When must I use Variable-Design-Point instead?

When D > 60 m (200 ft), API 650 requires the Variable-Design-Point Method (§5.6.4) for final shell design. This calculator still shows a screening thickness but raises a diameter-limit warning.

How does Table 5.2a minimum thickness interact with t_d?

t_required = max(t_d, t_t, t_min). If calculated thicknesses fall below the diameter-based floor, the course is raised to t_min before commercial plate selection.

Is shell weight a certified MTO?

No. Mass is a screening take-off from π·D·h·t_nom·ρ. Fabrication scrap, weld overlay, and wind-girder steel are not included.

API 650 Tank Shell Thickness Calculator (1-Foot Method)

Engineering Reference & ASME Code Basis

1. Core Formula & Variable Definitions

API Standard 650 — Welded Tanks for Oil Storage, 13th Edition — Section 5.6.3 Calculation by 1-Foot Method · API Standard 650 — Section 5.6.1.1 / Table 5.2a & 5.2b Minimum Shell Thickness · API Standard 650 — Section 5.6.4 Variable-Design-Point Method (out of scope — diameter screen)

Design thickness (SI): t_d = 4.9·D·(H−0.3)·G / (S_d·E) + CA

Hydrotest thickness (SI): t_t = 4.9·D·(H−0.3) / S_t + CA

USC: replace 4.9→2.6 and (H−0.3)→(H−1) with D, H in ft and stresses in psi

Governing plate: t_required = max(t_d, t_t, t_min,API), t_nom = CeilToCommercialPlate(t_required)

API Standard 650 — Section 5.6.3 1-Foot Method · Table 5.2a / 5.2b minimums

Hero t_nom · massMethod 1-foot §5.6.3Floor Table 5.2a/bLimit D ≤ 60 m

H is the height from the bottom of the course under consideration to the design liquid level. Course-by-course screening uses successive H reductions by course height. Shell mass uses π·D·h_course·t_nom·ρ with ρ = 7850 kg/m³. Screening only — confirm with the project API 650 edition.

t_d (Design shell thickness) — Product design thickness including CA (mm / in).
t_t (Hydrostatic test thickness) — Hydrotest condition thickness including CA (mm / in).
D, H (Diameter / liquid height) — Nominal inside diameter and liquid height above course bottom (m / ft).
G, E (Specific gravity / joint efficiency) — Design specific gravity (water = 1.0) and weld joint efficiency (0.70–1.0).
S_d, S_t (Allowable stresses) — Product design and hydrotest allowables from API 650 Table 5.2a/5.2b for the selected plate.
t_min, t_nom (Code floor / commercial plate) — Table 5.2a/b minimum by diameter; t_nom is next mill plate ≥ t_required.

2. Screening Rules Matching This Calculator

API 650 §5.6.3 1-foot equations, Table 5.2a/5.2b minimum shell thickness by diameter, and commercial plate ceil. Variable-Design-Point, wind girders, and seismic buckling are out of scope.

Default duty20 m · 15 m · A36 · E 0.85 · CA 2 mm

Bottom course hero ≈ 12 mm · 61.7 t with t_d ≈ 11.00 mm and t_t ≈ 10.42 mm.

1-foot diameter screenD ≤ 60 m (200 ft)

Larger tanks require Variable-Design-Point Method (§5.6.4) for final design.

Minimum thickness floorTable 5.2a/5.2b

D < 15 m → 5 mm; 15–36 m → 6 mm; 36–60 m → 8 mm; D > 60 m → 10 mm.

Out of scope hereVDP · wind girder · seismic

No intermediate wind girders, annular plates, or appendix seismic checks.

Quick Reference Lookup Table

Default duty (D = 20 m · H = 15 m · course 2.5 m · G = 0.85 · A36 · E = 0.85 · CA = 2 mm) — engine assert
Quantity	Value	Unit	Notes
Bottom t_d	11.00	mm	4.9 D (H−0.3) G /(S_d E) + CA
Bottom t_t	10.42	mm	4.9 D (H−0.3)/S_t + CA
API 650 t_min	6	mm	15 ≤ D < 36 m
Bottom t_nom	12	mm	Ceil commercial plate
Courses	6	—	15 / 2.5
Shell mass	61.7	t	π D h t_nom ρ · Σ courses
40 m · 18 m · A516-70 · G=1 · CA=3 · E=1	t_d 22.0 · t_nom 22	mm	S_d = 183 MPa
60 ft · 48 ft · A36 · CA=1/16"	t_d 0.378 · t_nom 0.500	in	Course height 8 ft
100 ft · 50 ft · A283-C · CA=1/8"	t_d 0.766 · t_nom 0.875	in	D ≈ 30.5 m < 60 m screen
1-foot diameter limit	60 m / 200 ft	—	Above → §5.6.4 VDP

Hero reports bottom-course commercial plate and total shell mass. Default duty asserts t_d ≈ 11.00 mm, t_t ≈ 10.42 mm, t_nom = 12 mm, mass ≈ 61.7 t. Spec LLM heroes that omit course mass or mis-state t_t are discarded.

3. Plate Metallurgy & Allowable Stress Limits

API 650 Table 5.2a/5.2b screening allowables for common carbon and HSLA tank plate. Confirm group, thickness category, and design metal temperature against the project edition.

Material Group	Temperature Range	Allowable Stress / Limit	Engineering Notes
ASTM A283 Grade C	Ambient design metal ≤ ~90 °C screening	S_d = 137 MPa · S_t = 154 MPa	USC: 20,000 / 22,500 psi. Lower-cost CS plate.
ASTM A36	Ambient design metal ≤ ~90 °C screening	S_d = 160 MPa · S_t = 171 MPa	USC: 23,200 / 24,900 psi. Default calculator material.
ASTM A516 Grade 70	Ambient design metal ≤ ~90 °C screening	S_d = 183 MPa · S_t = 196 MPa	USC: 26,600 / 28,500 psi. Pressure-vessel quality CS.
ASTM A537 Class 1	Ambient design metal ≤ ~90 °C screening	S_d = 200 MPa · S_t = 214 MPa	USC: 29,000 / 31,100 psi. HSLA quenched & tempered plate.

Code Applicability & Safety Boundaries

Not a substitute for Variable-Design-Point (§5.6.4) when D > 60 m
Allowables are ambient screening — check elevated-temperature tables when required
Joint efficiency E must match the radiographic / examination plan

4. Step-by-Step Worked Example

Field VerificationShow

Worked Example — 20 m × 15 m A36 Tank

Size bottom and upper shell courses for a 20 m diameter tank with 15 m design liquid height, 2.5 m courses, G = 0.85, ASTM A36, E = 0.85, CA = 2 mm using the API 650 1-foot method.

Geometry:D = 20 m · H = 15 m · course = 2.5 mFluid / joint:G = 0.85 · E = 0.85Material:A36 · S_d = 160 MPa · S_t = 171 MPaCA:2 mm

Bottom-course design thickness

Formula: t_d = 4.9 D (H−0.3) G /(S_d E) + CA

4.9·20·14.7·0.85/(160·0.85) + 2

Result:t_d ≈ 11.00 mm

Hydrotest thickness

Formula: t_t = 4.9 D (H−0.3)/S_t + CA

4.9·20·14.7/171 + 2

Result:t_t ≈ 10.42 mm

Code floor and commercial plate

Formula: t_required = max(t_d, t_t, t_min)

max(11.00, 10.42, 6) → ceil plate

Result:t_nom = 12 mm

Course stack and shell mass

Formula: Σ π D h_i t_nom,i ρ

6 courses · ρ = 7850 kg/m³

Result:Shell mass ≈ 61.7 t

Conclusion: Bottom course screens at 12 mm commercial plate (governed by t_d ≈ 11.00 mm). Total shell plate mass ≈ 61.7 t. Confirm wind girders and, if D exceeds 60 m, switch to Variable-Design-Point.

5. How to calculate API 650 tank shell thickness (1-foot method)

1
Enter tank diameter, liquid height, and course height
Use inside diameter D and design liquid height H. Course height sets how many shell rings are stacked.
2
Select plate material, joint efficiency, G, and CA
Choose A283-C / A36 / A516-70 / A537-1, set E from the RT plan, specific gravity, and corrosion allowance.
3
Review bottom-course t_nom and shell mass
Hero shows commercial bottom-course thickness and total shell weight. Summary lists t_d, t_t, and t_min.
4
Check course MTO table and diameter screen
Use the course step chart/table for plate take-off. If D > 60 m, apply Variable-Design-Point instead of 1-foot.

6. Frequently Asked Questions & Technical References

Show

Section 5.6.3 calculates each shell course using liquid height H measured from the bottom of that course, with the (H − 1 ft) or (H − 0.3 m) offset built into the code equations.