How many gallons is my tank?
Measure the inside diameter and length of a round tank — or length, width and height of a box tank — and read the exact US gallons, litres and cubic feet. Set the fill slider to your dipstick reading for what is in it now.
Exact segment geometry — see methodology. Dished-head propane math is documented there too.
What to measure
The quick formula for cylinder volume in gallons
US gallons = D² × L × 0.0034US gallons = D² × L × 5.875US gallons = L × W × H ÷ 231Both cylinder constants come from the same two facts: a circle's area is π × D² ÷ 4, and a US gallon is exactly 231 cubic inches. In inches that gives π ÷ 4 ÷ 231 = 0.00339999…, which rounds to 0.0034; in feet, with 1,728 cubic inches per cubic foot, it gives 5.8752. It works the same whether the tank stands up or lies on its side — only the partly full figure depends on orientation.
Worked example. A round tank 48 in across and 72 in long: 48 × 48 × 72 = 165,888 → × 0.0034 = 564.0 US gallons (2,135 L). Enter 48 and 72 above to check it.
Gallons per inch of a round tank
For a standing cylinder, each inch of depth holds the same amount, so this table turns a tape-measure reading straight into gallons. For a horizontal tank it gives the gallons per inch of length — useful for sizing, but not for reading a dipstick.
| Inside diameter | Per inch of length | Per foot of length | Per inch, litres |
|---|---|---|---|
| 12 in (1 ft) | 0.49 gal | 5.9 gal | 1.85 L |
| 18 in (1.5 ft) | 1.10 gal | 13.2 gal | 4.17 L |
| 24 in (2 ft) | 1.96 gal | 23.5 gal | 7.41 L |
| 30 in (2.5 ft) | 3.06 gal | 36.7 gal | 11.58 L |
| 36 in (3 ft) | 4.41 gal | 52.9 gal | 16.68 L |
| 42 in (3.5 ft) | 6.00 gal | 72.0 gal | 22.70 L |
| 48 in (4 ft) | 7.83 gal | 94.0 gal | 29.65 L |
| 54 in (4.5 ft) | 9.91 gal | 119.0 gal | 37.53 L |
| 60 in (5 ft) | 12.24 gal | 146.9 gal | 46.33 L |
| 72 in (6 ft) | 17.63 gal | 211.5 gal | 66.72 L |
| 84 in (7 ft) | 23.99 gal | 287.9 gal | 90.81 L |
| 96 in (8 ft) | 31.33 gal | 376.0 gal | 118.61 L |
| 120 in (10 ft) | 48.96 gal | 587.5 gal | 185.33 L |
| 144 in (12 ft) | 70.50 gal | 846.0 gal | 266.88 L |
Lying on its side? A dipstick reading on a horizontal tank is not linear in depth. Use the horizontal tank volume calculator or a printed tank chart.
FAQ
TVC-113/FAQHow do I work out how many gallons are in a round tank?
Measure the inside diameter and the length (or height) in inches, then multiply: diameter × diameter × length × 0.0034. A 48-inch tank that is 72 inches long holds 48 × 48 × 72 × 0.0034 ≈ 564.0 US gallons. The 0.0034 is π ÷ 4 ÷ 231, because a US gallon is exactly 231 cubic inches.
How many gallons is a cylinder measured in feet?
Multiply diameter × diameter × length in feet by 5.875. A 4 ft × 6 ft tank holds 4 × 4 × 6 × 5.875 ≈ 564 US gallons.
How many gallons is a rectangular tank?
Length × width × height in inches, divided by 231. A 48 × 24 × 30 inch tank holds 34,560 ÷ 231 ≈ 149.6 gallons.
My tank is partly full — how many gallons are in it?
For a vertical (standing) cylinder, gallons are proportional to depth: multiply the depth in inches by the gallons-per-inch in the table. For a horizontal (lying) cylinder they are not, because the middle inches hold far more than the top and bottom ones — enter the dipstick depth in the calculator above, or use the horizontal tank volume calculator.
Should I use inside or outside dimensions?
Inside. Outside dimensions include the wall, so on a thick plastic or concrete tank they overstate capacity. If you can only measure the outside, subtract twice the wall thickness from the diameter (and from the height, for the floor and lid).
Is this US gallons or imperial gallons?
US gallons (231 cubic inches). One imperial gallon is about 1.2 US gallons, so divide the US figure by 1.20095 for imperial gallons. Litres and Imperial gallons are shown in the results.
Sources and standards
- Handbook 44, Appendix C — General Tables of Units of MeasurementNational Institute of Standards and Technology, U.S. Department of Commercehttps://www.nist.gov/document/2026-nist-handbook-44-appendix-cDefines the US liquid gallon as exactly 231 cubic inches — the constant behind every formula on this page.
- Methodology — the partial-fill formulas and liquid densities used on this sitetankvolumecalc.comhttps://tankvolumecalc.com/methodology/
Every linked source above was retrieved and checked on 2026-11-01.