How far will the fuel take you, keeping a reserve in the tank? Enter tank size, the reserve you want to keep, the burn rate at cruise and your cruise speed. The calculator returns hours of running and range in nautical and statute miles.
Before a run to a fuel dock you have not used, before an offshore leg, or any time the gauge says “quarter” and you need to decide whether to stop. It is also the quickest way to compare cruise settings: the same tank at a lower burn rate and lower speed often goes farther.
Use a burn rate you have measured, not a brochure figure. The best number comes from filling the tank, running at your usual cruise setting for a logged number of hours, and refilling: gallons taken ÷ hours = gallons per hour. A fuel-flow meter, or an engine display that reports it, is the next best source.
The reserve is taken off the top first. What remains is divided by the burn rate to give hours, and hours times speed gives distance.
usable = tank × (1 − reserve ÷ 100)
hours = usable ÷ GPH
range (nm) = hours × knots; statute miles = nm × 1.15078
Center console, 100 gal, one-third reserve. 100 × (1 − 0.33) = 67.0 gal usable. At 8 GPH that is 8.38 h, and at 18 kn the range is 150.8 nm (173.5 statute miles).
Same boat, throttled back. At 5.5 GPH and 14 kn: 12.18 h and 170.5 nm. Slower but farther, because the burn rate fell faster than the speed.
Trawler, 400 gal, 20% reserve. 320 gal usable at 3.2 GPH is 100.00 h; at 7.5 kn that is 750.0 nm.
| Reserve | Usable (gal) | Hours | Range (nm) |
|---|---|---|---|
| 0% | 100.0 | 12.50 | 225.0 |
| 10% | 90.0 | 11.25 | 202.5 |
| 20% | 80.0 | 10.00 | 180.0 |
| 25% | 75.0 | 9.38 | 168.8 |
| 33% | 67.0 | 8.38 | 150.8 |
| 50% | 50.0 | 6.25 | 112.5 |
Each row comes from the same formula as the calculator.
A long-standing habit is one third of the fuel out, one third back, one third in reserve. The calculator’s reserve field handles the last third. For an out-and-back trip with no fuel at the far end, the turn-around point is half the range shown, not the whole of it.
A third is the conservative standard and suits unfamiliar water. Twenty percent is common on known routes with fuel on the way. Never plan to arrive with less than you would need to reach a second fuel stop.
Enter the combined GPH for both engines.
Yes, as long as the tank and burn rate are both in litres. The hours and distance come out the same.
Without a speed the page reports hours only, which is the useful number for a generator or a boat idling on a mooring.