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Battery Life Calculator

See how long a battery will run your device, from its capacity (mAh, Ah or Wh) and the device's draw (mA, A or W). Also converts mAh to Wh.

Level: Grades 6–7

Printable worksheet

Battery Life Calculator

Know the answer you need? Pick an input to work backward.

Inputs

The rated capacity printed on the battery or in its specifications.

Used to convert between mAh and Wh. A single lithium-ion cell is typically 3.6–3.7 V.

The average current or power the device draws while running.

The share you can really use after conversion losses, cut-off voltage, age and temperature. 100% is the ideal case.

Result

Estimated runtime

10.2 h

3,000 mAh at 250 mA with 85% usable lasts about 10 h 12 min.

Runtime
10 h 12 min
Capacity in watt-hours
11.1 Wh
Capacity in milliamp-hours
3,000 mAh
Usable energy
9.44 Wh
Load as current
250 mA
Load as power
0.925 W

See the step-by-step solution ↓

Step-by-step solution

  1. Keep only the usable share of the capacity, since some is lost in real use

    3,000 mAh×85%=2,550 mAh

  2. Divide the usable charge by the current the device draws. This gives the runtime in hours

    t=2,550 mAh250 mA=10.2 h

  3. Write the runtime in days, hours and minutes

    10.2 h=10 h 12 min

  4. For reference, the capacity in watt-hours at this voltage: Wh = mAh × V ÷ 1,000

    3,000×3.71,000=11.1 Wh

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Find out how many hours a battery will keep your device running. A battery's capacity is a store of charge or energy, and a device uses it up at a steady rate. Divide one by the other and you get the runtime. A 3,000 mAh battery feeding a 250 mA load would, in a perfect world, last 12 hours.

Real batteries give somewhat less. So this calculator also takes a usable-capacity percentage, to allow for the losses.

Capacities and loads come in two families. Milliamp-hours (mAh) and amps measure charge and current. Watt-hours (Wh) and watts measure energy and power. Mixing them needs the battery's voltage. Enter whichever units you have, and the calculator converts as needed. It always shows the capacity both ways, so it also works as a mAh-to-Wh converter.

Where it's used

Why this math matters outside the classroom:

  • Phone runtime. A 3,000 mAh phone battery feeding a steady 300 mA load runs for roughly 3,000 ÷ 300 = 10 hours.
  • Power banks. Travelers convert a 10,000 mAh power bank at 3.7 V into 37 Wh to check it against airline carry-on rules.
  • Camping lights. Campers running a 6 W light string from a 12 V, 50 Ah battery estimate 600 Wh ÷ 6 W, or about 100 hours before losses.
  • Electronics projects. Hobbyists find that a sensor drawing 20 mA from a 2,000 mAh pack should last about 100 hours.

Grade level and standards: Grades 6–7; Common Core State Standards for Mathematics 6.RP.A.3.b, 6.RP.A.3.d. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.

How to use it

  1. Enter the battery capacity and choose mAh, Ah or Wh.
  2. Enter the battery's rated voltage. It matters when you mix charge and energy units, and for the mAh/Wh conversion.
  3. Enter the device's average load in mA, A or W.
  4. Set the usable capacity (85% is a reasonable starting point for many devices). Then read the runtime.

What the inputs mean

Battery capacity
The rated capacity. Phone and power-bank capacities are usually given in mAh, at the voltage of the cells inside (about 3.6–3.7 V). Laptop batteries and larger packs are often given in Wh.
Battery voltage (rated)
The rated (stated) voltage of the battery itself. That's about 3.6–3.7 V per lithium-ion cell, 1.2 V per NiMH cell, 1.5 V for an alkaline cell and 12 V for a typical lead-acid battery.
Device load
The device's average draw. If it changes, use an average over a typical spell of use. Short peaks matter less than the long-run average.
Usable capacity
An allowance for losses. These include energy wasted in voltage converters, and the cut-off voltage at which the device stops. They also include cold temperatures, high drain and battery ageing. Lower it for old batteries or cold conditions.

Formula and method

runtime (h) = capacity (mAh) × usable% ÷ load (mA)
runtime (h) = capacity (Wh) × usable% ÷ load (W)
Wh = mAh × V ÷ 1,000      mAh = Wh ÷ V × 1,000
W = A × V

Runtime is in hours (h). Capacity is in mAh or Wh, and load is in mA (milliamps) or W (watts). Usable% is the share of the capacity you can really use. V is the battery voltage in volts, and A is current in amps.

When both capacity and load are in charge units (mAh and mA), the voltage cancels out. Then runtime is just charge divided by current. When either one is in energy units, both are changed to watt-hours and watts using the battery voltage. Then runtime is energy divided by power.

The usable percentage scales the rated capacity down to what the device can really draw before it shuts off. It's one number standing in for several real-world losses.

Worked example

A device draws 250 mA from a 3,000 mAh, 3.7 V battery, and you expect about 85% of the rated capacity to be usable.

  1. Usable charge: 3,000 mAh × 85% = 2,550 mAh.
  2. Runtime: 2,550 mAh ÷ 250 mA = 10.2 hours.
  3. Capacity in watt-hours: 3,000 × 3.7 ÷ 1,000 = 11.1 Wh.

Result: The battery should run the device for about 10.2 hours (10 h 12 min). It stores 11.1 Wh.

Understanding the result

Treat the result as an estimate for a steady load. Some devices sleep and wake. Radios send in bursts, and motors start and stop. For all of these, the average draw is hard to know without measuring it.

When comparing batteries with different voltages, compare watt-hours, not milliamp-hours. A 10,000 mAh power bank at 3.7 V stores 37 Wh. Delivering that at 5 V over USB, with conversion losses, gives noticeably fewer than 10,000 mAh at the output.

Assumptions and limitations

  • The load stays the same (or the value you enter is a true average).
  • The voltage is the battery's rated voltage, and it's treated as staying the same as the battery drains.
  • 1 Ah = 1,000 mAh and Wh = Ah × V exactly.
  • It doesn't model the Peukert effect, where lead-acid and some other batteries give less capacity at high currents.
  • It doesn't model self-discharge (a battery slowly losing charge on its own). That matters for very small loads over months or years.
  • Temperature, age and how charged the battery is can move the real runtime well away from the estimate.

Frequently asked questions

How do I convert mAh to Wh?

Multiply the milliamp-hours by the voltage and divide by 1,000. A 5,000 mAh battery at 3.7 V holds 5,000 × 3.7 ÷ 1,000 = 18.5 Wh. To go the other way, divide watt-hours by the voltage and multiply by 1,000.

Why does my device run for less time than the calculation?

The rated capacity is measured under gentle, ideal conditions. Voltage converters waste some energy, and devices shut down before the battery is fully empty. Cold, age and high current all cut what you get too. Lowering the usable percentage brings the estimate closer to real life.

Why is the voltage needed?

Milliamp-hours measure charge, not energy. To compare charge with a load in watts, or to give capacity in watt-hours, you need the voltage the charge is delivered at. If both capacity and load are in amps or milliamps, the voltage cancels out and doesn't change the runtime.

What usable percentage should I use?

There's no single right value. Around 80–90% suits many lithium-ion devices in mild conditions. Use less for old batteries, cold weather and devices that draw a lot of power. Use less too for lead-acid batteries you don't want to drain deeply.

How this calculator is checked

Method: Definitions of charge (Ah), energy (Wh = Ah × V) and power (W = A × V); runtime as capacity ÷ rate. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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