Field calculator
Tank volume calculator (cylindrical, gallons)
Sizing a cistern, a storage or buffer tank, or a chemical vessel starts with how much it actually holds. For a straight vertical cylinder the volume is pi divided by four, times the diameter squared, times the height, converted to gallons at 7.48 gallons per cubic foot. Enter the diameter and height in feet and a fill level percentage to get both the full capacity and the volume at the working level. This formula is for a vertical cylinder; a horizontal cylindrical tank, or one with dished or coned heads, needs a different calculation, and the usable volume is always less than the raw geometric volume once you subtract the outlet height, the freeboard, and any dead space below the draw. Confirm the tank geometry and the real working level before you rely on a number for a fill, a flush, or a chemical charge.
Result
Vertical cylindrical tank volume: V = pi/4 × diameter squared × height, converted to gallons at 7.48 gallons per cubic foot. Enter the diameter and height in feet and a fill percentage. This is for a straight vertical cylinder (a cistern, storage tank, or buffer tank); a horizontal cylinder or a tank with dished or coned heads needs a different calculation, and the usable volume is less than the geometric volume once you account for the outlet height and any freeboard. Confirm the tank geometry and the working level for a real charge.
Worked example
How much does a vertical cylindrical tank 8 ft in diameter and 10 ft tall hold?
- Diameter8 ft
- Height10 ft
- Volume = π÷4 × diameter² × height = 0.7854 × 64 × 10 = 502.7 cu ft.
- Gallons = 502.7 × 7.48 = 3,760 gallons when full.
About 502.7 cu ft, or 3,760 gallons full. Derate for freeboard and the real fill level for usable volume.
anvilfield.com/calculators/tank-volume-cylindrical-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Tank volume FAQ
What is a water supply fixture unit?
A water supply fixture unit, or WSFU, is a dimensionless number assigned to a fixture that represents its load on the water system, folding together how much it draws, how long, and how often. Totaling WSFU and reading the demand curve gives a probable peak flow in gpm. Verify the values against the adopted code.
How do you size water supply pipe?
Total the WSFU for each segment, convert to a probable demand in gpm off the Hunter curve, then build a pressure budget by subtracting elevation, meter, backflow, and fixture residual from the source pressure. Size each segment to carry its demand under both the leftover friction budget and the velocity limit. The adopted code controls.
What velocity is too high for water pipe?
Common design limits are about 8 ft per second for cold water and 5 ft per second for hot, with hot lower because it erodes copper faster. Above 140 degrees F, drop copper to 2 to 3 ft per second. Past these limits you get erosion corrosion, pinholes at the elbows, noise, and water hammer.
Why not size water pipe for every fixture running at once?
Because that moment never happens, and sizing for it builds oversized pipe that then sits stagnant. Not every fixture draws at the same second, and the odds fall as the building grows. That diversity is built into the demand curve, which flattens as fixture units climb, so the curve, not the summed flow, is the demand.
How much pressure do I lose going up a building?
About 0.43 psi for every foot of height, the static head, with a precise figure of 0.433 psi per foot. A fixture 40 ft up has lost roughly 17 psi to elevation before any flow starts. On a high-rise the elevation alone can outrun the street pressure, which is why tall buildings need pressure zones and booster pumps.