How this instrument works
A cylindrical tank — a water tank, cistern, fuel tank or rain barrel standing upright — holds liquid in the shape of a simple cylinder, so its capacity follows the ordinary cylinder-volume formula: the circular base area, pi times the radius squared, multiplied by the tank's height. What separates a genuinely useful tank-capacity figure from a bare geometry answer is the unit at the end — tanks are bought, filled and rated in US gallons, not cubic feet.
This instrument takes the tank's diameter and height, both in feet, since that's how most standing tanks are specified and measured, computes the cylinder's volume in cubic feet, and multiplies by 7.48052 — the exact number of US gallons in one cubic foot — to return the capacity directly in gallons. That single multiplication is the entire difference between a raw geometry answer and the number printed on a tank's own capacity label.
Sizing a tank's true capacity matters before a purchase, since a supplier's stated 'nominal' size sometimes rounds to a marketing-friendly number rather than the tank's exact geometric volume, and it matters afterward too, for working out how long a pump will take to empty it, how much water a cistern can actually bank between refills, or how close to full a rain barrel is running from a measured water depth.
- Enter Tank diameter (ft) — the tank's inside diameter, in feet.
- Enter Tank height (ft) — the tank's usable height, in feet, from the base to the fill line or top.
- Read Volume (US gallons) directly beneath both fields; it recalculates instantly as either measurement changes.
- Use the tank's actual usable height rather than its outer shell height if there's dead space at the top or bottom that liquid never reaches.
- For a partially filled tank, enter the current liquid depth into Tank height (ft) instead of the tank's full height to estimate the volume currently held.
Worked example — a 4 ft diameter, 6 ft tall tank
Enter 4 into Tank diameter (ft) and 6 into Tank height (ft) — a compact vertical water storage tank. Radius is 4 ÷ 2 = 2 ft, so volume works out to π × 2² × 6 = π × 24 ≈ 75.3982 ft³.
Volume (US gallons) reads 564.018 — that figure multiplied by 7.48052, the exact number of US gallons packed into one cubic foot. This is the tank's true full-height capacity, the number to check against whatever capacity a supplier's spec sheet claims for the same nominal size.
Questions
Why multiply by 7.48052 specifically?
Because one cubic foot holds exactly 7.48052 US gallons — a fixed conversion that follows directly from the legal definitions of the foot and the gallon, not an approximation. Once the tank's volume is computed in cubic feet from its diameter and height, multiplying by that figure converts it straight to gallons.
Should I use the tank's full height or the liquid's current depth?
Use full height for the tank's total capacity, or substitute the current liquid depth into the same Tank height (ft) field to estimate how much is in the tank right now — the formula works identically either way, since it's simply computing the volume of whatever cylinder of liquid is currently present.
Does the tank's wall thickness affect this figure?
Only if you measure the outside diameter instead of the inside. This instrument assumes Tank diameter (ft) is the tank's inside, liquid-holding diameter; measuring across the outer shell instead slightly overstates capacity, more noticeably on a tank with thick walls, like a steel or heavily insulated vessel.
Why doesn't a supplier's 'nominal' capacity always match this instrument's answer?
Manufacturers sometimes round a tank's stated capacity to a clean marketing figure — a '500-gallon' tank, for example — rather than its exact geometric volume at true diameter and height, and some capacity ratings also build in a margin below the physical top of the tank to leave room for expansion or venting. Measuring the tank's actual dimensions and running them through this instrument gives the true, non-rounded figure.
How is this different from the site's pipe volume instrument?
Both use the same cylinder-volume math and report US gallons, but the shape and typical proportions differ: a tank is usually short and wide relative to its height, entered here by diameter and height as a single standing vessel, while a pipe is long and thin, entered by diameter and a much greater length, sized as a flowing run rather than a static reservoir.