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Battery Life Calculator

Estimate how long a battery will run a device. Enter the capacity in milliamp-hours and the load, either as current in milliamps or as power in watts, and apply a usable-capacity factor for the losses every real battery has.

Quick examples
mAh
V

3.7 V for a lithium-ion cell, 1.2 V NiMH, 1.5 V alkaline, 12 V lead-acid.

mA
%

Allows for conversion losses, cut-off voltage and aging. 70–90% is realistic.

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Battery life17 h
In hours
17 h
Battery energy
18.5 Wh
Load on the battery
250 mA

Saved setups

Save a set of inputs you reuse — your usual rate, your loan, your room sizes — and load it back in one tap.

    Your recent calculations

    Results you calculate here are kept on this device so you can come back to them.

      Formula

      Battery life (hours) = capacity (mAh) × usable % ÷ load (mA)
      From watts: load (mA) = power (W) ÷ voltage (V) × 1,000
      Energy (Wh) = capacity (mAh) × voltage (V) ÷ 1,000

      How to use it

      1. Enter the capacity printed on the battery.
      2. Enter its nominal voltage.
      3. Enter the device’s average draw in mA or W.
      4. Adjust usable capacity: about 85% is a fair default.

      Worked examples

      A 5,000 mAh battery feeding 250 mA with 85% usable: 4,250 mAh ÷ 250 mA = 17 hours

      In hours
      17 h
      Battery life
      17 h
      Battery energy
      18.5 Wh

      A 10,000 mAh, 3.7 V power bank (37 Wh) running a 5 W load at 85% usable

      In hours
      6.29 h
      Battery life
      6 h 17 min
      Battery energy
      37 Wh
      Load on the battery
      1,351 mA

      Why you never get the full rating

      Rated capacity is measured under gentle, ideal conditions. In use, voltage converters waste some energy as heat, devices shut off before the cell is truly empty, heavy loads and cold reduce what a cell can deliver, and capacity fades with age. Planning on 70–90% of the label is realistic.

      Power banks and mAh

      A power bank’s mAh rating is at its internal cell voltage of about 3.7 V, but it charges your phone at 5 V or more. A 10,000 mAh bank holds 37 Wh; after conversion losses it delivers roughly 6,000–7,000 mAh at 5 V. Comparing watt-hours avoids the confusion.

      Flying with batteries

      Airline limits are set in watt-hours. Under FAA rules, lithium-ion batteries up to 100 Wh may go in carry-on bags, and 101–160 Wh need airline approval. 100 Wh is about 27,000 mAh at 3.7 V.

      Questions people ask

      How long will a 5,000 mAh battery last?

      It depends on the load. At 250 mA with 85% usable, 17 hours. At 500 mA, 8.5 hours. At 1,000 mA, 4.25 hours.

      How do I convert mAh to Wh?

      Multiply by the voltage and divide by 1,000. 10,000 mAh at 3.7 V is 37 Wh.

      How do I find my device’s current draw?

      Check its label or specification for mA or W. If only watts are given, choose the power option and the current is worked out from the battery voltage.

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