Calculating fuel reserve for your boat
By Tyler Wilson · September 2026
Start with the rule of thirds as your floor: a third of your fuel to get there, a third to get back, a third held in reserve. Then do the real math, because thirds of a tank is not a plan. Fuel needed is burn rate times hours, hours come from distance divided by speed over ground, and speed over ground includes the current. Compute the reserve against the slowest realistic version of the trip, never the fastest.
The arithmetic, worked
Take a displacement cruiser burning 3 gallons per hour at an 8-knot cruise, on a 34 nautical mile run. In slack water that is 34 divided by 8, about 4.3 hours, and roughly 13 gallons each way. Now run the slow case: a 1.5-knot adverse current drops speed over ground to 6.5 knots, stretches the same leg to about 5.2 hours, and the burn to nearly 16 gallons. The boat did not get thirstier per hour; the trip got longer. Reserve math that ignores current is fiction with a calculator.
One trip, three numbers
| Case | How to compute it | What it is for |
|---|---|---|
| Planned case | Distance over expected speed over ground, times burn rate | The number you fuel against |
| Slow case | Same math with adverse current, weather slowdown, and a detour to your bailout | The number your reserve must cover |
| Point of no return | The fuel level at which continuing costs more than turning back | Decided at the dock, so it is a fact underway instead of a debate |
Run your own numbers
Same arithmetic, your boat. Type your numbers, tap a current, and the verdict line is the one to plan against.
Your boat, your numbers
Type your numbers, then tap a current. The hours, gallons, and the verdict react to every change.
Adverse current on the slow leg
Planned leg
12.8 gal
4.3 hr at 8 kn
Slow leg, 1.5 kn against
15.7 gal
5.2 hr at 6.5 kn over ground
Round trip, slow case
28.4 gal
vs 40 gal budget (two thirds of tank)
Inside the rule of thirds: the reserve stays untouched on paper.
Estimates from the numbers you enter, not measurements. Your logged burn beats the brochure, and conditions underway beat both.
Where the estimate leaks
Burn rates from the brochure assume a clean bottom, a light load, and calm water. A fouled bottom, full water and gear, headwinds, and the extra throttle you use in chop all push burn up. If you have a fuel flow meter, trust it over the manual; if you do not, log real fill-ups against engine hours for a season and use your own number.
Reserve is a floor you never plan to touch
The reserve third is not slack for optimistic planning; it is the budget for the version of the day you did not forecast. If the slow case already eats into it on paper, the trip needs more fuel, a shorter leg, or a different day. And on tidal water, remember the cheapest fuel strategy is the timing one: the current guide below is worth more gallons than any throttle discipline.
Sources
- US Coast Guard Auxiliary float plan resources
- NOAA CO-OPS tide and current predictions
- NWS marine forecasts
Planning, not navigation
Everything on this page is homework for before you leave the dock. Plan here; navigate on your charts and your plotter. And what the water is actually doing always outranks a forecast, this page included.
TidePilot is a planning advisor, not a navigation system. Always confirm conditions with current observations and use a marine chartplotter for navigation underway.
Where TidePilot comes in
Everything on this page is the homework a TidePilot brief does for your specific boat, route, and crew. Share a start and a destination and the brief comes back with the read at each leg's arrival time, your wind and sea limits applied, and the reasoning shown. The call stays yours. Start a brief.
More guides
- Choosing a departure window
- How wind, swell, tide, and current interact along a route
- What each forecast model tells boaters
- How a TidePilot brief gets made
The read before you leave™. Plan with TidePilot.