Printable reference
API 653 Formula Cheat Sheet
Every formula behind the free TankForge calculators, US and metric, with clause citations — one printable page.
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.
Shell minimum thickness (t-min) calculator
One-foot-method t-min per shell course — and its inverse, maximum fill height from a measured thickness.
US customary
t-min = max( 2.6 · D · (H − 1) · G / (S · E), 0.1 )
H = t · S · E / (2.6 · D · G) + 1 (inverse — max fill height)
Metric (SI)
t-min = max( 4.9 · D · (H − 0.3) · G / (S · E), 2.5 )
H = t · S · E / (4.9 · D · G) + 0.3 (inverse — max fill height)
| Symbol | Meaning | US unit | Metric unit | Source |
|---|---|---|---|---|
D | Tank diameter | ft | m | Tank record / As-built |
H | Fill height above this course | ft | m | 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 | MPa | Table 4.1 |
E | Joint efficiency | Table 4.2 | ||
t | Measured (or as-built) course thickness | in | mm | UT reading / mill cert |
API 653 §4.3.3.1API 653 Table 4.1API 653 Table 4.2
Bottom repair-threshold calculator
Repair every bottom reading at or below X so the next internal interval reaches N years — closed form and per-plate table.
US customary
T = (f · age · MRT + N · t0) / (f · age + N) (closed form, single governing thickness)
interval = min( (min − MRT) / rate × f, cap ) (f = 1 code form, f = 0.5 conservative comparison)
Metric (SI)
T = (f · age · MRT + N · t0) / (f · age + N) (closed form, single governing thickness)
interval = min( (min − MRT) / rate × f, cap ) (f = 1 code form, f = 0.5 conservative comparison)
| Symbol | Meaning | US unit | Metric unit | Source |
|---|---|---|---|---|
T | Repair threshold — patch every reading at or below T | in | mm | Solved |
MRT | Minimum remaining thickness at end of interval | in | mm | Table 4.4 |
t0 | Original / nominal bottom thickness | in | mm | As-built / mill cert |
age | Years the bottom has been in service | yr | yr | Tank record |
N | Target next internal interval | yr | yr | Owner / RBI target |
f | Life-to-interval factor | Interval basis toggle |
API 653 §4.4.5.1API 653 Table 4.4API 653 §6.4.2.2.1
Tank bottom remaining life and inspection interval calculator
Bottom corrosion rate, remaining life to the minimum remaining thickness, and the next internal inspection interval.
US customary
rate = (t0 − min) / age
remaining life = (min − MRT) / rate
interval = min( remaining life, cap ) (cap 20 yr; 30 yr with a release prevention barrier)
MRT = .100 in bare steel · .050 in with an RPB or reinforced lining
Metric (SI)
rate = (t0 − min) / age
remaining life = (min − MRT) / rate
interval = min( remaining life, cap ) (cap 20 yr; 30 yr with a release prevention barrier)
MRT = 2.540 mm bare steel · 1.270 mm with an RPB or reinforced lining
| Symbol | Meaning | US unit | Metric unit | Source |
|---|---|---|---|---|
min | Governing (least) remaining bottom thickness | in | mm | UT grid / MFL prove-up / pit depth |
t0 | Original / nominal bottom thickness | in | mm | As-built / mill cert |
age | Years the bottom has been in service | yr | yr | Tank record |
MRT | Minimum remaining thickness at the end of the interval | in | mm | Table 4.4 |
cap | Maximum internal inspection interval | yr | yr | §6.4.2.2.1 |
API 653 §4.4.5.1API 653 Table 4.4API 653 §6.4.2.2.1
Shell corrosion rate, remaining life and inspection interval calculator
Long-term and short-term shell corrosion rates, remaining life against t-min, and the external UT and visual inspection intervals.
US customary
LT = (t_nominal − t_current) / years in service
ST = (t_previous − t_current) / years between readings
N = max(LT, ST) (inspection practice — not a formula printed in API 653)
RCA = t_current − t-min; remaining life = RCA / N
external UT interval = min( RCA / 2N, 15 yr ); 5 yr when N is not known
external visual interval = min( RCA / 4N, 5 yr )
Metric (SI)
LT = (t_nominal − t_current) / years in service
ST = (t_previous − t_current) / years between readings
N = max(LT, ST) (inspection practice — not a formula printed in API 653)
RCA = t_current − t-min; remaining life = RCA / N
external UT interval = min( RCA / 2N, 15 yr ); 5 yr when N is not known
external visual interval = min( RCA / 4N, 5 yr )
| Symbol | Meaning | US unit | Metric unit | Source |
|---|---|---|---|---|
t_current | Current thickness at the location | in | mm | UT reading |
t-min | Minimum required thickness | in | mm | §4.3.3.1 |
t_nominal | As-built thickness | in | mm | As-built / mill cert |
t_previous | Same location, previous inspection | in | mm | Prior report |
N | Governing shell corrosion rate | mils/yr | mm/yr | Calculated |
RCA | Remaining corrosion allowance | in | mm | Calculated |
API 653 §6.3.3.2API 653 §6.3.2.1API 653 §4.3.3.1
Tank bottom critical-zone minimum thickness calculator
The critical-zone minimum bottom thickness at the next inspection from the lower shell course t-min — plus the remaining life and interval against it.
US customary
limit = max( min( .118 in, 0.5 × t-min course 1 ), .100 in )
t-min course 1 = max( 2.6 · D · (H − 1) · G / (S · E), 0.1 ) (one-foot method)
years to limit = (CZ min − limit) / rate; rate = (nominal − CZ min) / age
interval = min( years to limit, cap ) (cap 20 yr; 30 yr with a release prevention barrier)
Metric (SI)
limit = max( min( 3 mm, 0.5 × t-min course 1 ), 2.500 mm )
t-min course 1 = max( 4.9 · D · (H − 0.3) · G / (S · E), 2.5 ) (one-foot method)
years to limit = (CZ min − limit) / rate; rate = (nominal − CZ min) / age
interval = min( years to limit, cap ) (cap 20 yr; 30 yr with a release prevention barrier)
| Symbol | Meaning | US unit | Metric unit | Source |
|---|---|---|---|---|
t-min course 1 | Minimum thickness of the lower shell course | in | mm | §4.3.3.1 |
CZ min | Least remaining thickness in the critical zone | in | mm | UT / MFL prove-up in the critical zone |
nominal | As-built critical-zone plate thickness | in | mm | As-built / mill cert |
age | Years the bottom has been in service | yr | yr | Tank record |
cap | Maximum internal inspection interval | yr | yr | §6.4.2.2.1 |
API 653 §4.4.5.4API 653 §4.3.3.1API 653 §6.4.2.2.1
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.
US customary
S = min(0.8·Y, 0.429·T) (C1–C2)
S = min(0.88·Y, 0.472·T) (courses above C2)
St = min(0.88·Y, 0.472·T) (C1–C2, hydrotest)
St = min(0.9·Y, 0.519·T) (courses above C2, hydrotest)
Metric (SI)
S = min(0.8·Y, 0.429·T) (C1–C2)
S = min(0.88·Y, 0.472·T) (courses above C2)
St = min(0.88·Y, 0.472·T) (C1–C2, hydrotest)
St = min(0.9·Y, 0.519·T) (courses above C2, hydrotest)
| Symbol | Meaning | US unit | Metric unit | Source |
|---|---|---|---|---|
Y | Specified minimum yield strength | psi | MPa | Material spec / mill cert |
T | Specified minimum tensile strength | psi | MPa | Material spec / mill cert |
S | Allowable product design stress | psi | MPa | Table 4.1 |
St | Allowable hydrostatic test stress | psi | MPa | §4.3.3.2 |
API 653 Table 4.1API 653 §4.3.3.2
Joint efficiency (Table 4.2 / 4.3) calculator
Shell joint efficiency E by construction standard and build-year edition band, welded or riveted.
US customary
E is read directly from Table 4.2a / 4.2b (welded) or Table 4.3 (riveted) by construction standard and build-year edition — there is no formula to compute; the calculator selects the right row.
Metric (SI)
E is read directly from Table 4.2a / 4.2b (welded) or Table 4.3 (riveted) by construction standard and build-year edition — there is no formula to compute; the calculator selects the right row.
| Symbol | Meaning | US unit | Metric unit | Source |
|---|---|---|---|---|
E | Joint efficiency used in the one-foot method / t-min formula | Table 4.2 |
API 653 Table 4.2API 653 Table 4.3
Unit converter for API 653 inspectors
Thickness (in / mils / mm), corrosion rate (mpy / mm per yr), length (ft / m), and pressure (psi / MPa).
US customary
1 in = 25.4 mm, 1 mil = 0.0254 mm
1 ft = 0.3048 m
1 psi = 0.006894757293168361 MPa
mpy (mils/yr) to mm/yr uses the same factor as mils to mm (0.0254)
Metric (SI)
1 in = 25.4 mm, 1 mil = 0.0254 mm
1 ft = 0.3048 m
1 psi = 0.006894757293168361 MPa
mpy (mils/yr) to mm/yr uses the same factor as mils to mm (0.0254)
| Symbol | Meaning | US unit | Metric unit | Source |
|---|---|---|---|---|
in / mils / mm | Thickness | in, mils | mm | Exact SI unit definition |
mpy / mm/yr | Corrosion rate (thickness per year) | mpy | mm/yr | Exact SI unit definition |
ft / m | Length | ft | m | Exact SI unit definition |
psi / MPa | Pressure / stress | psi | MPa | Exact SI unit definition |
Built by TankForge — API 653 inspection software
The Annex C checklist, tank drawings, UT readings and the finished report, on one iPad, from the tank deck — even with no signal.
Free printable guides to other tank parts are coming — shell, bottom & foundation, nozzles.
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/formulas — TankForge