Phones are usually in mAh, laptops in Wh
mAh alone can’t give you a time
mAh printed on a battery is a quantity of charge, not energy — a charger’s watt rating is a rate of energy delivery, so the units don’t match until you convert. Multiply by voltage to get watt-hours (Wh). Lithium-ion cells run a nominal voltage around 3.85V, so Wh = mAh × 3.85 ÷ 1000. A 5,000mAh battery holds roughly 19Wh.
The rated wattage rarely goes in whole
A 20W charger doesn’t deliver a steady 20W the entire time — some energy is lost as heat during voltage conversion, and the device throttles itself as it warms up. This calculator assumes about 70% of rated power actually reaches the battery over a wired connection, and 50% over wireless (the coil-to-coil gap loses more).
The last 20% takes longer than the first 80%
Lithium-ion charging happens in two phases: constant current (CC) pushes as much charge in as possible early on, then around 80% it switches to constant voltage (CV) and tapers the current down. That’s why 0→80% is faster than 80→100%, and why "50% in 30 minutes" marketing claims don’t extrapolate to a full charge.
This is an estimate
Battery temperature, remaining charge level, and whether the device is in use while charging all shift the real number. Charging while actively using the screen is much slower, since energy goes out almost as fast as it comes in.
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Frequently asked questions
QWhy do I need to pick mAh or Wh?
Phones are usually labeled in mAh, laptops in Wh. mAh needs a voltage assumption to convert to energy, so choosing the right unit avoids that extra step and keeps the estimate closer to the real number.
QWhy is wireless charging slower in this calculator?
Wireless charging loses more energy as heat in the gap between the two coils than a direct wired connection does, so a smaller share of the charger’s rated power actually reaches the battery.