Generator Runtime Calculator

Estimate how long a generator runs on a tank.

Fuel tank size

gal

Fuel consumption

gal/h

Results update as you type.

Estimated runtime

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How it works

Knowing how long a generator will run on a tank of fuel is essential for power outages, camping, and job sites, and this calculator estimates it. You enter the fuel tank size in gallons and the generator's fuel consumption in gallons per hour, and it returns the estimated runtime. The calculation is straightforward: divide the tank capacity by the consumption rate, since a generator burning fuel at a steady rate empties its tank in tank size divided by that rate. The crucial detail is that fuel consumption depends heavily on the electrical load: a generator powering just a few lights sips fuel and runs a long time, while one running near its maximum output, driving air conditioners or power tools, burns fuel much faster and runs for fewer hours. Manufacturers usually publish consumption figures at half load and full load, so use the rate that matches your expected usage. For reference, a typical portable generator might use a fraction of a gallon per hour at light load and over a gallon at heavy load. This helps you plan refueling, decide whether a generator can last through the night or a work shift, and compare models. Real runtime varies with load, altitude, fuel type, and generator age. Enter the tank size and fuel consumption to estimate the runtime.

Formula

Runtime = tank size / fuel consumption per hour

Generator runtime is the fuel tank size in gallons divided by the fuel consumption rate in gallons per hour. The result is how many hours the generator can run on a full tank at that consumption rate. Higher electrical loads increase fuel consumption and shorten runtime, so the rate should match your expected load.

Source:Everyday estimation

We review all our tools from time to time to ensure accuracy.

Worked example

Fuel tank size:8Fuel consumption:0.75

8 / 0.75 ≈ 10.7 hours on a full tank at that consumption rate.

How to use this calculator

1
Enter your values
Enter fuel tank size and fuel consumption.
2
Adjust the options
Fine-tune any options to match your situation.
3
Read your result
Estimated runtime updates instantly as you type — no button to press.

Frequently asked questions

How do I calculate generator runtime?

Divide the fuel tank size in gallons by the generator's fuel consumption in gallons per hour. An 8-gallon tank with a generator using 0.75 gallons per hour runs about 10.7 hours. The calculator does this instantly. Use a consumption rate that matches your expected electrical load for the most accurate estimate.

How does load affect runtime?

Fuel consumption rises with the electrical load. A lightly loaded generator, powering a few lights or small devices, uses much less fuel and runs far longer than one near full capacity driving heavy appliances. Manufacturers list consumption at different loads, so match the rate to your expected usage, since heavy loads can cut runtime dramatically.

Where do I find fuel consumption figures?

The generator's manual and specifications list fuel consumption, often at both half load and full load, in gallons or liters per hour. Use the figure closest to your expected load. If only a full-load rate is given but you will run a lighter load, actual runtime will be longer than the calculation suggests.

Can I run a generator continuously overnight?

It depends on tank size and load. Calculate the runtime, and if it is shorter than the period you need, you will have to refuel, which usually requires shutting the generator off and letting it cool first for safety. Some larger or extended-runtime models can run through the night on one tank at moderate loads.

Why might actual runtime differ from the estimate?

Real runtime varies with the changing electrical load, altitude, temperature, fuel type and quality, and the generator's age and condition. The estimate assumes a constant consumption rate, but loads fluctuate as devices turn on and off. Treat the result as a planning figure and keep a fuel reserve to avoid unexpectedly running out.