Phone Charging Speed Calculator

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)

Efficiency Factor: this calculator uses 0.80. Some of the wall power becomes heat in the brick, the cable, and the phone’s own charging circuit rather than stored charge, and the loss is larger than most people expect on a fast charger.

Two phases, not one

The single formula above is only half the story, and it is why simple charge-time estimates come out far too optimistic. Lithium cells charge at constant current up to roughly 80%, then switch to constant voltage and taper off. The important detail is that the taper is the battery refusing power, not the charger offering less. Above 80% the cell will accept only a fraction of its peak rate no matter what you plug it into. Below that point, the limit is whichever is smaller: what the charger can supply, or what the cell will take (this calculator assumes a peak of about 2C, which is typical of phones that are not marketed on charging speed).

Worked Example, an iPhone 15 (3,349 mAh at 3.85V nominal):

• Battery capacity: 3,349 × 3.85 / 1,000 = 12.89 Wh
• Peak the cell will take: 12.89 × 2 = about 26W
• 0% to 80% is 10.31 Wh at the full 20W: 10.31 / (20 × 0.80) = 38.7 minutes
• 80% to 100% is 2.58 Wh, but the cell now takes only about 10.3W: 2.58 / (10.31 × 0.80) = 18.8 minutes
• Total: 57.4 minutes, which the calculator reports as 57 min

Add the two phases first and round once at the end. Rounding each phase separately gives 39 and 19, which add to 58, and that is a minute the charger never spends.

Per percentage point, that last stretch runs about twice as slow as the first 80%.

Charger Speed Reference (iPhone 15, 0 to 100%, straight out of this calculator):

• 5W old-style brick: 3h 13m
• 20W USB-C: 57 min
• 30W: 49 min
• 65W GaN: also 49 min, because the battery caps out near 26W long before the charger does

That last line is the practical takeaway. Past a certain point you are buying headroom the phone will never use. Going from the 20W brick to the 65W one is three times the charger for eight minutes, and from 30W to 65W it is nothing at all.

Where this estimate is optimistic: the two-phase model here switches cleanly at 80%, while a real charge curve starts tapering well before that. Apple quotes about 90 minutes for a full charge where this calculator says 57. Treat the number as a floor rather than a promise, and expect a heavily fast-charging Android with a hot battery or a warm room to run longer still.

Battery Health Note: Charging to 100% and holding it there is what degrades a lithium cell, more than the charging itself. Most flagship phones now offer optimized charging, which parks at 80% overnight and finishes just before your alarm.
Leave it on. It costs you nothing and manufacturers put the benefit at roughly 20% more useful life over two years.