Quick answer
With the default setup, 20,000 mAh gives about 2.94 charges from 0-100%, or 3.27 charges from 10% to 100%. The simple mAh ratio says 4 charges, but the realistic estimate is lower after voltage conversion, charging efficiency, and reserve.
- Full-charge equivalent
- 2.94 charges
- Selected top-ups
- 3.27 charges
- Usable energy
- 56.6 Wh
- Travel range
- Usually under 100 Wh
How to use this power bank phone charges calculator
Enter the power bank capacity from the label. If Wh is printed, use Wh. If the label only shows mAh, enter the battery voltage shown near the capacity rating. Many power banks advertise mAh at 3.7 V or 3.85 V, not at 5 V USB output.
Enter your phone battery capacity, then choose whether you want to estimate full 0-100% equivalents or practical top-ups such as 10% to 100%, 20% to 80%, or 20% to 90%. The top-up range often matches real travel use better than full-cycle math.
For charging speed, use the Battery Charging Time Calculator. For laptop runtime, use the Laptop Power Bank Runtime Calculator.
Power bank phone charges formula
power bank Wh = power bank mAh x power bank voltage / 1000usable Wh = power bank Wh x efficiency % / 100 x (1 - reserve % / 100)phone battery Wh = phone mAh x phone battery voltage / 1000full equivalent charges = usable Wh / phone battery Whselected top-up energy = phone battery Wh x (target % - start %) / 100selected top-ups = usable Wh / selected top-up energyThe key is converting both the power bank and phone battery into watt-hours. mAh by itself is not a complete energy value unless the voltage is known.
Assumptions and methodology
This calculator is designed for real-world phone charging, not ideal lab capacity. It subtracts efficiency loss and optional reserve before estimating phone charges.
- Power bank mAh is converted to Wh using the battery voltage printed on the label.
- If you do not know the phone battery voltage, 3.85 V is a practical default for many modern phones; the phone battery capacity usually has a larger effect on the result.
- Usable energy is reduced by charging efficiency and reserve.
- Phone battery mAh is converted to Wh using the phone battery nominal voltage.
- The result separates 0-100% equivalent charges from your selected top-up range.
- Real results can change with fast charging heat, phone use while charging, cable quality, battery age, and charge slowdown near 100%.
What phone battery size should you enter?
Use the exact rated capacity when you know it. If not, choose a realistic range below. A larger phone battery means fewer full charges from the same power bank.
| Device type | Typical battery | Use note |
|---|---|---|
| Compact phone | 3,000-3,800 mAh | Older or smaller phones may need less energy per charge. |
| Typical modern phone | 4,000-5,000 mAh | A good default range when you do not know the exact battery size. |
| Large phone or gaming phone | 5,000-6,500 mAh | Large batteries reduce the number of full charges a power bank can provide. |
| Small tablet or foldable | 6,000-10,000+ mAh | Use the larger device battery capacity if the power bank will charge more than a phone. |
Example calculations
20,000 mAh power bank phone charging example
A 20,000 mAh power bank rated at 3.7 V stores about 74 Wh. With 85% charging efficiency and a 10% reserve, usable phone-charging energy is about 56.6 Wh.
A 5,000 mAh phone at 3.85 V is about 19.3 Wh. That gives about 2.94 charges from 0-100%, or about 3.27 charges from 10% to 100%.
The simple mAh ratio would suggest 4 charges, but the realistic estimate is lower because some energy is lost before it reaches the phone battery.
Common power bank phone charge examples
These examples use watt-hours, efficiency loss, and reserve rather than simple mAh division.
| Setup | Power bank | Phone battery | 0-100% charges | Selected top-ups | Travel range | Result |
|---|---|---|---|---|---|---|
| 5,000 mAh slim bank, 5,000 mAh phone | 18.5 Wh | 5,000 mAh | 0.74 charges | 1.05 charges | Usually under 100 Wh | Enough for target |
| 10,000 mAh bank, 5,000 mAh phone | 37 Wh | 5,000 mAh | 1.47 charges | 1.63 charges | Usually under 100 Wh | Enough for target |
| 20,000 mAh bank, 5,000 mAh phone | 74 Wh | 5,000 mAh | 2.94 charges | 3.27 charges | Usually under 100 Wh | Enough for target |
| 20,000 mAh bank, 4,000 mAh phone | 74 Wh | 4,000 mAh | 3.68 charges | 5.25 charges | Usually under 100 Wh | Enough for target |
| 27,000 mAh travel bank | 99.9 Wh | 5,000 mAh | 3.97 charges | 4.41 charges | Usually under 100 Wh | Enough for target |
| 30,000 mAh large travel bank | 111 Wh | 5,000 mAh | 4.41 charges | 4.9 charges | Airline approval range | Enough for target |
Why a 20,000 mAh power bank does not mean four 5,000 mAh phone charges
The simple ratio ignores voltage and conversion loss. A power bank rated at 20,000 mAh usually means 20,000 mAh at its internal cell voltage. The phone receives energy after USB conversion, cable loss, charging-circuit loss, heat, and charge management.
That is why this calculator shows both the simple mAh ratio and the realistic charge estimate. The difference is often the reason a power bank feels smaller than the number printed on the case.
Related calculators
Battery Charging Time Calculator
Estimate charging time from battery capacity, charger power, and charging efficiency.
Storage Capacity Calculator
Estimate how many photos, videos, apps, songs, and documents fit into phone or tablet storage.
Power Bank Flight Limit Calculator
Convert power bank mAh and voltage to watt-hours, then check common carry-on, airline-approval, and over-limit flight ranges.
Laptop Power Bank Runtime Calculator
Estimate how long a USB-C power bank can run a laptop, including usable Wh, output limits, cable rating, laptop load, and travel range.
USB-C Charger Wattage Calculator
Check whether a USB-C charger, cable, and port can power phones, tablets, laptops, handhelds, and multiple devices.
AI Glasses Battery Life Calculator
Estimate AI smart glasses battery life from audio, video, photos, AI requests, live assist time, standby drain, and charging-case capacity.
FAQ
How many times can a 10,000 mAh power bank charge a phone?
For a 5,000 mAh phone, a 10,000 mAh power bank is often closer to about 1.4-1.6 full charges, not 2 full charges. The exact number depends on voltage, charging efficiency, reserve, cable quality, heat, and whether the phone is being used while charging.
How many phone charges do I get from a 20,000 mAh power bank?
With the default assumptions, a 20,000 mAh power bank and a 5,000 mAh phone give about 2.9 full 0-100% equivalent charges. If you usually charge from 10% to 100%, that is about 3.2 selected top-ups.
Is a 5,000 mAh power bank enough for one full phone charge?
For many modern phones, not usually. A 5,000 mAh power bank can be useful for an emergency top-up, but after voltage conversion and charging loss it may be below one full 0-100% charge for a 5,000 mAh phone.
Why does mAh divided by phone mAh overestimate charges?
Power bank mAh is usually rated at internal battery voltage, often around 3.7 V. Phones charge through USB conversion and internal charging circuitry, so some energy is lost as heat. Watt-hours plus efficiency gives a more realistic estimate.
Should I use mAh or Wh for a power bank?
Use Wh if the label shows it because watt-hours measure stored energy directly. Use mAh only when you also know the voltage printed near the capacity rating.
What efficiency should I use?
Use 80-90% for a practical estimate. A short high-quality cable, moderate temperature, and slow-to-medium charging can be more efficient than hot fast charging with a poor cable.
Does fast charging change the number of phone charges?
Fast charging can reduce efficiency because more energy is lost as heat. It may charge faster, but the number of total charges can be slightly lower than a slower, cooler charge.
Is a 20,000 mAh power bank enough for travel?
For one phone, usually yes for a weekend or travel day. It may be enough for two phones if you are topping up rather than charging from empty to full. For laptops, tablets, cameras, or multiple people, use a larger Wh-based estimate.
Can I use this for tablets or earbuds?
Yes, if you enter the device battery capacity and voltage. For earbuds or small accessories, the result may be many charges; for tablets, the result will be much lower than for a phone.