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.