Cylindrical tank volume sketches show up when you are estimating usable water in a vertical or horizontal freshwater tank for fire protection learning. The geometric volume of a right circular cylinder is V = π(d/2)²H with diameter and height (or length) in consistent units. This page explains that sketch and gives a free browser calculator that accepts diameter and height/length in feet (or inches), then reports volume in cubic feet and U.S. gallons (ft³ × 7.4805 for freshwater), with an optional fill percentage. It is for learning and quick checks—not a substitute for manufacturer tank data, freeboard and suction geometry, the criteria adopted by your authority having jurisdiction, or review by a licensed design professional.
How tank volume is used
- Enter the internal diameter of the cylinder (tank shell ID), not a painted outside dimension if they differ.
- Enter the cylinder height for a vertical tank, or the cylinder length for a horizontal tank treated as a full cylinder. This page does not solve partial-fill horizontal-cylinder geometry beyond a simple fill % of total volume.
- Compute V = π(d/2)²H with d and H in the same unit system (here: convert inches to feet when needed).
- Convert ft³ to gallons with the freshwater factor 7.4805 gal/ft³ when you need a gallon sketch. Multiply by fill % / 100 if you only want a partial-fill sketch of total capacity.
- Treat the result as geometry only. It does not size pumps, confirm listing data, net positive suction head, usable volume above the suction outlet, or replace hydraulic calculations in the adopted edition.
Free calculator
Educational approximation only. Freshwater gallons = ft³ × 7.4805. Nothing you enter leaves your browser. For pipe segment volume see the pipe volume calculator; for elevation pressure see elevation pressure; for pump demand sketches see the fire pump curve check. Full pump and tank tables live in the Fire Pump & Tank Sizing workbook.
Worked example
A vertical freshwater tank with an internal diameter of 10 ft and a water height of 12 ft: V = π(5)²×12 = 942.5 ft³ ≈ 7,050 gallons (using 7.4805 gal/ft³). At 80% of that geometric volume the sketch is about 5,640 gallons. Always confirm the published internal dimensions, freeboard, and suction geometry for the tank you are modeling, and check the result against the adopted edition, manufacturer data, and AHJ—not this page alone.
Next steps: convert elevation and pressure with the free elevation pressure calculator, sketch pump demand with the fire pump curve check, estimate available supply with the hydrant flow test calculator or residual pressure calculator, and run full pump and tank tables in the Fire Pump & Tank Sizing workbook ($49). For sprinkler segment volume, use the pipe volume calculator.