Battery Charge Time Calculator

How charging time is calculated

Charging time comes from one comparison: how much energy the battery still needs, and how fast the charger can deliver it. Battery capacity is printed in milliamp hours, or mAh. Chargers are sold by wattage. Those are different units, so the first job is to put both on the same footing, and watt hours is that footing.

The steps are short enough to do on paper:

• Divide mAh by 1,000 to get amp hours. A 5,000 mAh battery is 5.0 Ah.
• Multiply amp hours by the battery's nominal voltage. Most phone cells sit near 3.85 V, so 5.0 Ah x 3.85 V = 19.25 Wh.
• Multiply by the share of the battery you need to fill. Going from 10% to 100% means 90% of 19.25 Wh, which is 17.33 Wh.
• Divide by the charger's wattage, then allow for losses. A 20 W charger does not put 20 W into the battery, so divide the energy by the efficiency factor first.

The result is hours. Multiply by 60 for minutes. The calculator on this page does all of that, and it also shows the watt hours and the wall draw so you can see which input is driving the answer. The wattage figures used in the examples follow the USB Power Delivery ranges published by the USB Implementers Forum, the body that maintains the USB specification.

Worked example: a 5,000 mAh phone from 10% to full

Take a phone with a 5,000 mAh battery at 3.85 V, plugged into a 20 W USB-C charger, starting at 10% and charging to 100%. Assume 90% efficiency, which is a fair middle figure for a wired charger and the charging circuit inside the phone.

Step one, capacity in watt hours: 5.0 Ah x 3.85 V = 19.25 Wh. Step two, energy to add: 19.25 Wh x 0.90 = 17.325 Wh. Step three, energy pulled from the wall: 17.325 Wh / 0.90 = 19.25 Wh. Step four, time: 19.25 Wh / 20 W = 0.9625 hours, or about 58 minutes.

Notice that the wall draw lands back on the battery's full capacity. That is not a coincidence in this example. The phone needs 90% of its energy and the charger is 90% efficient, so the two factors cancel and the wall sees one full battery's worth of energy. Change the starting point to 50% and the same phone needs only 9.625 Wh in the battery, 10.69 Wh from the wall, and 32 minutes on the same charger.

A 20,000 mAh power bank at 3.7 V holds 74.0 Wh. Charging it from empty on an 18 W charger takes 74.0 / 16.2 = 4.57 hours, or 4 hours 34 minutes. That is the number people underestimate most often, because a power bank holds four phone batteries and the wall charger does not get any faster.

Charging time by charger wattage

This table uses the same 5,000 mAh, 3.85 V battery and the same 90% efficiency, and it charges from 10% to 100%. The only thing that changes is the charger. The last column shows the energy that has to come out of the wall socket.

The energy column never moves, and that is the point. A bigger charger does not use more electricity to fill the same battery. It moves the same watt hours in a shorter window. The savings from a fast charger show up on your clock, not on your power bill.

Doubling the wattage does not halve the time in the real world, because the phone decides how much current it will accept. A 65 W charger on a phone capped at 25 W charging behaves like a 25 W charger, and the extra capacity sits idle. Match the charger to what the device will take before you spend money on a bigger brick.

Why the real number runs longer: efficiency losses

Every conversion between the wall and the battery turns some energy into heat. The charger converts 120 V AC to a low DC voltage, the cable drops a little along its length, and the charging circuit inside the phone steps the voltage again. The U.S. Department of Energy regulates these losses directly through its battery charger efficiency standards, which exist because the waste is large enough to matter across millions of devices.

A wired charger that keeps 90% of what it draws is a reasonable working assumption. Cheap bricks and thin cables can fall to 80%, and wireless charging is worse because the coil pair loses more in the air gap. The table shows what that does to the same 58 minute charge.

Ten percentage points of efficiency cost about 7 minutes on a phone and much more on a laptop. A 60 Wh laptop battery charged from 20% to 100% on a 65 W charger takes 48 Wh / 58.5 W = 0.82 hours, or 49 minutes, at 90% efficiency. Drop to 80% and the same charge takes 55 minutes. The heat is also why a phone gets warm in your hand while it charges, and why charging under a pillow is a bad idea.

Why the last 20% takes so long

The straight-line math above assumes the charger pushes power at a constant rate until the battery is full. Lithium batteries do not work that way. A fast charger runs at full power while the battery is empty, then steps down as the voltage rises. The final stretch, usually the last 15% to 20%, runs at a fraction of the peak rate to protect the cell.

This is why a phone that reaches 50% in 20 minutes can take another 40 minutes to reach 100%. The first half is the fast half. The published claim on the box, often something like "50% in 30 minutes", describes that first half and says nothing about the rest.

You can see the effect in the numbers. A 5,000 mAh phone on a 30 W charger from 0% to 50% takes 9.625 Wh / 27 W = 0.36 hours, or 21 minutes. The same phone from 50% to 100% needs the same 9.63 Wh, but if the charger has dropped to an average of 12 W for that stretch, it takes 9.625 / 10.8 = 0.89 hours, or 53 minutes. The second half is more than twice as slow even though the energy is identical.

Practical result: when you are in a hurry, charge to 50% or 60% and leave. The calculator's answer is closest to reality when the target is below 80%. For a full charge, add 15 to 25 minutes to whatever the arithmetic gives you on a fast charger.

Reading a battery spec: mAh, Wh and voltage

Phone batteries are labeled in mAh, laptop and power station batteries in Wh. The two numbers describe the same thing at different levels, and mixing them up is the most common source of a bad estimate. Milliamp hours measure charge. Watt hours measure energy, and energy is what a charger delivers.

Voltage matters more than most people expect. A laptop cell at 15.4 V stores almost four times the energy of a phone cell with the same mAh rating. That is why a 4,000 mAh laptop battery takes far longer to fill than a 4,000 mAh phone battery on the same wattage. If a spec sheet gives you only mAh, look for the voltage before you trust a time estimate.

Power banks add one more wrinkle. The 20,000 mAh printed on the case is measured at the internal cell voltage, not at the USB port. Conversion to 5 V and the cable loss eat roughly a third of that before your phone sees it, so a 20,000 mAh bank delivers about 13,000 mAh of usable charge in practice. Treat the printed number as gross capacity, not as phone charges.

Power banks, laptops and other devices

The same arithmetic works for anything with a battery, but the inputs change. A laptop charger often runs at 20 V and 3.25 A, which is 65 W, and the battery is listed in Wh already, so you can skip the first conversion. A cordless drill or a robot vacuum uses a battery pack rated in Ah and volts, and the pack's watt hours are printed on the label.

Two adjustments matter for larger devices. First, a laptop that is running while it charges sends part of the charger's output to the motherboard, so the battery fills more slowly than the arithmetic suggests. Add 20% to the time if the screen is on. Second, some laptops refuse to charge at full speed from a charger below their rating and will instead hold the battery steady or drain slowly while plugged in. A 45 W charger on a 65 W laptop is not a slow charge, it is often no charge at all under load.

For a power bank charging a phone, the bank's output rating is the ceiling, not the phone's. A 20,000 mAh bank with a 20 W port charging a 5,000 mAh phone gives you about 13,000 mAh of usable energy, which is roughly 2.6 full phone charges, and each one takes about an hour at that wattage. The bank itself then needs 4 to 5 hours on the wall to refill.

Mistakes that make the estimate wrong

Four inputs cause most of the error. Check them before you trust a number.

• Using the charger's maximum wattage as a constant. A 65 W charger only delivers 65 W to a device that asks for it, and only during the fast phase. Use the device's accepted wattage instead.
• Forgetting the cable. A thin or long cable can drop the delivered power by 10% to 20%, and a damaged one can cut it in half. The charger is not the only part of the path.
• Charging while using the phone. Games, video and GPS pull power at the same time the battery is filling. The net rate can fall by half, and the estimate will be far too optimistic.
• Treating mAh as comparable across voltages. A 5,000 mAh battery at 3.85 V and a 5,000 mAh pack at 12 V are not the same amount of energy.

The calculator gives a clean estimate under normal conditions. If your phone is hot, the screen is on, or the cable is old, expect the real time to run 20% to 40% longer. That gap is not a flaw in the math, it is the difference between a specification and a Tuesday afternoon.

This is a planning estimate, not a measurement of your specific device. Charger behavior varies by model and firmware, and manufacturers publish their own charge curves.