Water Tank Volume Calculator
Find how many gallons and liters your water tank holds, from its shape and dimensions.
Reviewed by the ToolNestr Editorial Team — July 2026
How the water tank volume calculator works
A tank's capacity is just its internal volume expressed in the units you buy or measure water in. The method is always two steps: find the volume in cubic feet from the tank's geometry, then convert cubic feet to gallons or liters with a fixed constant. One cubic foot of water is 7.48 US gallons, 6.23 imperial gallons, or 28.32 liters.
The geometry depends on the shape. A rectangular tank is length × width × height. A cylinder — whether it stands upright or lies on its side — is π × radius² × the length along its axis; for a vertical cylinder that axis is the height, and for a horizontal cylinder it is the barrel length. Because we work in inches, the calculator divides the cubic-inch result by 1728 to get cubic feet before converting to gallons.
For the most accurate capacity, measure the inside of the tank rather than the outside, since the walls displace water. The geometric volume is the tank filled to the brim; the usable volume is slightly less because of the overflow air gap at the top and the dead space below the outlet at the bottom.
The cubic-foot-to-gallon conversion (7.48 US gal/ft³, 28.32 L/ft³) follows the standard US customary and SI water-volume equivalents published by NIST.
Pick the shape
Vertical cylinder, horizontal cylinder, or rectangular.
Measure inside, in inches
Use internal dimensions for a true capacity.
Read the capacity
Get gallons, liters, and imperial gallons at once.
The formula explained
Cylinder volume
= π × (dia⁄2)² × length in cubic inches, then ÷ 1728 for cubic feet. A 36 in diameter, 60 in tall vertical tank: π × 18² × 60 = 61,073 in³ ÷ 1728 = 35.3 ft³.
Convert to gallons
gal = ft³ × 7.48 = 35.3 × 7.48 = 264 US gallons.
Rectangular volume
= L × W × H in cubic inches, same ÷ 1728 and × 7.48 to reach gallons.
Worked example
A rectangular cistern 72 in long × 30 in wide × 48 in high (inside).
Use cases for the water tank volume calculator
Rainwater harvesting
Size a cistern to capture runoff from your roof, and compare its capacity against your household's daily water use to plan for dry-season storage.
Livestock and irrigation
Calculate horizontal tank capacity for a trailer-mounted water hauler or stationary stock tank, and confirm it fits your delivery truck's payload limits.
Fire suppression storage
Verify a vertical storage tank meets a required minimum gallon capacity for a rural fire-suppression water supply or sprinkler system reserve.
Aquaculture and ponds
Find the gallon capacity of a rectangular holding tank for fish or aquaponics, which determines stocking density and filtration sizing.
Real-world context
A common vertical tote-style storage tank, 48 inches in diameter and 72 inches tall, works out to π × 24² × 72 = 130,288 cubic inches, or 75.4 cubic feet — that's 564 US gallons, right in line with the widely used 550-gallon IBC tote size found on many rural properties for backup water storage.
A horizontal water-hauling tank mounted on a utility trailer, 24 inches in diameter and 96 inches long, holds π × 12² × 96 = 43,429 cubic inches, or 25.1 cubic feet, which converts to 188 gallons — light enough at roughly 1,570 lb of water that it stays well within a half-ton pickup's typical payload rating even fully loaded.
A rectangular rainwater cistern built into a basement or crawlspace, 96 inches long, 48 inches wide, and 60 inches deep (measured inside the walls), holds 96 × 48 × 60 = 276,480 cubic inches, or 160 cubic feet — that's 1,197 US gallons, enough to cover several weeks of household use for a family practicing water conservation during a dry spell.
Common misconceptions
"A tank's rated capacity on the label is exactly how much water it holds." Rated capacity is usually rounded down from geometric volume. Manufacturers typically label a tank below its brim-full geometric volume to account for the overflow port, fill neck, and a safety margin, so the calculator's filled-to-the-brim figure will often read slightly higher than the printed rating.
"US gallons and imperial gallons are close enough to use interchangeably." The difference is about 20%. One imperial gallon equals roughly 1.2 US gallons, so a tank rated at 500 imperial gallons actually holds about 600 US gallons — mixing up the two units when ordering a tank or estimating fill time causes a meaningful miscalculation.
"A horizontal cylindrical tank holds the same amount whether it's full to the top or only half full by height." Volume does not scale linearly with fill height in a horizontal cylinder. Because the cross-section is a circle rather than a rectangle, a tank filled to half its diameter actually holds exactly half the volume, but any other partial fill level requires circular-segment geometry, not a simple percentage of height.
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Frequently asked questions
How do I calculate the volume of a water tank in gallons?
First find the tank volume in cubic feet from its shape — length × width × height for a rectangular tank, or π × radius² × height for a cylinder. Then multiply cubic feet by 7.48 to get US gallons.
How many gallons is a cubic foot of water?
One cubic foot holds 7.48 US gallons (or about 28.32 liters). This is a fixed conversion, so any tank volume in cubic feet can be turned into gallons by multiplying by 7.48.
Does tank wall thickness matter?
For an accurate capacity you should measure the inside dimensions, since the walls take up space the water cannot. On thin plastic or steel tanks the difference is small, but on thick concrete cisterns it is worth subtracting the wall thickness from each dimension.
Why is my tank never completely full?
Most tanks have an overflow or air gap near the top and a small dead space at the bottom below the outlet, so the usable volume is a few percent less than the full geometric volume this calculator gives.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.