What is the battery life?
Enter the battery’s capacity and the device’s current or power. The battery life calculator shows how many hours and days it runs, and you can lower the usable share to allow for real-world losses.
- Battery life (hours)
- 20
The battery lasts about 20 hours (20 hours).
- In days, hours and minutes
- 20 hours
- Battery life (days)
- 0.833
Battery life (hours): 20. The battery lasts about 20 hours (20 hours).
Battery life (hours) by usable capacity
How to calculate
Works out how long a battery runs a load: capacity in mAh ÷ current in mA, or capacity in Wh ÷ power in W, with an optional share of the capacity that is usable.
Example with the default inputs (I know mAh and current, Capacity (mAh) 2,000, Load current 100 mA, Usable capacity 100%): The battery lasts about 20 hours (20 hours).
Method: Hours = capacity (mAh) ÷ 1,000 × usable share ÷ load (A), or capacity (Wh) × usable share ÷ load (W). Days = hours ÷ 24.
- A steady average load. Real batteries give less at high currents and low temperatures (Peukert’s law), and less as they age; lower the usable share to allow for this.
- mAh mode assumes the load runs at the battery’s own voltage. For a device behind a voltage converter, use Wh and watts.
- Exact on the numbers as typed; rounded only to show.
Worked examples
Each example is checked against the calculator on every build.
- I know mAh and current, Capacity (mAh) 2,000, Load current 100 mA, Usable capacity 100% gives Battery life (hours) 20, In days, hours and minutes 20 hours, Battery life (days) 0.833333.Source: Wikipedia, "Ampere hour" (a battery of C ampere hours supplies a current of I amperes for C ÷ I hours), https://en.wikipedia.org/wiki/Ampere_hour (retrieved 2026-10-05)
- I know mAh and current, Capacity (mAh) 3,000, Load current 250 mA, Usable capacity 80% gives Battery life (hours) 9.6, In days, hours and minutes 9 hours, 36 minutes.Source: Wikipedia, "Ampere hour" (a battery of C ampere hours supplies a current of I amperes for C ÷ I hours), https://en.wikipedia.org/wiki/Ampere_hour (retrieved 2026-10-05)
- I know Wh and watts, Capacity 50 Wh, Load power 12 W, Usable capacity 100% gives Battery life (hours) 4.166667, In days, hours and minutes 4 hours, 10 minutes.Source: Wikipedia, "Kilowatt-hour" (energy = power × time; 1 Wh is 1 W for 1 hour), https://en.wikipedia.org/wiki/Kilowatt-hour (retrieved 2026-10-05)
- I know mAh and current, Capacity (mAh) 220, Load current 0.015 mA, Usable capacity 100% gives Battery life (hours) 14,666.666667, In days, hours and minutes 611 days, 2 hours, 40 minutes.Source: Wikipedia, "Ampere hour" (a battery of C ampere hours supplies a current of I amperes for C ÷ I hours), https://en.wikipedia.org/wiki/Ampere_hour (retrieved 2026-10-05)
How it works
With u the usable share (the percent ÷ 100):
- mAh and current: hours = capacity (mAh) ÷ 1,000 × u ÷ load current (A)
- Wh and watts: hours = capacity (Wh) × u ÷ load power (W)
- days = hours ÷ 24
Units
- Current: µA, mA or A. Energy: Wh or kWh (1 Wh = 3,600 J). Power: mW, W or kW.
Rules
- Every value must be above 0; the usable share is from 1% to 100%.
- Every step is exact on the numbers as typed. Hours are shown to 2 decimals, days to 3.
- In days, hours and minutes: the life rounded half up to the whole minute, with parts that are 0 left out ("4 hours, 10 minutes"); under half a minute shows "under 1 minute".
Worked examples by hand
2,000 mAh at 100 mA. 2,000 ÷ 100 = 20 hours = 0.833 days.
3,000 mAh at 250 mA, 80% usable. 3,000 × 0.8 ÷ 250 = 2,400 ÷ 250 = 9.6 hours = 9 hours, 36 minutes.
50 Wh at 12 W. 50 ÷ 12 = 4.1667 hours = 4 hours, 10 minutes.
220 mAh at 15 µA. 15 µA = 0.015 mA. 220 ÷ 0.015 = 14,666.67 hours = 611.11 days = 611 days, 2 hours, 40 minutes.
Other questions people ask
How do I calculate battery life?
Divide the capacity by the load. In milliampere hours: hours = mAh ÷ mA, so a 2,000 mAh battery at 100 mA lasts 20 hours. In watt hours: hours = Wh ÷ W, so a 50 Wh battery at 12 W lasts about 4 hours 10 minutes.
Why does my battery run out sooner than the calculator says?
The rated capacity is measured at a gentle current and room temperature. High currents, cold, age and the battery’s cut-off voltage all reduce what you get out. Set the usable capacity below 100%, for example 80%, to plan with a margin.
Should I use mAh or Wh?
Use mAh when the device runs straight from the battery at its voltage and you know its current. Use Wh when a converter changes the voltage (phones, laptops and power banks with USB outputs), because watt hours count energy whatever the voltage.
How do I turn mAh into Wh?
Multiply by the battery’s nominal voltage and divide by 1,000. A 3,000 mAh cell at 3.7 V holds 3,000 × 3.7 ÷ 1,000 = 11.1 Wh.
How do I find a device’s average current?
Measure it with a multimeter in series, or read the power from the label and divide by the voltage. If the device sleeps and wakes, average it over time: 10 mA for 1% of the time and 0.01 mA the rest averages about 0.11 mA.
What is Peukert’s law?
A rule that a battery gives less total charge the faster it is drained, which matters most for lead-acid batteries. The calculator does not model it; a lower usable share stands in for it.