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USB-C Charging Time Calculator: How long does a phone, tablet or laptop take to charge?

A 5,000 mAh smartphone that accepts 25 W is at 80% from 0% in about 45 minutes – the last 20% then take a good 20 minutes more. This calculator estimates the charging time for USB-C devices from battery capacity, charger and the device's limit, which usually restricts charging power more than the adapter does.

Units

Converted to Wh at 3.85 V cell voltage.

USB-A: 5–18 W, USB-C PD: 20, 30, 45, 65, 100 W. 12 V car adapters often 18–45 W.

Smartphones usually accept 15–45 W, tablets 20–30 W, laptops 45–100 W.

Advanced settings

Losses in charger, cable and the device's charging electronics: typically 80–90%.

Result

Estimated charging time

54 min

from 20% to 100%

Energy to recharge
15.4 Wh
Effective charging power
21.3 W
Fast phase
33 min
Slow phase (above 80%)
22 min
  • The device limits to 25 W – a stronger charger doesn't help.
  • The last 20% charge much more slowly (CV phase). For everyday use 80% is often enough – and it's easier on the battery.
Show calculation
  1. 1Battery content: 5,000 mAh × 3.85 V ÷ 1000 = 19.3 Wh
  2. 2To recharge: 19.3 Wh × (100% − 20%) = 15.4 Wh
  3. 3Effective power: min(30 W, 25 W) × 85% = 21.3 W
  4. 4Fast phase to 80%: 0.54 h
  5. 5Slow phase from 80% (factor 2): 0.36 h

 

How it's calculated

Battery [Wh] = mAh × 3.85 V ÷ 1000 to recharge [Wh] = battery × (target − start) effective power = min(charger, device limit) × efficiency time = to recharge ÷ effective power (above 80% SoC × 2)
Example: 5,000 mAh ≈ 19 Wh; 20% → 100% is 15 Wh; at 25 W × 85% = 21 W: 0.54 h to 80% + 0.36 h for the rest ≈ 55 minutes

Lithium batteries first charge at constant current (fast) and from about 80% at constant voltage with decreasing current (slow). The calculator doubles the time needed for the range above 80% – an approximation that fits most devices well.

Worked example

A tablet with 8,000 mAh is to be charged from 30% to 100% on a 12 V USB-C adapter (45 W); the tablet accepts at most 30 W.

Inputs

  • Battery rating: mAh (smartphone, tablet, power bank)
  • Battery capacity: 8,000 mAh
  • Charger power: 45 W
  • Maximum charging power of the device: 30 W
  • State of charge at start: 30 %
  • Target state of charge: 100 %
  • Charge efficiency: 85 %

Result

1 h 5 min

Estimated charging time

Energy to recharge
21.6 Wh
Effective charging power
25.5 W
Fast phase
36 min
Slow phase (above 80%)
29 min

Calculation

  1. Battery content: 8,000 mAh × 3.85 V ÷ 1000 = 30.8 Wh
  2. To recharge: 30.8 Wh × (100% − 30%) = 21.6 Wh
  3. Effective power: min(45 W, 30 W) × 85% = 25.5 W
  4. Fast phase to 80%: 0.6 h
  5. Slow phase from 80% (factor 2): 0.48 h

The variables explained

Battery capacity (mAh/Wh)
Manufacturer rating. mAh refers to the cell voltage (3.7–3.85 V); laptops usually state Wh.
Charger power (W)
USB-C PD adapter or 12 V adapter. Through weak cables or USB-A often only 10–18 W.
Maximum charging power of the device (W)
Negotiated by the device; many smartphones accept 18–27 W, some 45–65 W.
Charge efficiency (%)
Losses in adapter, cable and charging electronics; part is released as heat in the device.

Common mistakes

  • Looking only at the adapter's power: the device determines how much it accepts.
  • Using cheap cables without an e-marker: above 60 W, USB-C needs a suitable cable.
  • Using the device while charging: display and radios noticeably extend the charge time.
  • Charging in a hot vehicle: many devices throttle charging power from 95–104 °F (35–40 °C).

Assumptions and limits

  • Cell voltage 3.85 V for mAh ratings.
  • CV phase from 80% with double the time needed (approximation).
  • No use of the device while charging.