Device Battery Life Estimator
What does this Battery Life Estimator do?
This tool helps you quickly estimate how long your device (phone, tablet, laptop, flashlight, etc.) battery will last based on the battery capacity and your device's power usage. Enter your battery's mAh (or Wh), your device's typical power drain, and get the expected hours and minutes of use.
It works for gadgets, electronics, power banks—anything with a rechargeable battery!
How to calculate device battery life
- Find the battery capacity (e.g. "5000 mAh" or "50 Wh"). It's printed on the device or in the specs sheet.
- Get average device power usage (in mA or Watts) when active. Use the typical load (not peak/idle).
- Enter both values above, then estimate.
- For most devices, stopping at ~15% remaining (reserve) is best for battery health. For emergencies, you may run to 0%.
Example: Typical Phone and Laptop
Example 1: Phone
Battery: 4000 mAh | Usage: 350 mA (active avg)
→ Estimated life: ~11 hr 25 min
Battery: 4000 mAh | Usage: 350 mA (active avg)
→ Estimated life: ~11 hr 25 min
Example 2: Laptop
Battery: 50 Wh | Usage: 12 W
(Switch units at top of tool!)
→ Estimated life: ~4 hr 10 min
Battery: 50 Wh | Usage: 12 W
(Switch units at top of tool!)
→ Estimated life: ~4 hr 10 min
Example 3: Headphones
Battery: 600 mAh | Usage: 50 mA
→ Estimated life: ~12 hr
Battery: 600 mAh | Usage: 50 mA
→ Estimated life: ~12 hr
Assumptions & Technical Notes
- mAh calculation: Battery life (hours) = Battery capacity (mAh) ÷ Device usage (mA)
- Wh/Watt calculation: Battery life (hours) = Battery capacity (Wh) ÷ Power usage (Watt)
- Reserve mode means ~15% of battery is left unused (for longer battery lifespan)
- Real-world usage may vary (Wi-Fi on/off, screen brightness, CPU load, background tasks, battery wear, temperature)
- This tool is for estimation only; actual time may be shorter or longer.
Frequently Asked Questions
- What is battery capacity?
- The total charge a battery can hold, usually measured in milliamp-hours (mAh) for phones or amp-hours (Ah) for large batteries. Sometimes shown as watt-hours (Wh) for laptops or power banks.
- What if my battery is in Wh, but my device draws power in mA?
- Switch units with the toggle next to usage, or convert using:
1 Wh = 1000 mAh at 1 V; for 3.7V batteries, Wh = (mAh × 3.7) / 1000.
For best accuracy, use the same units. - Can I use this for rechargeable AA/AAA batteries?
- Yes! Just enter their mAh and your device's draw. This also works for flashlights, toys, UPSes, and more.
- Does it work for lithium-polymer or nickel-metal hydride batteries?
- Yes, provided you use accurate mAh or Wh ratings. Results are similar for all types.
- How precise is this estimate?
- This is an ideal estimate. True results depend on real-world conditions, standby drain, device age, etc.