Phone Charging Time Calculator
Time to charge a phone from its current level, with the battery's energy in watt-hours.
Results
What this tool does
Capacity is printed in milliamp-hours but chargers are rated in watts, and the two only meet through the cell voltage — nominally 3.85 V for a modern lithium phone battery. A 5000 mAh phone therefore holds about 19 watt-hours, which is why a 20 W charger cannot fill it in an hour no matter what the box says.
Formula
t = energy still needed ÷ (charger power × efficiency)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
cp | Battery capacity | mAh |
vb | Cell voltage | V |
pw | Charger power | W |
lv | Current level | % |
ef | Efficiency | % |
MI | Minutes | min |
HH | Hours | h |
WH | Energy | Wh |
NE | Energy | Wh |
CY | Cost | — |
Worked example
- Battery capacity5000 mAh
- Cell voltage3.85 V
- Charger power20 W
- Current level20 %
- Efficiency80 %
- Minutes58 min
- Hours0.96 h
- Energy19.250 Wh
- Energy15.400 Wh
- Cost1.2647
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
Frequently asked questions
Why does the last twenty percent take so long?
Because lithium batteries charge at full current only up to around eighty percent, then switch to holding a constant voltage while the current tapers off. That taper protects the cell and extends its life, but it means the charge curve flattens badly at the end. This calculation assumes a steady rate, so it is closest to reality in the fast middle part and optimistic near full.