Allowable stress (Table 4.1) calculator
Allowable product stress S and hydrotest stress St by material and shell course, with the full Table 4.1.
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.
Every t-min calculation needs an allowable stress, and API 653 gets it from the shell material's specified yield and tensile strength through a table lookup — a smaller number for the bottom two courses, a larger one for the courses above them, and a separate pair of numbers for the hydrostatic test. Pick a material and a course below to see both, plus the full table for comparison.
- S = min(0.8·Y, 0.429·T) for C1–C2, min(0.88·Y, 0.472·T) above
- Course 1, Unknown material: Y = 30000 psi, T = 55000 psi -> S = 23600 psi, St = 26000 psi
| Material | S lower psi | S upper psi | Y psi | T psi |
|---|---|---|---|---|
| Unknown material * | 23600 | 26000 | 30000 | 55000 |
| A283 Gr. C | 23600 | 26000 | 30000 | 55000 |
| A285 Gr. C | 23600 | 26000 | 30000 | 55000 |
| A36 | 24900 | 27400 | 36000 | 58000 |
| A516 Gr. 55 | 23600 | 26000 | 30000 | 55000 |
| A516 Gr. 60 | 25600 | 28200 | 32000 | 60000 |
| A516 Gr. 65 | 27900 | 30700 | 35000 | 65000 |
| A516 Gr. 70 | 30000 | 33000 | 38000 | 70000 |
| A573 Gr. 58 | 24900 | 27400 | 32000 | 58000 |
| A573 Gr. 65 | 27900 | 30700 | 35000 | 65000 |
| A573 Gr. 70 | 30000 | 33000 | 42000 | 70000 |
The formula
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
Y | Specified minimum yield strength | psi | Material spec / mill cert |
T | Specified minimum tensile strength | psi | Material spec / mill cert |
S | Allowable product design stress | psi | Table 4.1 |
St | Allowable hydrostatic test stress | psi | §4.3.3.2 |
Worked example
Using the pre-filled example above (US customary units):
- S = min(0.8·Y, 0.429·T) for C1–C2, min(0.88·Y, 0.472·T) above
- Course 1, Unknown material: Y = 30000 psi, T = 55000 psi -> S = 23600 psi, St = 26000 psi
- Allowable product stress S: 23600 psi
- Allowable hydrotest stress St: 26000 psi
- Specified minimum yield Y: 30000 psi
- Specified minimum tensile T: 55000 psi
Where the inputs come from
- Material comes from the mill certification or the original construction record for that specific shell course — courses are sometimes mixed materials on older tanks.
- Course number counts from the bottom: the bottom two courses use the lower-course value, and every course above them uses the higher upper-course value.
- When the material record is missing or unreadable, the unknown-material row is the API 653 fallback, not a shortcut to use when the real record is available.
Built by TankForge — API 653 inspection software
TankForge looks up the right row per course automatically from the material you set for that course, so the allowable stress in a t-min calculation always matches what the report cites.
Related
Frequently asked
Why does a hydrotest value here differ from the table in Addendum 4?
Every value on this page is computed from the material's specified yield and tensile strength with the factors in the formula above, then rounded the way the table's notes direct. In Addendum 4 two printed hydrotest cells do not match that arithmetic: the metric lower-course value for the grades with the unknown-material strengths, and the US upper-course value for A516 Grade 60. The page keeps the computed value, which matches Addendum 2 and Addendum 4's own rows for materials with the same strengths. Check against your edition if a report depends on it.
Why is the allowable stress higher above the bottom two courses?
The table reflects the lower design margin API 653 allows for courses that carry less structural consequence than the bottom two — the same material gets a smaller allowable stress low on the shell and a larger one higher up.
What is the difference between the product stress and the hydrotest stress?
The product stress governs the tank in normal service with its stored product. The hydrotest stress is a separate, higher allowance used only for checking a hydrostatic test with water, which is denser than most stored products and briefly loads the shell harder.
What if my material is not in the list?
Use the unknown-material row unless you can find the specific ASTM grade in the construction records — do not guess a grade that is not documented, since that overstates the allowable stress and understates the required thickness.
Does course number 1 always mean the bottom shell course?
Yes, by the convention this calculator (and TankForge) uses: course 1 is the lowest shell course, immediately above the bottom.
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).
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Ask for a guide, calculator or tank part we haven’t covered. Email is optional — anonymous requests are welcome.
tankforge.us/guide/calc/allowable-stress — TankForge