Estimate device runtime in hours from battery capacity (mAh) and average current draw (mA).
Calculator verified • Last updated: August 2026
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Battery capacity in milliamp-hours (mAh) describes how much current a battery can theoretically supply over one hour before its rated charge is used up. Dividing capacity by the device's average current draw gives a straightforward runtime estimate:
$$\text{Battery Life (hours)} = \frac{\text{Capacity (mAh)}}{\text{Current Draw (mA)}}$$
Capacity: the battery's rated capacity, in milliamp-hours (mAh).
Current Draw: the device's average operating current, in milliamps (mA).
For example, a 3,000 mAh battery powering a device that draws 150 mA on average gives 3000 ÷ 150 = 20 hours of estimated runtime.
This simple division assumes the battery delivers its full rated capacity at a perfectly constant rate, which isn't how real batteries behave. Voltage sags as a battery discharges, and many devices become less efficient (or stop working entirely) once voltage drops below a threshold, even though some rated capacity technically remains. Higher discharge currents also reduce a battery's effective capacity below its nameplate rating (a phenomenon called the Peukert effect in lead-acid batteries, and a milder version of the same idea applies to lithium-ion cells), and temperature — especially cold — further reduces usable capacity. Treat this calculator's result as an optimistic upper bound, not a guarantee.
Check the device's datasheet or product specifications for a typical or average operating current — many devices list this directly. Where it isn't published, a USB power meter or multimeter can measure actual current draw directly. Average draw is usually meaningfully lower than peak draw (the current spikes a device pulls momentarily, e.g. during a wireless transmission or screen wake), so using a peak figure here would significantly underestimate runtime.
Using peak current instead of average current is the most common input mistake, producing an artificially pessimistic runtime estimate. Confusing mAh (a capacity/charge unit) with mA (a current/rate unit) is another frequent mix-up — they're related but not interchangeable, and mixing them up in a spreadsheet formula silently produces a nonsense number. Ignoring temperature and discharge-rate effects, as covered above, is the main reason a calculated estimate and real-world runtime diverge.
mAh (Milliamp-Hour) — a unit of electric charge capacity; a 2000 mAh battery can theoretically supply 2000 mA for one hour, or smaller currents for proportionally longer.
mA (Milliamp) — a unit of electric current, describing a rate of charge flow rather than a total capacity.
Peukert Effect — the phenomenon where a battery's effective usable capacity decreases at higher discharge currents, most pronounced in lead-acid batteries but present to some degree in most battery chemistries.
Discharge Curve — the graph of a battery's voltage over time as it drains, rarely flat in practice, which is why a linear capacity/current calculation is an approximation rather than an exact prediction.
This calculator uses a simple linear division, but real batteries don't deliver their full rated capacity at a constant rate — voltage sags as the battery drains, efficiency losses increase at higher discharge currents, and temperature affects usable capacity. Actual runtime is commonly somewhat lower than the simple division suggests.
Check the device's datasheet or specifications for a typical/average operating current, or measure it directly with a multimeter or USB power meter if you have access to the device. Peak current draw is usually higher than average, so use the average figure for a realistic runtime estimate.
Milliamp-hour — a unit of electric charge capacity. A 2000 mAh battery can theoretically supply 2000 milliamps for one hour, or 1000 milliamps for two hours, and so on, before its rated capacity is exhausted.