mAh · Wh · runtime

Battery Life Estimator

Enter a battery's capacity, voltage and average draw to get a rough estimate of how long it will run.

Find this on the battery label or device spec sheet.

Typical for a phone Li-ion cell is 3.7–3.85V. Only affects the result when capacity and draw use different unit systems.

The average current or power the device pulls while in use.

Rough example draws only — actual usage varies a lot by device and settings.

—Estimated runtime
Capacity in Wh—
Capacity in mAh—
Draw in watts—
Draw in mA—

Rough estimate only. Real-world runtime depends on battery age, temperature and how draw changes during actual use.

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mAh or Wh

Enter capacity however your device lists it — milliamp-hours or watt-hours.

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Current or power draw

Use average current in mA or average power in watts, whichever you know.

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Private by design

No accounts, no uploads — every calculation stays on your device.

How to use the battery life estimator

Enter what you know about the battery and its typical draw to get an estimated runtime.

  1. Enter capacity and voltageType the battery's capacity in mAh or Wh, and its nominal voltage.
  2. Enter average drawType the average current (mA) or power (W) the device uses, or pick a preset.
  3. Read the estimateSee the estimated runtime in hours and minutes, updated instantly.

About estimating battery life

Runtime is simply how long a battery's stored energy lasts at a given rate of use: runtime (hours) equals energy capacity divided by draw. When capacity is in watt-hours and draw is in watts, or capacity is in milliamp-hours and draw is in milliamps, the calculation is direct and voltage does not matter. Voltage only becomes necessary to bridge between the two systems, since energy (Wh) equals charge (Ah) multiplied by voltage.

The result here is necessarily a simplification. Real batteries do not discharge at a perfectly constant rate, their usable capacity shrinks with age and cold temperatures, and a device's actual draw fluctuates constantly with screen brightness, network activity and background processes. Treat the estimate as a planning figure, not a guarantee.

Why voltage sometimes cancels out

If both capacity and draw are already in the same system (mAh with mA, or Wh with W), the voltage terms cancel and runtime is just capacity divided by draw.

Typical smartphone draw

Screen-on use often draws 300–1000+ mA depending on brightness and activity, while standby can drop to just a few mA.

Battery aging

Lithium-ion batteries typically lose usable capacity over hundreds of charge cycles, so a two-year-old battery may run shorter than its rated capacity suggests.

Cold weather effects

Low temperatures temporarily reduce a battery's effective capacity, which is why devices often report shorter runtime in cold conditions.

Battery life estimator FAQ

Common questions about estimating runtime from capacity and draw.

How accurate is this estimate?

This is a simplified estimate based on the nameplate capacity and a constant average draw. Real-world runtime varies with battery age, temperature, the battery's discharge curve and how draw changes over time, so treat the result as a rough guide rather than a guarantee.

How do I find my device's average current draw?

Check the device's spec sheet, a power-monitoring app or utility, or start from one of the preset example draws. Screen-on smartphone use commonly draws roughly 300 to 1000+ mA depending on brightness and activity, while standby draws only a few milliamps.

Why does the calculator ask for voltage?

Voltage is only needed to convert between mAh/mA and Wh/W. If capacity and draw are both given in the same system (mAh with mA, or Wh with W), the voltage cancels out and does not change the runtime result.

What voltage should I use for a phone or laptop battery?

Most single-cell lithium-ion and lithium-polymer batteries used in phones and many gadgets have a nominal voltage around 3.7 to 3.85 volts. Check the battery label or device specifications for the exact figure.

Does current draw stay constant during real use?

No. Screen brightness, wireless activity, background tasks and temperature all change current draw continuously, so real runtime commonly differs from a constant-draw estimate by 20% or more.

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