How much fuel does the tank hold?
Add a fuel depth to see how much fuel is in the tank at that level.
The result is the volume of the shape you measure. A tank takes less fuel than that: walls and fittings take space, and a gasoline tank keeps an air space at the top (see below).
To find how many gallons a rectangular fuel tank holds, multiply its inside length, width and height in inches and divide by 231, the number of cubic inches in a US gallon. A tank 36 inches long, 24 inches wide and 12 inches high holds 36 × 24 × 12 = 10,368 cubic inches, or 10,368 ÷ 231 = 44.9 gallons (169.9 liters). The calculator above does the same for a box, a tank with one sloping side and a cylinder, in inches or centimeters, and tells you how much fuel is in the tank at the depth you read on a dipstick.
Box = L × W × H · One sloping side = L × H × (top width + bottom width) ÷ 2 · Cylinder = π × (D ÷ 2)² × L · US gallons = cubic inches ÷ 231 · Liters = cubic inches × 0.016387064 · Imperial gallons = liters ÷ 4.54609. L, W, H and D are the inside length, width, height and diameter.
How to Calculate Fuel Tank Capacity
A US gallon is defined as exactly 231 cubic inches, and a cubic inch is exactly 16.387064 cubic centimeters, so a gallon is 3.785411784 liters (The American Practical Navigator, Bowditch; NIST Handbook 44). Work out the tank’s volume in cubic inches from its shape, then divide by 231. A cubic foot, 1,728 cubic inches, is 7.48 gallons. Measured in centimeters, the volume in cubic centimeters divided by 1,000 is liters. The imperial gallon used in the UK is larger: 4.54609 liters (NIST), so one US gallon is 0.833 imperial gallon.
The result is the volume of the shape you measure. Use the inside dimensions if you can reach them; measured outside, the tank’s walls are counted as fuel space. Tanks that are not simple shapes can be split into parts, a box and a wedge for example, and the parts added up.
Examples of rectangular tanks, not standard sizes:
| Inside size | Cubic inches | US gallons | Liters |
|---|---|---|---|
| 24 × 12 × 10 in | 2,880 | 12.5 | 47.2 |
| 30 × 20 × 10 in | 6,000 | 26.0 | 98.3 |
| 36 × 24 × 12 in | 10,368 | 44.9 | 169.9 |
| 48 × 24 × 12 in | 13,824 | 59.8 | 226.5 |
| 48 × 30 × 14 in | 20,160 | 87.3 | 330.4 |
| 60 × 36 × 16 in | 34,560 | 149.6 | 566.3 |
Tanks With a Sloping Side or a Round Shape
For a tank with one sloping side, narrower at the bottom than at the top or the other way round, the cross-section is a trapezoid: average the top and bottom widths and multiply by the height and the length. A tank 40 inches long and 14 inches high, 30 inches wide at the top and 16 at the bottom, holds 40 × 14 × (30 + 16) ÷ 2 = 12,880 cubic inches, 55.8 gallons. For a cylinder, the cross-section is a circle of area π × (D ÷ 2)²: a tank 16 inches across and 48 inches long holds 41.8 gallons, and the same diameter standing 24 inches high holds 20.9 gallons.
How Much Fuel Is in the Tank at a Given Depth
Enter the fuel depth you read on a dipstick, measured up from the bottom, and the calculator gives the fuel at that level. In a box or an upright cylinder the fuel is in proportion to the depth: half the depth is half the tank. In other shapes it is not. The sloping-side tank above holds 23.6 gallons at half its depth, only 42.4% of the tank, because the bottom half is the narrow half. In a cylinder lying on its side, the fuel below a depth h is a circular segment, with area R² × cos⁻¹((R − h) ÷ R) − (R − h) × √(2Rh − h²) for a radius R (MathWorld); at a quarter of its diameter, the 16-inch tank holds 8.2 gallons, 19.6% of the whole. The share for any cylinder on its side:
| Fuel depth, share of the diameter | Fuel, share of the tank |
|---|---|
| 10% | 5.2% |
| 20% | 14.2% |
| 25% | 19.6% |
| 30% | 25.2% |
| 40% | 37.4% |
| 50% | 50.0% |
| 60% | 62.6% |
| 70% | 74.8% |
| 75% | 80.4% |
| 80% | 85.8% |
| 90% | 94.8% |
Tank Capacity and the Fuel You Can Use
A tank takes less fuel than its geometry. On boats with gasoline engines other than outboards, U.S. Coast Guard rules require every fuel tank to carry a label that gives its capacity in U.S. gallons (33 CFR 183.514), the figure to check against the calculation. Naval architect Dave Gerr, writing with information from the American Boat and Yacht Council (ABYC), explains that a gasoline tank has always needed an air space at the top, called ullage, for the fuel to expand. Fuel systems on new gasoline boats must also shut the fill nozzle off automatically when the tank is filled to the proper ullage: that is not part of the EPA’s evaporative emission rules but an industry requirement, made through ABYC standards H-24 and H-25 alongside them, and for most installations leaving 5% to 8% of the tank as ullage is one simple way to meet it. On the 44.9-gallon example tank, an ullage of 5% to 8% leaves room for about 41.3 to 42.6 gallons of fuel.
Plan a trip on the fuel you carry, not the tank’s size. The Coast Guard’s one-third rule: use one third of the fuel to go out, one third to get back and hold one third in reserve. With the 41.3 to 42.6 gallons the example tank carries, that is about 13.8 to 14.2 gallons each. The Boat Fuel Calculator works out the fuel a trip takes from the distance, speed and fuel burn, and the Fuel and Water Weight Calculator what a full tank weighs; for the boat’s weight with fuel aboard, use the Boat Weight Calculator.
Frequently Asked Questions
How do you calculate the gallons in a boat fuel tank?
Multiply the inside length, width and height in inches and divide by 231, the number of cubic inches in a US gallon. A tank 36 by 24 by 12 inches holds 10,368 cubic inches, or 44.9 gallons. For a tank with one sloping side, use the average of the top and bottom widths.
How many liters are in a gallon?
A US gallon is exactly 3.785411784 liters, 231 cubic inches. An imperial gallon, used in the UK, is 4.54609 liters, so a US gallon is 0.833 imperial gallon.
How do you work out the capacity of a cylindrical tank?
Multiply π by the radius squared and by the length, all in inches, and divide by 231 for US gallons. A tank 16 inches in diameter and 48 inches long holds 3.1416 × 8 × 8 × 48 = 9,651 cubic inches, about 41.8 gallons.
How much fuel is in my tank at a given depth?
In a box-shaped tank the fuel is in proportion to the depth: half the depth is half the tank. In a cylinder lying on its side it is not: at a quarter of the diameter the tank is only 19.6% full. Enter the depth in the calculator to get the gallons.
Can I fill a boat gas tank to the top?
No. A gasoline tank needs an air space at the top, called ullage, for the fuel to expand, and fuel systems built to ABYC standards H-24 and H-25 shut the fill nozzle off before the tank is full. Naval architect Dave Gerr gives an ullage of 5% to 8% of the tank as one simple way to meet that requirement.
Where do I find my fuel tank capacity?
On boats with gasoline engines other than outboards, Coast Guard rules require each fuel tank to carry a label with its capacity in U.S. gallons (33 CFR 183.514). The owner’s manual often lists it too. If neither is available, measure the tank and use the calculator.
Sources
- NGA, The American Practical Navigator (Bowditch), Vol. II (2019), Calculations and Conversions: 1 US gallon = 231 cubic inches and 1 cubic inch = 16.387064 cubic centimeters (exact), 1 cubic foot = 7.480519 US gallons (read Oct 22, 2026).
- NIST, Handbook 44 (2026), Appendix C: General Tables of Units of Measurement: US gallon = 231 cubic inches = 3.785411784 liters (read Oct 22, 2026).
- NIST, Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically: gallon (Canadian and U.K., imperial) = 4.546 09 liters (read Oct 22, 2026).
- Eric W. Weisstein, Circular Segment, MathWorld, Wolfram Research: the area of a segment from the radius and its height (read Oct 22, 2026).
- eCFR, 33 CFR 183.514, Fuel tanks: Labels: the label gives the capacity in U.S. gallons; 33 CFR 183.501: Subpart J applies to boats with gasoline engines, except outboard engines (read Oct 22, 2026).
- Dave Gerr, naval architect, New Gasoline Fuel-Installation Requirements (Gerr Marine, from information provided by John Adey and the ABYC technical department): ullage, the air space at the top of the tank, required for expansion; the automatic fill-nozzle shut-off at the proper ullage, an industry requirement of ABYC H-24 and H-25 and not part of the EPA regulation; 5% to 8% ullage as one simple way to comply (read Oct 22, 2026).
- U.S. Coast Guard, A Boater’s Guide to the Federal Requirements for Recreational Boats (November 2023), Fuel Management: the one-third rule (read Oct 22, 2026).