Phone Battery Calculator
Battery runtime follows directly from physics: capacity divided by current draw gives time. This calculator takes your battery capacity in milliamp-hours, the average current your phone draws, the charging current, and the battery voltage, then returns the runtime in hours, the ideal charge time, and the energy in watt-hours. Watt-hours matter for airline carry-on limits, while runtime and charge time help you plan a day away from a charger. The formulas are exact definitions, so accuracy depends only on the inputs you measure.
Battery formulas
Runtime (hours) = capacity (mAh) / current draw (mA)
Charge time (hours) = capacity (mAh) / charging current (mA)
Energy (Wh) = capacity (mAh) * voltage (V) / 1,000
Runtime (minutes) = runtime hours * 60
Capacity divided by current gives time, the definition of an amp-hour. Multiplying capacity by voltage converts charge to energy. Real charging tapers near full, so the ideal charge time is a lower bound.
Battery context
- An amp-hour is one amp of current sustained for one hour, by definition.
- Watt-hours equal charge times voltage; airlines often cap carry-on cells.
- The U.S. FAA limits spare lithium batteries to 100 Wh without approval.
- Higher screen brightness and weak signal raise the current draw.
- Battery capacity declines as cells age over many charge cycles.
Phone battery: frequently asked questions
How is battery runtime calculated?
Runtime in hours is the battery capacity in milliamp-hours divided by the average current draw in milliamps. For example, a 4,000 mAh battery drawing 500 mA on average runs for 8 hours. This is the standard charge-over-current relationship and needs no empirical estimate.
What is the difference between mAh and Wh?
Milliamp-hours (mAh) measure charge capacity, while watt-hours (Wh) measure energy. To convert, multiply mAh by the battery voltage and divide by 1,000. A 4,000 mAh battery at 3.85 volts holds about 15.4 watt-hours. Airlines often state limits in watt-hours.
How long does charging take?
Charge time in hours is the battery capacity divided by the charging current, both in the same unit. A 4,000 mAh battery charged at 2,000 mA takes about 2 hours in the ideal case. Real charging is slower near full because the current tapers, so treat this as a lower bound.
Why is actual battery life often shorter than calculated?
The calculation assumes a constant current draw and 100 percent efficiency. In practice, screen brightness, signal strength, background apps, and battery aging all change the draw. Use your measured average current for a realistic figure; the formula itself is exact.
What voltage should I enter?
Most single-cell lithium-ion phone batteries have a nominal voltage near 3.7 to 3.85 volts, often printed on the battery or in the device specifications. Enter the value your device states. The voltage is only used to convert capacity to watt-hours, not to compute runtime.
Official sources
- U.S. Federal Aviation Administration: Lithium battery carriage limits.
- U.S. National Institute of Standards and Technology: SI units of charge, current and energy.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.