Tank and water-supply duration sketches answer a simple question: at a steady discharge flow, how long does a known volume of water last? The teaching relation is duration (minutes) = usable volume (gallons) ÷ flow (gpm), or hours = minutes ÷ 60. This page explains that sketch and gives a free browser calculator for volume and flow (with an optional reverse: volume from flow × duration). 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, a sealed hydraulic calculation, or review by a licensed design professional.
How duration is used
- Start from usable volume (gallons above the suction outlet / freeboard), not the painted shell capacity, when that distinction matters on your project.
- Pick a steady flow in gpm that matches the meaning you are studying (sprinkler demand, hose stream, pump rated flow, or a teaching example).
- Compute tmin = Vgal / Qgpm. Convert to hours with ÷ 60 when you need a longer-duration sketch.
- Treat the result as arithmetic only. It does not size pumps, confirm listing data, net positive suction head, fill time, or replace duration rules in the adopted edition.
Free calculator
Educational approximation only. Duration = gallons ÷ gpm (minutes). Nothing you enter leaves your browser. For cylinder geometry see the tank volume calculator; for flow unit conversion see GPM to CFS / CFS to GPM; 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 tank with about 10,000 gallons of usable water discharging at a steady 250 gpm: 10,000 ÷ 250 = 40 minutes (about 0.67 hours). Conversely, 250 gpm for 60 minutes sketches 15,000 gallons. Always confirm usable volume (not shell capacity alone), and check the result against the adopted edition, manufacturer data, and AHJ—not this page alone.
Next steps: estimate cylinder geometry with the free tank volume calculator, convert elevation and pressure with the elevation pressure calculator, sketch pump demand with the fire pump curve check, estimate available supply with the hydrant flow test calculator, and run full pump and tank tables in the Fire Pump & Tank Sizing workbook ($49).