Battery Runtime CalculatorFree online other calculator — instant results, no sign-up
Estimate runtime using capacity, load current and usable capacity.
Sign in to save results and favorites.
About the Battery Runtime Calculator
The Battery Runtime Calculator estimates how long a battery can supply a steady load using its capacity in mAh, load current in mA, and usable capacity percentage. It adjusts the rated capacity to the portion you expect to use, then divides that amount by the current draw. With the default inputs of 2000 mAh, 200 mA, and 80% usable capacity, the estimated runtime is 8 hours.
- Capacity (mAh)
- The battery's rated charge capacity in milliamp-hours (mAh), with a default of 2000 mAh.
- Load current (mA)
- The current drawn from the battery in milliamps (mA), with a default of 200 mA.
- Usable capacity (%)
- The percentage of rated battery capacity expected to be available before stopping discharge, with a default of 80%.
Formula and method
t = C × (u / 100) / I; with the defaults: t = 2000 × (80 / 100) / 200 = 8 hours.
- tEstimated battery runtime in hours.
- CRated battery capacity in mAh.
- uUsable capacity expressed as a percentage.
- ILoad current drawn from the battery in mA.
Convert the usable capacity percentage to a decimal and multiply it by the rated capacity to find the available charge. Divide that available charge by the load current; mAh divided by mA gives hours. For the defaults, 80% becomes 0.8, the available charge is 2000 × 0.8 = 1600 mAh, and runtime is 1600 / 200 = 8 hours.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Capacity2000 mAh
- Load current200 mA
- Usable capacity80 %
Results
- Result8 hours
How to use the Battery Runtime Calculator
- 1Enter the battery's rated capacity in mAh; the default is 2000.
- 2Enter the current drawn from the battery in mA; the default is 200.
- 3Enter the usable capacity percentage; the default is 80.
- 4Use a positive load current, a nonnegative capacity, and a usable capacity between 0% and 100%.
- 5Read the estimated runtime in hours; the default inputs produce 8 hours.
Reading your result
The result row reports estimated runtime in hours. A result of 8 hours means the battery is predicted to supply the specified 200 mA load for 8 hours when 80% of its 2000 mAh rated capacity is usable. This is an estimate, not a guaranteed operating time or a recommended discharge limit.
Limitations
The calculation assumes a steady current draw and a fixed usable capacity fraction. It does not independently model battery chemistry, discharge-rate effects, temperature, aging, self-discharge, voltage sag, or device cutoff voltage. Conversion losses and device standby draw must be reflected in the battery-side load current or otherwise accounted for without double-counting. Capacity in mAh measures charge, not energy, so the formula should not directly combine battery capacity with current measured at a different voltage. A zero load current does not yield a finite runtime with this formula.
Common questions
Why does the default calculation give 8 hours?+
Only 80% of the 2000 mAh rated capacity is treated as available, giving 1600 mAh. Dividing 1600 mAh by the 200 mA load gives 8 hours.
Is usable capacity the same as the battery's current charge percentage?+
Not necessarily. Usable capacity is the fraction of rated capacity available for the planned discharge. It may reflect the starting charge level, a reserved charge level, reduced capacity from aging, or cutoff behavior; the default 80% is an assumption, not a measurement.
Can I use the current printed on a USB-powered device?+
Only if it represents the current drawn from the battery at the battery's voltage. A voltage converter changes the relationship between device current and battery current, so use measured or estimated battery-side current that includes conversion losses.
What should I enter if the device's current draw changes?+
Use a time-weighted average battery current over a representative operating cycle. The estimate can still be optimistic if high-current peaks cause voltage sag or trigger the device's low-voltage cutoff.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
You might also like
IP Subnet Calculator
Network, broadcast and host range of an IPv4 subnet.
Password Generator
Create a strong random password.
Voltage Drop Calculator
Voltage drop on a copper conductor.
GDP Calculator
GDP by the expenditure approach.
Horsepower Calculator
Horsepower from torque and RPM.
Engine Horsepower Calculator
Estimate horsepower from weight and quarter-mile time.
Results are estimates for informational purposes and are not financial, medical or legal advice.