Phone Charging Speed Calculator
Estimate phone charging time from battery capacity (mAh) and charger wattage.
Compare 5W, 18W, 45W, and 65W speeds to see when you reach 50% and 80%.
Phone charging time is determined by battery capacity, charger power output, and charging efficiency losses. The fundamental formula is:
Charge Time (hours) = Battery Capacity (mAh) × Voltage (V) / (Charger Wattage × Efficiency)
Since Power (W) = Voltage (V) × Current (A), and battery capacity in Wh = mAh × V / 1000:
Charge Time = Battery Capacity (Wh) / (Charger Power × Efficiency Factor)
Typical Efficiency Factor: 0.85–0.90 (accounts for heat loss and charging circuit losses)
For USB-C PD and fast charging protocols (PPS), efficiency stays around 85–88%.
Worked Example — iPhone 15 (3,349 mAh, 3.87V):
- Battery capacity in Wh: 3,349 × 3.87 / 1,000 = 12.96 Wh
- Charger: 20W USB-C
- Efficiency: 0.87
- Charge time (0–100%): 12.96 / (20 × 0.87) = 12.96 / 17.4 = 0.745 hours = 44.7 minutes
In practice, fast charging slows after ~80% to protect battery chemistry (CC/CV charging curve), so real-world 0–100% typically takes 70–90 minutes for a 20W charger.
Charger Speed Reference:
- 5W (old iPhone brick): ~3 hours for iPhone 15
- 20W USB-C: ~75 minutes (0–100%)
- 30W: ~60 minutes
- 65W GaN: ~45 minutes for mid-size phones
Battery Health Note: Charging above 80% regularly degrades lithium-ion cells. Many flagship phones now offer optimized charging that holds at 80% overnight and tops off before your alarm — extending battery lifespan by up to 20% over 2 years.
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.
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