Fuel Burn Calculator
Fuel planning starts with a basic product: multiply how fast you burn fuel by how long you run, and you get the total fuel used. This calculator takes a fuel flow rate, in gallons per hour, and a duration, in hours, and returns the fuel consumed, the figure you compare against tank capacity and required reserves before any trip. It applies just as well to an aircraft cruising at a planned power setting, a boat motoring at a steady throttle, or a generator running for a set period. Enter the burn rate and the time, and the tool returns the total fuel needed, so you can check that the planned amount, plus a sensible reserve, fits your tanks. Real consumption varies with power setting, load, altitude, temperature and conditions, so a wise operator always adds a reserve and never plans the trip down to the last drop in the tank. The relationship itself is exact and linear: double the time or the rate and the fuel used doubles. Every figure is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator defaults so you can follow each step.
Fuel burn is flow rate x time. Burning 12 gallons per hour for 3.5 hours uses 42.00 gallons of fuel before reserves.
Fuel burn formula
Fuel used = rate x time
rate = fuel flow in gallons per hour
time = duration in hours
(add a reserve to the planned total)
The product is the planned fuel before any reserve. Actual burn varies with power, load and conditions, so operators add a margin and confirm against tank capacity.
Worked example
Suppose the fuel flow is 12 gallons per hour and the run lasts 3.5 hours.
- Fuel used: 12 x 3.5 = 42.00 gallons
- Per half hour: 12 x 0.5 = 6.00 gallons
The total fuel used is 42.00 gallons, before any reserve. These are the calculator's default inputs, so the result matches the widget exactly.
Fuel Burn Calculator: frequently asked questions
How do I calculate fuel burn?
Multiply the fuel flow rate by the time. If you burn 12 gallons an hour and run for three and a half hours, you use 42 gallons. The same method works for any steady-rate consumer, whether an engine, generator or pump, as long as the rate and time use matching units.
Should I add a fuel reserve?
Yes. Prudent planning always adds a reserve above the calculated trip fuel to cover diversions, holding, headwinds or unexpected delays. Aviation rules require minimum reserves, and mariners carry extra for the same reasons. This tool gives the base requirement; add your reserve on top.
What if my burn rate changes?
This calculator assumes a single steady rate. If consumption varies, for example a high burn during climb and a lower burn in cruise, split the trip into segments, calculate each, and add them. The sum is more accurate than a single average for variable profiles.
Can I use this for boats or generators?
Yes. Any device that consumes fuel at a roughly steady rate fits the rate-times-time model. Enter the consumer's gallons-per-hour figure and the run time to get total fuel. Just keep the units consistent throughout.
Is fuel burn the same as fuel efficiency?
No. Fuel burn is the total quantity used over time. Efficiency, such as miles per gallon or nautical miles per gallon, relates fuel to distance and also depends on speed. This tool computes total burn, not distance-based efficiency.
Official sources
- Aeronautics, propulsion and flight planning resources: US National Aeronautics and Space Administration (NASA). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.