Storage Tank Sizing (API 650)

Quick pick (TL;DR)

Working volume = throughput × storage days; divide by the max fill (~90%) for geometric volume.
Roof: fixed-roof (cone) for low-volatility products (API 650); external/internal floating roof to cut breathing losses on volatile products; sphere/bullet for pressurised.
Choose a shell height (often 12 m) and the diameter falls out of V = (π/4)D²H.
Provide proper venting (API 2000) and 110% bund containment.

Step 1 — Roof / tank type

Type	Use for	Why
Fixed-roof	Low-volatility	Simple
Floating (EFR)	Volatile	Less loss
Floating (IFR)	Volatile + weather	Roof + deck
Sphere / bullet	Pressurised	Pressure

Step 2 — Worked sizing

Diesel, 1200 m³/day, 3 days

Step 1 — Working volume V_work = 1200 × 3 = 3600 m³ Step 2 — Geometric volume max fill 90% → V = 3600 / 0.9 = 4000 m³ Step 3 — Height & diameter choose H = 12 m A = V / H = 4000 / 12 = 333.3 m² D = √(4A/π) = √(424.4) = 20.6 m Step 4 — Round up select D = 21 m (capacity ~4156 m³) Answer → fixed-roof, D 21 m × H 12 m (API 650)

Capacity is a planning choice; ÷0.9 fill leaves headroom; fix H, solve for D.

Step 3 — Venting & containment

Vent per API 2000 (normal + emergency).
Bund ≥ 110% of largest tank.
Ringwall foundation; seismic; roof seals.

Tank sizing calculator

Working volume → diameter

Working V = throughput × days ÷ fill; D from chosen height.

Throughput (m³/day)

Storage days

Max fill

Height H (m)

Fixed-roof/API 650 screening — fix height, solve for diameter.

Common mistakes

Sizing to 100% fill.
Fixed roof on volatile product.
Undersized venting.

Requisition & datasheet

Product, throughput, days, volume.
Type, H, D, venting, bund, material.

Frequently asked questions

What should I know about Storage Tank Sizing (API 650)?

Working volume = throughput × storage days; divide by the max fill (~90%) for geometric volume. Roof: fixed-roof (cone) for low-volatility products (API 650); external/internal floating roof to cut breathing losses on volatile products; sphere/bullet for pressurised. Choose a shell height (often 12 m) and the diameter falls out of V = (π/4)D²H. Provide proper venting (API 2000) and 110% bund containment.

Which standards or codes apply?

API 650 (welded tanks), API 620 (low-pressure tanks), API 2000 (venting), API 653 (inspection), NFPA 30 (flammable liquids).

Standards — API 650/620/2000/653; NFPA 30.