This calculator divides a generator’s fuel tank capacity by its fuel consumption rate to estimate how many hours it will run on a full tank.
Generator Run Time Calculator
Estimated run time for a generator on a full fuel tank, from tank capacity and fuel consumption rate.
What the Generator Run Time Calculator Covers
This calculator covers generator run time estimates, fuel tank duration calculations, and refueling interval planning — a simple but essential calculation for anyone relying on a portable or standby generator during an outage, camping trip, or job site.
This is useful for planning how long a generator will run unattended, scheduling refueling before it runs dry, and comparing different generators’ effective runtime based on their tank size and fuel efficiency.
How Run Time Is Calculated
Formula
Run Time = Fuel Tank Capacity ÷ Fuel Consumption Rate
Where:
- Fuel Tank Capacity = the generator’s total fuel tank size, in gallons
- Fuel Consumption Rate = how much fuel the generator burns per hour, in gallons per hour
Worked Example
A generator with a 5-gallon tank, consuming fuel at 0.75 gallons per hour.
Run Time = 5 ÷ 0.75 ≈ 6.7 hours
This generator would run for roughly 6 hours 40 minutes on a full tank at this consumption rate.
Why the Consumption Rate Depends on Load
Fuel consumption rate is not fixed — it depends heavily on how much load the generator is powering. A generator running near its maximum rated output burns significantly more fuel per hour than the same generator running a light load. Check your generator’s spec sheet for consumption rates at your typical load level, often given at 25%, 50%, and 100% load.
Frequently Asked Questions
Where do I find my generator’s fuel consumption rate?
Check the owner’s manual or manufacturer’s spec sheet, which typically lists consumption rates at different load percentages — use the rate that best matches what you’re actually powering, not just the maximum-load figure.
How much shorter is run time under full load versus light load?
This varies by generator model, but running near maximum rated output typically consumes noticeably more fuel per hour than light-load operation, which can meaningfully shorten run time compared to a light-load estimate.
Should I factor in a safety margin before refueling?
Many generator owners refuel before the tank runs completely empty to avoid an unexpected shutdown, since running dry can also introduce air into the fuel system on some engines — check your specific generator’s manufacturer guidance.
Does altitude or temperature affect fuel consumption?
Yes, engine performance and fuel efficiency can vary with altitude and ambient temperature, so real-world run time may differ somewhat from a consumption rate measured under different conditions.
How do I estimate total fuel needed for an extended outage?
Divide the total hours you expect to run the generator by the run time per tank, then round up to find how many full tanks (and how much total fuel) you’ll need to cover that period.
Does a larger external fuel tank change this calculation?
Yes — if you’re using an auxiliary or external fuel tank instead of the generator’s built-in tank, enter that larger tank’s actual capacity in place of the standard tank size to get an accurate extended run time estimate.
