How it’s calculated
A capsule is a cylinder of length a = L − 2r capped by two hemispheres, which together make one sphere of radius r.
Example (a horizontal tank): r = 1 ft, L = 5 ft, so a = 3 ft. V = π × 1 × 3 + (4/3)π = (13/3)π ≈ 13.614 ft³ ≈ 101.8 US gal; A = 6π + 4π = 10π ≈ 31.42 ft².
Real tanks have wall thickness and often elliptical rather than hemispherical heads, so use the inside dimensions and treat the result as an estimate.
Frequently asked questions
What if I know the length of the straight section instead?
Add the diameter to it to get the overall length: L = a + 2r.
Is a propane tank a capsule?
Many horizontal propane tanks are close to a capsule, but heads are often shallower (semi-elliptical), so the true capacity is a little less. Use the nameplate water capacity when you have it.
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Sources
- Prealgebra 2e, 9.6 Solve Geometry Applications: Volume and Surface Area (cylinders and spheres) — OpenStax
- Capsule — Wolfram MathWorld
Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.
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