Phone Charging Time Calculator

The arithmetic behind charging time

Charging time is an energy problem. A battery stores energy in watt-hours, a charger delivers energy at a rate measured in watts, and time is energy divided by rate. The only wrinkle is that the battery's capacity is usually printed in milliamp-hours, so you have to convert.

Energy (Wh) = capacity (Ah) × voltage (V) × fraction of charge needed
Effective power (W) = charger watts × efficiency
Time (hours) = energy ÷ effective power

A typical phone battery runs at a nominal 3.7 volts, and its capacity is given in milliamp-hours, so 4,500 mAh is 4.5 amp-hours and about 16.65 watt-hours. Charging from 20% to 100% means adding 80% of that, or 13.32 Wh. A 20 W charger does not put all 20 W into the battery; some is lost as heat in the brick, the cable and the phone's power circuitry, so the calculator applies an efficiency figure, usually 80% to 90%.

The result is the straight-line estimate: 13.32 Wh ÷ 17 W = 0.78 hours, or about 47 minutes. The USB Implementers Forum publishes the USB Power Delivery specification that governs how a phone and charger negotiate that wattage in the first place.

Worked example: a 4,500 mAh phone on a 20 W charger

The defaults in the calculator above, a 4,500 mAh battery at 20% charge, a 20 W charger and 85% efficiency.

• Battery energy: 4.5 Ah × 3.7 V = 16.65 Wh
• Charge to add: 100% − 20% = 80% → 16.65 × 0.8 = 13.32 Wh
• Effective charging power: 20 W × 0.85 = 17 W
• Time: 13.32 ÷ 17 = 0.78 hours = 47 minutes
• Per 10% of battery: about 6 minutes

In the real world, expect closer to 55 to 60 minutes, because the charging curve tapers above roughly 80% and the phone may also be running apps while it charges. If the screen is on and you are using the phone, that 20 W is being shared between the battery and the processor, and the time can stretch by 30% or more.

Note how the fraction matters. Charging 0% to 50% is only 8.33 Wh, or about 29 minutes on the same charger, noticeably less than half the time of a full charge, because you avoid the slow top end entirely.

Worked example: an older phone on a 5 W brick

Now an older 3,000 mAh phone at 5% charge on the old 5 W USB-A brick everyone has in a drawer.

• Battery energy: 3.0 Ah × 3.7 V = 11.10 Wh
• Charge to add: 95% → 11.10 × 0.95 = 10.545 Wh
• Effective power: 5 W × 0.85 = 4.25 W
• Time: 10.545 ÷ 4.25 = 2.48 hours = 149 minutes
• Per 10%: about 16 minutes

Put the same phone on a 20 W charger and the math drops to 10.545 ÷ 17 = 0.62 hours, or 37 minutes, a fourfold improvement from nothing but the brick. That is the entire case for fast charging in one comparison.

It is also the reason a phone that "used to charge overnight" now needs to be plugged in during the day: batteries lose capacity with age, so the same 3,000 mAh pack may hold only 2,400 mAh after a couple of years, and the phone also throttles charging more aggressively as it ages. Enter your current battery health figure if your phone reports one.

Charging time by charger wattage

Straight-line estimates for a 4,500 mAh phone charged from 0% to 100% at 85% efficiency. Real times are 15% to 25% longer because of the taper at the top of the curve.

Notice the diminishing returns. Going from 5 W to 20 W cuts the time by 75%, but going from 20 W to 65 W cuts it by only about half again in theory, and far less in practice, because almost every phone caps how much power it will accept at any moment. A charger above your phone's maximum is not wasted, but it will not charge faster than the phone allows.

Energy use is a separate question worth a thought: a slow charger left plugged in for four hours can waste more energy overall than a fast one used for 40 minutes. The ENERGY STAR programme covers charger efficiency standards, and the US Department of Energy has general guidance on device energy use.

Why real charging time is longer than the math

Every figure above assumes constant power from start to finish, and no phone works that way. Four things stretch the real number.

• The taper. Above roughly 80%, the phone deliberately slows the current to protect the cell, so the last fifth of the charge takes a disproportionate share of the time.
• Heat. A hot battery charges more slowly, and on a hot day or under a pillow the phone may throttle hard. Charging in a cool spot genuinely is faster.
• Use while charging. If the screen is on, the phone is gaming or it is running navigation, part of the charger's output feeds the processor instead of the battery.
• Older cells. Internal resistance rises with age, so a three-year-old phone accepts power more slowly than the same model did when new.

That is why a straight-line estimate is a floor, not a promise. Use it to compare chargers and to plan around a 30-minute top-up, then expect the phone to add its own margin on top.

Using the estimate well

The most practical use of a charging-time estimate is topping up, not full charges. If you have 25 minutes before you leave and a 20 W charger, the math says about 42% of a 4,500 mAh battery, realistically 30% to 35% once the phone is warm and you have used it. That is enough to change a decision about whether to grab the charger at all.

It is also the right tool for judging a charger purchase. If your phone's maximum charging rate is 18 W, buying a 65 W brick buys nothing except future-proofing for your next phone. If your phone supports 45 W and you are still on a 5 W brick, the upgrade is the single biggest charging-speed improvement you can make.

Treat the number as an estimate built on the figures you enter, and check your phone's own specifications before you buy anything. Battery capacity, maximum charging wattage and supported standards are all published by the manufacturer, and they decide the real answer. Nothing you type into this page leaves your browser.