 2.6 × 120 × (48 − 1) × 1 = 14664.00
- S · E = 23600 psi × 0.7 = 16520.00
- t-min = 14664.00 / 16520.00 = 0.8877 in (floor 0.1)
- Minimum thickness (t-min): .888 in
Where the inputs come from
- Diameter and course heights come from the tank record or an as-built survey — measure at the shell, not the foundation ring.
- Fill height is measured from the bottom of the course you are checking to the maximum liquid level the tank is rated for, not the top of the tank.
- Specific gravity comes from the product data sheet; use the heaviest product the tank is rated to hold if that is not the current one.
- Allowable stress and joint efficiency come from the material and construction records for that course — the calculator defaults to the values API 653 assigns when those records are missing.
Built by TankForge — API 653 inspection software
TankForge runs this per course from your UT grid and carries it straight into the report — no separate spreadsheet to keep in sync.
Related
Frequently asked
Why subtract the one-foot datum from the fill height?
The formula finds the stress one foot above the course's own bottom, not at the bottom itself — that's the reference point the method is built around, and it's baked into the coefficient the page prints above.
What if the fill height is at or below the one-foot datum?
The formula would go negative, so the code floor governs instead — the calculator switches to the floor value automatically and shows that step.
Which allowable stress and joint efficiency should I use?
Use the values the material and weld records for that specific course actually document. The pre-filled defaults are the API 653 fallback for when a course has no identifiable material or construction record — treat them as a last resort, not a shortcut.
Is this the same formula as the API 650 design calculation?
No — API 650 sizes a new shell course for design loads. This is the API 653 in-service check: the smallest a course is allowed to thin down to before it needs attention. They share a family resemblance but answer different questions.
Does this apply to every tank diameter?
The one-foot method has an upper diameter limit; above it the code calls for the variable-design-point method instead. The calculator flags this automatically when your diameter is over the limit.
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/shell-tmin — TankForge