How this tank volume calculator works
A tank’s capacity is its interior volume converted to gallons — and the gallon is a fixed 231 cubic inches, so the only variable is the geometry:
gallons = in³ ÷ 231 · liters = in³ × 0.01639 · ft³ = in³ ÷ 1,728
At the defaults — a 24 in diameter, 48 in long cylinder — that is 21,715 in³, about 94 gallons, 356 liters or 12.57 ft³; the full chain is in the worked example. Measure the inside, since the water fills the interior, not the walls.
Cylinders and rectangular tanks
Two shapes cover most tanks; pick the one that matches and read the inside dimensions:
- Vertical cylinder: diameter and height — water totes, pressure tanks, silos. Volume is π r² × height.
- Horizontal cylinder: same formula with length instead of height — the full-tank capacity is identical.
- Rectangular: length × width × height — cisterns, sumps, stock tanks and troughs.
- Interior dimensions: subtract the wall thickness; a nominal size overstates capacity.
- Odd shapes: break a stepped or L-shaped tank into pieces and add — the same trick as the square footage calculator.
A capsule or oval tank — a cylinder with domed or bulged ends, common in stock tanks and propane — reads a little high on the plain-cylinder formula, because the ends aren’t flat. For a rough capacity that is close enough to order water or size a stand, treat it as a cylinder of the straight-wall length; for a billing-grade number, figure the end caps separately. Most utility tanks sit close enough to a plain cylinder that the difference falls inside the freeboard you’d leave at the top anyway.
Gallons, liters, and cubic feet
The same tank reads several ways — know which unit the job wants:
- US gallon: 231 in³ — what you fill, dose and pay for.
- Cubic foot: 1,728 in³, or 7.48 gallons — for flow and engineering.
- Liter: the metric measure, 3.785 per US gallon.
- Imperial gallon: larger than a US gallon (1.2×) — check which a UK spec means.
- Weight: water is 8.34 lb per gallon, so capacity also tells you the load on a stand.
Weight is the part people forget. Water is 8.34 pounds a gallon, so the default 94-gallon tank is about 784 pounds full — more than one person can shift, and a real load on whatever holds it up. Scale that to a 300-gallon tote and you are past a ton of water sitting on the stand or the floor, which is why a tank’s capacity is a structural number as much as a plumbing one. Size the support to the full tank, not the empty one.
Cylinder tank capacity, and the conversions
Table 1 is the capacity of common cylindrical tanks; Table 2 is the constants behind the conversions.
| Ø × length (in) | Gallons |
|---|---|
| 12 × 24 | 12 |
| 18 × 36 | 40 |
| 24 × 48 | 94 |
| 30 × 60 | 184 |
| 36 × 72 | 317 |
| Unit | Equals |
|---|---|
| 1 US gallon | 231 in³ |
| 1 cubic foot | 7.48 gal |
| 1 US gallon | 3.785 L |
| 1 gallon water | 8.34 lb |
How to measure a tank
- Pick the shape — cylinder or rectangular — to match the tank.
- Measure the inside dimensions in inches — diameter and length, or length, width and height.
- Subtract the walls if you only have the outside size, so the capacity isn’t overstated.
- Read gallons, liters and cubic feet, whichever the job needs.
- Scale for a partial fill — the height fraction for a vertical tank; a segment for a horizontal one.
Worked example: a 24 × 48 in cylinder
The default, worked line by line — a cylinder 24 in in diameter and 48 in long:
volume = π × 12² × 48 = 21,715 in³
gallons = 21,715 ÷ 231 = 94 gal
also = 356 L · 12.57 ft³ · about 784 lb of water
Notice how fast diameter grows the volume: it is squared, so a 30 in tank of the same length holds half again as much as a 24 in one — 147 versus 94 gallons — not the 25% the diameter suggests. That is worth remembering when a “slightly bigger” tank turns out to be a much heavier one on the stand.
What this tank estimate doesn’t account for
It is a clean geometric capacity — a few real-world things sit outside it:
- Partial horizontal fill. A part-full horizontal cylinder is a curved segment, not a simple fraction.
- Dished and coned ends. Rounded tank heads add or subtract from a plain cylinder.
- Fittings and baffles. Internal hardware displaces a little volume.
- Wall thickness. Nominal outside sizes overstate the interior — measure inside.
- Usable vs total. Outlets sit above the floor, so drawdown is less than the full capacity.
Tank volume FAQ
How do you calculate the volume of a tank?
Find the interior volume in cubic inches, then divide by 231 for US gallons. A cylinder is π × radius² × length; a rectangular tank is length × width × height. A 24 in diameter, 48 in long cylinder is 21,715 in³ — about 94 gallons. The calculator does the geometry and the conversions.
How many gallons is a cylindrical tank?
π × (diameter ÷ 2)² × length, in inches, divided by 231. So a 24 × 48 in tank holds 94 gallons; a 30 × 60 in tank about 184. The gallon is defined as 231 cubic inches, which is the only constant you need once you have the interior volume.
How do I find the volume of a rectangular tank?
Length × width × height in inches, divided by 231 for gallons. A 48 × 24 × 24 in tank is 27,648 in³, or about 120 gallons. Use the inside dimensions — the water fills the interior, not the outside of the walls — and switch the shape selector to rectangular.
How many gallons in a 24x48 tank?
About 94 US gallons for a cylinder 24 in across and 48 in long — 356 liters, or 12.57 cubic feet. If the 24 × 48 is a rectangular footprint you also need the height; at 24 in deep that rectangular tank would hold about 120 gallons instead.
What is the difference between gallons and cubic feet?
A US gallon is 231 cubic inches; a cubic foot is 1,728 cubic inches, or 7.48 gallons. So the same tank reads 94 gallons or 12.57 cubic feet — cubic feet for engineering and flow, gallons for what you fill and pay for. The calculator shows both, plus liters.
How do I calculate a partially filled tank?
This tool gives the full-tank capacity. A partly filled vertical tank is just the fraction of the height — half full is half the gallons. A partly filled horizontal cylinder is harder: the water forms a curved segment, so the fill volume is not proportional to depth — figure that segment separately.
Sources and standards
The volume is geometry pinned to exact unit definitions:
- The US gallon: exactly 231 cubic inches — the defining constant.
- Cylinder and prism volume: π r² × length and L × W × H — plane geometry.
- Exact conversions: 1,728 in³ per ft³, 3.785 L per gallon, 8.34 lb per gallon of water.
- Measure inside: capacity is the interior volume — subtract the walls.
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