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Generator fuel & runtime calculator
Estimate how long a generator runs on a tank, how much gasoline or propane a multi-day outage takes and what it costs, from your generator's fuel-use spec.
Results
| Runtime per tank | 10 hours |
|---|---|
| Fuel needed for the outage | 14.4 gal |
| Fuel cost for the outage | $50.4 |
| Tank fills needed | 4 fills |
| Fuel per day | 4.8 gal/day |
- Store fuel only in approved containers, with stabilizer added for gasoline older than a month. Check local limits on how much you can store at home.
- Never run a generator indoors, in a garage or near windows. Carbon monoxide kills. Place it at least 20 ft from the house with the exhaust pointing away.
About this calculator
Fuel is what a generator plan usually runs short on. Stations lose power in widespread outages too, and gasoline goes stale in storage. Knowing how many gallons a three-day outage takes tells you how much to keep on hand and whether a dual-fuel or propane setup makes more sense.
Manufacturers publish fuel use as a runtime table, for example "8 hours at 50% load on a full tank". Enter that rate, how long outages last where you live and how many hours a day you'll run the generator. You get runtime per tank, total fuel and cost.
How to use it
- Find fuel use in the manual or spec sheet at a load close to yours. Divide tank gallons by rated hours to get gal/h.
- Enter your tank size and local fuel price.
- Enter the outage length you're planning for and the hours you'll run per day.
- Plan to store the 'fuel needed' figure, with stabilizer, rotated every few months.
The formula
Runtime per tank = tank gallons ÷ gallons per hour.
Fuel needed = days × hours per day × gallons per hour.
Fuel use isn't proportional to load. A generator at 25% load typically burns roughly half of what it burns at full load, so always use the rate from the manual at a load close to yours.
Worked example (default values)
Inputs
| Fuel tank size | 4 gal |
|---|---|
| Fuel use at your typical load | 0.4 gal/h |
| Fuel price | 3.5 $/gal |
| Outage length | 3 days |
| Hours run per day | 12 h/day |
Results
| Runtime per tank | 10 hours |
|---|---|
| Fuel needed for the outage | 14.4 gal |
| Fuel cost for the outage | $50.4 |
| Tank fills needed | 4 fills |
| Fuel per day | 4.8 gal/day |
Tips and common mistakes
Propane stores indefinitely, but most dual-fuel generators lose about 10% of their rated output on it and burn it faster by volume. A 20 lb tank holds about 4.7 gallons. Convert your manual's propane runtime table the same way.
Run in cycles. A fridge stays cold for hours with the door closed. Running 2 hours on and 2 off halves fuel use with little food risk. Check food safety guidance for your situation.
Frequently asked questions
How long does a generator run on 5 gallons?
Most 5,000-7,500 W portable generators burn about 0.4-0.8 gal/h at moderate load, so about 6-12 hours. Small 2,000 W inverter generators can run 8-10 hours on about 1 gallon at 25% load.
How much gas do I need for a 3-day outage?
Running 12 hours a day at 0.5 gal/h is 18 gallons. Cycling the generator can cut that to 8-12 gallons.
Does a bigger generator use more fuel?
At the same load, usually yes, because its engine is larger and less efficient at light loads. Oversizing costs fuel.
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Estimates for planning only, based on the assumptions shown. Real-world performance varies with equipment, temperature and installation. Updated October 19, 2026. See our methodology.