Boat Fuel Range Calculator

A boat can reach a destination and still lack the fuel needed to return with its reserve intact. The tank’s rated capacity says how much it can hold, not how much is aboard today or how much can support the passage. A useful range estimate starts with fuel the engines can reliably draw and measured consumption at the intended cruising speed.

Unpumpable fuel

Fuel aboard that the engines cannot reliably draw. It is excluded from the available supply, not counted as an emergency reserve.

Reserve

Pumpable fuel deliberately held back rather than allocated to the planned journey.

Trip fuel budget

The pumpable supply remaining after the reserve is set aside. This is the fuel available for the planned cruising distance.

Endurance describes cruising time, while range describes distance. Hourly fuel burn turns an available volume into running hours, and speed over ground turns those hours into distance along a route. Speed over ground measures progress relative to the earth rather than through the water. An opposing current can therefore reduce the distance covered during the same running time. A knot is one nautical mile per hour.

The lowest hourly burn does not necessarily give the lowest fuel consumption for a journey. A lower speed also takes longer to cover the route, so useful comparisons pair the combined burn of all running engines with the speed observed at that same operating point. Combining a brochure speed with a burn figure measured at a different speed can overstate range.

For an out-and-back trip, both legs consume the cruising allocation. Half of the reserve-protected total distance is an outward limit only when route length, speed and burn are the same on the return. That planning radius follows the route travelled. It is not a straight-line circle on a chart or a guarantee that every destination inside such a circle is reachable.

The familiar one-third rule allocates a third of fuel to the outward journey, a third to the return and a third to reserve. Choosing a smaller reserve makes more fuel available for routine travel but leaves less for unexpected demands. A larger reserve reduces the distance that can be planned without using it.

Actual passage conditions can differ from the cruise measurements. Load, wind, current, idling and diversions can increase fuel needed or reduce progress. A fuel-range estimate supports route and refuelling decisions, but it cannot certify a safe passage. The fuel aboard, intended route and expected conditions still need checking before departure.

How to Use This Tool:

Replace the sample with measurements for your vessel and planned passage. Results update automatically as the inputs change.

1. Enter Fuel aboard before deductions, Combined cruise burn for all running engines, and the matching Cruise speed. Select each measurement’s unit independently. Changing a unit converts the existing number rather than merely relabelling it.
2. Choose One third or Custom under Reserve policy. The first choice holds exactly one third of pumpable fuel back. Custom reserves can range from 0 to 95%.
3. Enter the one-direction route length in Destination distance, then select Out and back or One way. Out-and-back travel automatically doubles the entered distance. Do not enter the already-doubled total in that mode.
4. Open Advanced to account for Unpumpable fuel or an Extra burn allowance. The former excludes a percentage of fuel aboard. The latter increases the entered burn without changing speed.
5. Enter Tank capacity to check the departure requirement against the tank, or Fuel price for cost estimates. Price is in USD per selected fuel-aboard unit, and changing that unit converts the price too. Zero omits the corresponding optional estimate.
6. Review Trip budget and the Fuel budget, particularly Margin above reserve and Departure fuel required. Check any tank warning before relying on a proposed fuel addition.
   If Check fuel inputs appears, correct the field named in its message. Blank or zero cruise burn or speed prevents a result. Invalid optional capacity or price entries instead show a warning and omit the affected estimates while leaving the main range available.

Interpreting Results:

The headline Reserve-protected radius is the destination-leg limit for an out-and-back trip. In one-way mode, Reserve-protected range instead allocates the whole trip budget to one direction, without return fuel. Fuel-exhaustion range and Runtime to exhaustion are theoretical comparisons that use the reserve too, not alternative trip targets.

Trip budget shows Within when planned burn does not exceed the cruising allocation. Equality is accepted, but leaves no margin above the retained reserve. A negative Margin above reserve means that allocation is exceeded. A positive Fuel after trip does not prove that usable fuel remains, because that figure includes unpumpable fuel. Check measured supply and matched cruise data rather than treating Within as a safety approval.

Departure fuel required is the total that would need to be aboard under the selected assumptions. Additional departure fuel is only the difference still needed above the current quantity. Neither means Refuel to full, which is simply tank capacity minus fuel aboard. An Exceeded result calls for reviewing that additional requirement against tank capacity, shortening the route or independently verifying a refuelling stop. Follow the vessel’s filling instructions rather than treating the arithmetic top-up as an instruction to fill to the brim.

Burn sensitivity shows how a higher burn reduces the protected destination distance while speed stays unchanged. Its starting point already includes any Extra burn allowance. It is a set of what-if calculations, not an engine performance curve, a weather forecast or a probability of completing the trip.

Technical Details:

Cruise burn is a volume-per-hour rate. Dividing the trip fuel budget by effective burn gives endurance; multiplying that time by speed over ground gives distance. Fuel availability and reserve reduce the cruising allocation, while an extra burn allowance increases the rate at which it is consumed.

Fuel, endurance and protected range

Use litres for fuel, litres per hour for burn, knots for speed and nautical miles for distance in these equations. Let F be Fuel aboard, b the observed Combined cruise burn and v the matched Cruise speed. The fractions u, r and a represent unpumpable fuel, reserve and extra burn allowance. Divide entered percentages by 100, so 25% becomes 0.25.

The available fuel is reduced before it is converted into cruising time and distance.

Here P is pumpable fuel, Q is the trip fuel budget, B is effective cruise burn, T is runtime to reserve and R is protected one-way range. E is cruise fuel economy in nautical miles per litre. Unpumpable volume is the difference between F and P.

The one-third setting uses a reserve fraction of exactly one third, not 30% or a percentage rounded to 33.33%. It adapts the usual outward/return/reserve guidance to pumpable fuel after the unpumpable deduction. A custom percentage uses the same pumpable-fuel basis. The initial custom value is 30%, but it has no effect while One third is selected.

Exhaustion estimates replace the trip budget with all pumpable fuel. These figures include fuel that was meant to remain in reserve.

Route demand and departure fuel

Let d be the entered destination distance and k the number of equal route legs. Use k = 1 for one way and k = 2 for out and back. Total trip distance is D, while the protected destination limit is the available range divided between those legs.

T_trip is planned cruise time, F_trip is planned trip burn and M is margin above reserve. The out-and-back radius is half of R. Fuel after the trip includes the original unpumpable quantity. Negative projected fuel or margin values remain visible as shortages rather than being replaced with zero.

Departure fuel must cover the route while still allowing for both deductions. The required total is calculated independently of the fuel currently aboard; only the additional amount depends on the current supply.

The same unpumpable and reserve percentages apply to the recalculated departure total. Their volumes are not frozen at today’s amounts. Consequently, correcting a 20 L trip-budget deficit can require more than 20 L of additional fuel.

Budget and tank boundaries

The trip-budget comparison uses fuel already aboard. Tank feasibility separately asks whether the required departure total could fit in a valid entered capacity C. A route can fit in the tank yet exceed the current fuel supply.

A capacity check is available only for a positive, valid capacity at least as large as fuel aboard. Zero omits it. A capacity below fuel aboard produces a warning and suppresses fill and top-up estimates; range still uses the entered fuel rather than silently reducing it to the tank size. The required departure total is not capped to capacity.

Burn sensitivity at unchanged speed

The sensitivity cases multiply the effective burn by 1, 1.1, 1.2, 1.3 and 1.5. These represent an additional 0%, 10%, 20%, 30% and 50% above the burn allowance already selected. For a multiplier f, the protected destination distance decreases in inverse proportion.

With a 20% burn allowance, the additional 20% case uses 1.2 times 1.2, or 1.44 times the observed burn, not 1.40 times. Speed and fuel allowances remain unchanged. Comparing different cruise speeds requires new matched burn measurements rather than assuming this curve predicts how an engine responds to speed.

Units, optional costs and precision

Volume conversions apply equally to fuel aboard, tank capacity and hourly burn. US and Imperial gallons are different units. The distance factors also convert speed because all supported speed units express distance per hour.

Convert km/h to knots by dividing by 1.852. Convert mph to knots by multiplying by 1.609344 and then dividing by 1.852. Thus 20 kn and 37.04 km/h describe the same speed, just as a 20 nmi destination and a 37.04 km destination describe the same route length. Equivalent unit changes preserve the physical result apart from numerical roundoff.

Displayed fuel quantities and effective burn use the fuel-aboard volume unit, even when burn was entered in another unit. Distances use the selected destination-distance unit. Fuel economy is expressed in that distance unit per fuel-aboard volume unit.

For optional estimates, let C be valid tank capacity in litres, p the entered USD price per selected fuel unit and c the litres in one such unit. The price per litre is q. Fill and refuelling quantities use current fuel aboard, while trip cost uses the planned burn.

Cost estimates use only the price entered. There is no live fuel-price lookup or currency conversion, and the currency label remains USD.

Calculations use unrounded numeric values, with unit-selector conversions retaining up to 16 significant digits. Most calculated values display at most two decimal places. Time rows display exactly two decimal places in decimal hours, so 1.50 h means one hour and 30 minutes. Budget and capacity comparisons use the unrounded values without an added safety tolerance. Numbers that look equal after rounding can therefore fall on different sides of a comparison. A rounded departure requirement is an estimate, not a guaranteed minimum supply.

Valid input ranges

These inclusive limits describe accepted inputs, not recommended operating values. The same physical limits apply when another unit is selected, with negligible roundoff protection at converted endpoints.

Inputs must be finite numbers, and blank required fields are not treated as zero. The custom reserve percentage matters only when Custom is selected. Zero fuel and zero destination distance are accepted; together they produce zero range and a zero-length trip that is within budget, not evidence of a usable passage. Zero custom reserve is also accepted, but retains no reserve at all.

Limitations and Privacy:

The cruise model holds speed and effective burn constant for the entered route. Out-and-back travel assumes identical conditions on both legs. It does not separately budget idling, manoeuvring, stops or diversions, predict weather or current, measure tank contents, or calculate an engine’s fuel curve. An extra burn allowance cannot determine how much speed would fall in worse conditions.

The unpumpable allowance is a fixed percentage of fuel aboard, including when the departure requirement is recalculated. It does not measure tank geometry, pickup behaviour or a fixed quantity trapped in a particular tank. A known capacity only checks the arithmetic requirement; it does not verify a fuel supply or a refuelling stop.

Fuel calculations run in your browser. Input values can be included in the page address and restored on reload, so sharing that address can also share your trip assumptions. Displaying the chart can load supporting software from an external service. Local arithmetic does not mean the page makes no network requests or is guaranteed to work offline.

Worked Examples:

References:

• A Boater’s Guide to the Federal Requirements for Recreational Boats, Fuel Management, page 43, U.S. Coast Guard, November 2023.
• Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically, National Institute of Standards and Technology, 2008.
• How Not To Run Out Of Fuel, BoatUS, November 2021.