Device Battery Runtime Calculator
Calculate how long a battery will last in your device based on battery capacity and power consumption.
Compare battery sizes.
Battery Runtime tells you how long a device will operate on a single charge based on the battery capacity and device power consumption.
The basic formula:
Runtime (hours) = Battery Capacity (mAh) / Device Current Draw (mA)
Or using watt-hours:
Runtime (hours) = Battery Energy (Wh) / Device Power (W)
Converting between units:
Watt-hours (Wh) = milliamp-hours (mAh) × Voltage / 1000
mAh = Wh × 1000 / Voltage
Common battery capacities:
- Smartphone: 3,000-5,000 mAh (3.7V = 11-18.5 Wh)
- Tablet: 5,000-10,000 mAh (3.7V = 18.5-37 Wh)
- Laptop: 40-100 Wh (10,800-27,000 mAh at 3.7V)
- Power bank: 5,000-26,800 mAh
- AA battery: 2,500 mAh (1.5V = 3.75 Wh)
- 18650 Li-ion cell: 2,500-3,500 mAh (3.7V = 9.25-12.95 Wh)
Common device power consumption:
- LED flashlight (low): 0.1W
- LED flashlight (high): 3-10W
- Bluetooth speaker: 2-5W
- Smartphone (active use): 2-4W
- Smartphone (standby): 0.1-0.3W
- Tablet: 3-8W
- Laptop (light use): 10-25W
- Laptop (heavy use): 30-65W
Efficiency factor: Real-world runtime is typically 70-85% of the theoretical calculation due to:
- Voltage conversion losses (5-15%)
- Battery chemistry not delivering full capacity under load
- Temperature effects on battery performance
- Battery degradation over charge cycles
The power bank mAh trap: This one catches nearly everybody. A power bank advertised as 20,000 mAh is quoting the rating of the lithium cells inside it, which sit at 3.7V. That is 74 Wh. The USB output is 5V, and pushing 74 Wh through a boost converter at 85% efficiency leaves roughly 63 Wh, or about 12,600 mAh measured at 5V. So the honest answer to “why does my 20,000 mAh bank only refill my 5,000 mAh phone three times instead of four?” is that it was never 20,000 mAh at the voltage you were using it. Enter the label figure with 3.7V selected here, not 5V.
Battery aging: Lithium-ion batteries typically retain 80% of their original capacity after 300-500 full charge cycles. After 500 cycles, a 5,000 mAh battery effectively becomes a 4,000 mAh battery.
The 80% rule: For practical estimates, multiply your theoretical runtime by 0.8 to get a realistic estimate.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.