Sphere & Capsule Tank Volume Calculator — Hemispherical Heads & Pure Spheres

A pure sphere has no straight cylindrical section — the entire tank is curved (like a giant ball). A capsule (or bullet) has a straight cylindrical body between two hemispherical heads (like a medicine capsule). Both shapes hold pressure well because the curve distributes stress through tension. Spheres have the most efficient volume-to-surface-area ratio of any shape (lowest wall cost per gallon), but are harder to transport. Capsules trade some efficiency for transportability — the straight body lets you mount the tank on a trailer frame.

Two hemispherical heads on a capsule tank together make exactly one full sphere. So the head contribution is (4/3)πr³ — and you can calculate the head volume independently from the straight shell. On a 36-inch diameter capsule the heads add (4/3)π×18³ = 24,429 in³ ≈ 105.8 gallons. That's about 21% of the total for a capsule with a 60-inch straight shell — always budget for this extra capacity when sizing a capsule or bullet tank.

Yes, spheres have the most extreme non-linear partial fill of any common tank shape. At 25% depth a sphere holds only about 10% of its volume — even less than a horizontal cylinder's 19.6%. At 50% depth it holds exactly 50% (symmetry again), and at 75% depth it holds about 90%. The spherical cap integral V = πh²(3r-h)/3 explains this extreme curve — the volume added per inch is smallest near the very bottom, grows through the middle, then shrinks again near the very top. Dip charts for spherical tanks must be computed point by point with the cap formula.

Spherical tanks are used for bulk LPG storage (large propane spheres at gas terminals), high-pressure laboratory vessels, and some cryogenic storage. Capsule tanks are everywhere: home propane bullets (250, 500, 1000 gallons), road tankers (LPG, ammonia, compressed gases), air receiver tanks, and portable water or fuel day tanks. The home heating-oil tanks that sit in basements are NOT capsules — they use dished heads or ovals, but the capsule is the shape of most compressed gas vessels.

If you know the total gallons of a tank but not whether it's a sphere or capsule, assume capsule — it's far more common for transport and field use. If you know the overall length (e.g., a 12-foot bullet tank has about 60 inches of straight shell plus two heads on a 36-inch diameter = 120 inches total) you can back into the straight length. If all you know is diameter and total gallons, this calculator lets you try different straight-length values until the capsule total matches your known figure.