Battery Life Calculator
Capacity Alone Doesn't Predict Runtime
A battery's rated capacity, whether in milliamp-hours or watt-hours, only tells half the story: runtime depends just as much on how fast the device draws power. A 4000 mAh phone battery lasts vastly different amounts of time depending on whether the screen is off and the radio idle, or the camera and display are both running at full brightness. This calculator divides capacity by draw — in either the mAh/mA or Wh/W convention — to get an estimated runtime.
The Formula
Both modes are the same relationship expressed in different units, adjusted for conversion efficiency:
Life (h) = Capacity (Wh) × Efficiency / Power Draw (W)
Efficiency accounts for the fact that not all stored energy converts into usable output — voltage regulation, heat, and battery age all reduce the effective capacity available. Leaving it at 100% gives the theoretical maximum runtime.
Where This Calculation Matters
- Comparing devices before buying — converting a manufacturer's mAh or Wh rating alongside a typical power draw figure into a real-world runtime estimate.
- Portable electronics design — sizing a battery to hit a target runtime for a known current draw, such as an IoT sensor or wearable.
- Power bank sizing — estimating how many charge cycles a portable battery pack can deliver to a device with a known draw.
- Field and travel planning — predicting how long equipment like a drone, radio, or camera will run on a given battery before needing a swap or recharge.
Runtime Examples
| Device Type | Capacity | Draw | Runtime |
|---|---|---|---|
| Smartphone, light use | 4000 mAh | 200 mA | 20.0 hours |
| Smartphone, heavy use | 4000 mAh | 500 mA | 8.0 hours |
| Laptop, light use | 60 Wh | 15 W | 4.0 hours |
| Laptop, heavy use | 60 Wh | 30 W | 2.0 hours |
Real-world runtime is typically shorter than the 100%-efficiency figure, since regulator losses, battery aging, and temperature all reduce usable capacity.
How to Use This Calculator
- Choose a mode: From Capacity (mAh) & Current Draw or From Energy (Wh) & Power Draw.
- Enter the battery's Capacity in mAh or Wh, matching the mode.
- Enter the device's Current Draw (mA) or Power Draw (W).
- Optionally set Efficiency % (defaults to 100 if left blank).
- Select Calculate to get the estimated runtime in hours.
Related Calculations
Need to know how long a recharge takes instead of a discharge? Use the Charging Time Calculator. Sizing a capacitor rather than a battery? See the Capacitor Charge Calculator.