Battery Life Calculator

Estimate runtime from battery capacity and current draw.

How the Math Works

The Battery Life Calculator uses the formula hours = capacity ÷ draw × efficiency to determine how long a battery will last. Capacity, measured in milliamp-hours (mAh), represents the total charge a battery can deliver. Draw, measured in milliamps (mA), indicates how much current a device consumes. Efficiency accounts for energy losses due to heat and other factors, typically expressed as a decimal (e.g., 85% efficiency = 0.85). To calculate, divide the battery's capacity by the device's current draw, then multiply by the efficiency factor to get the estimated runtime in hours.

Practical Applications

To use this calculator practically, first find your battery's capacity rating in mAh from its specifications or packaging. Next, determine your device's current draw in mA by checking its power adapter specifications, using a multimeter, or consulting the user manual. Apply an efficiency factor of 0.85 for most lithium-ion batteries or 0.70 for older nickel-based chemistry. For example, a 2000 mAh battery powering a 100 mA device with 85% efficiency yields 17 hours of runtime. You can also rearrange the formula to solve for required capacity when you know the desired runtime and power draw.

Day-to-Day Use

This calculation helps you make informed decisions about battery-powered devices in everyday situations. Before purchasing a camera, tablet, or emergency device, you can estimate how often you'll need to recharge it based on your usage patterns. When planning for extended trips or power outages, you can calculate whether your backup battery bank will sustain essential devices. For parents, it helps determine if a child's toy or tablet will last through a day-long outing without charging. Understanding these calculations also helps you compare device efficiency and make smarter choices about which gadgets best fit your lifestyle and needs.

Worked example

3000 mAh ÷ 200 mA × 85% → 12.75 h.

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