Boat Range and Reserve Fuel Calculator

Estimate reserve-adjusted range, cruise endurance, equal-conditions out-and-back distance, and planned-trip fuel margin. All inputs stay in this browser tab.

A fuel range estimate is not a passage planFuel burn and speed can change with wind, waves, current, load, trim, fouling, maneuvering, and engine condition. Verify actual usable fuel, vessel data, weather, tides, alternates, and applicable requirements before departure.

Fuel and cruise data

Fuel available

Enter measured fuel, not rated tank capacity.

Fuel not safe to rely on because of pickup or tank limits.

Include all engines and continuous fuel consumers.

Speed and reserve

Use the speed matching the entered burn rate.

33.3% matches the reserve third in the Coast Guard one-third practice.

Optional trip check

Use the entire intended route, including the return leg.

Inputs are processed only in your browser and are not added to analytics events.

Range and reserve result

Enter fuel, burn, speed, and reserve values to calculate.

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Boat range formula

The calculator first removes unusable fuel and the selected reserve. The remaining travel fuel is divided by total hourly burn, then multiplied by speed.

Usable fuel = onboard fuel − unusable fuel

Travel fuel = usable fuel − reserve fuel

Range = travel fuel ÷ hourly burn × cruise speed

The out-and-back value divides reserve-adjusted range by two and assumes identical outbound and return conditions. It is not an allowance for current or weather.

Percentage and time reserves

A percentage reserve holds back part of usable fuel. A time reserve holds enough fuel for the selected minutes at the entered cruise burn.

Percent reserve = usable fuel × reserve %

Time reserve = hourly burn × reserve minutes ÷ 60

Do not treat a preset as universal. A suitable reserve depends on the vessel, passage, conditions, alternatives, applicable rules, and uncertainty in the inputs.

How to use this calculator

  1. Enter the fuel actually onboard and separately estimate fuel that cannot be relied on.
  2. Enter measured total burn at the intended cruise setting. If several engines or a generator are running, include all continuous consumption.
  3. Enter a realistic speed matching that burn. Prefer speed through water for a vessel-performance estimate.
  4. Choose a percentage reserve or time at cruise burn. The one-third button sets a 33.3% holdback.
  5. Optionally enter the complete planned route. Compare its fuel requirement and margin above reserve with a conservative passage plan.

Example range calculation

A boat has 120 US gallons onboard, of which 5 gallons are treated as unusable. With 12 GPH total burn, 18 knots cruise speed, and a 33.3% reserve, approximately 76.71 gallons remain for planned travel.

76.71 gal ÷ 12 gal/h × 18 kn = 115.06 nautical miles

Under the calculator's equal-conditions assumption, the corresponding out-and-back limit is about 57.53 nautical miles from the starting point. This is a math example, not a recommended destination distance.

Frequently asked questions

How is boat range calculated from fuel burn?

Subtract unusable fuel and reserve fuel from fuel onboard. Divide the remaining travel fuel by total hourly burn to get endurance, then multiply endurance by cruising speed.

What is the one-third fuel rule for boats?

The U.S. Coast Guard recreational boating guide describes using one-third of fuel to go out, one-third to return, and holding one-third in reserve. This is a general fuel-management practice, not proof that a particular passage is safe or compliant.

Should I enter rated tank capacity as fuel onboard?

No. Enter the fuel actually onboard. Use the unusable-fuel field for fuel that may not reach the pickup reliably because of tank shape, trim, heel, contamination, or vessel-specific limits.

Does this calculator account for current?

No. It uses steady speed and fuel burn. Current changes distance over ground, while wind, waves, load, fouling, maneuvering, and throttle changes can alter both speed and consumption.

Why is out-and-back range half of one-way range?

The calculator reserves half of the travel budget for the return leg. This simple division assumes the same route, burn, and effective speed both ways; real passages rarely have perfectly equal conditions.

Can I use maximum range as a destination distance?

No. Maximum range is a mathematical boundary under unchanged inputs. Keep appropriate additional margins and account for weather, tides, alternatives, vessel limits, and applicable requirements.

Are my boat fuel inputs uploaded or tracked?

No. Calculations run locally in the browser, and this tool does not send, store, or attach input values to analytics events.

Limits and safety disclaimer

  • The model assumes burn rate and speed remain constant. It does not model acceleration, idle time, detours, traffic, current, wind, sea state, fouling, load changes, or equipment faults.
  • Published tank capacity is not necessarily usable fuel. Pickup position, tank geometry, trim, heel, contamination, and gauge error can reduce available fuel.
  • Range is through-water distance when speed is entered in knots or an equivalent steady speed. Ground distance can differ because of current.
  • The planned-trip check only compares arithmetic fuel demand with the selected holdback. It does not assess route safety or reserve adequacy.

Safety disclaimer: This educational estimate does not replace a passage plan, onboard fuel measurements, manufacturer guidance, official forecasts, navigation information, applicable regulations, or the judgment of the vessel operator.

Methodology and sources

Last reviewed: August 1, 2026. The model uses direct time, rate, distance, and volume relationships. Safety and unit guidance were checked against:

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