A power bank’s printed capacity is not the same as the energy your phone receives. This guide explains how to estimate real-world phone charges, includes a practical reference table for 5,000mAh, 10,000mAh, and 20,000mAh power banks, and shows how to recalculate the result for any phone.
Overview
The simple version is that a power bank usually provides fewer charges than its capacity suggests. A 10,000mAh power bank does not normally deliver two complete charges to a 5,000mAh phone. Energy is lost as the power bank’s internal battery voltage is converted to the output voltage required by the phone, and more can be lost through the cable, charging circuitry, heat, and wireless charging.
For a useful planning estimate, assume that around 75% to 85% of the advertised capacity becomes usable at the phone. The table below uses an 80% working estimate. It is not a guarantee, but it is a practical starting point for comparing a slim portable charger with a higher-capacity model.
| Phone battery capacity | 5,000mAh power bank | 10,000mAh power bank | 20,000mAh power bank |
|---|---|---|---|
| 3,000mAh | About 1.3 charges | About 2.7 charges | About 5.3 charges |
| 4,000mAh | About 1 charge | About 2 charges | About 4 charges |
| 4,500mAh | About 0.9 charge | About 1.8 charges | About 3.6 charges |
| 5,000mAh | About 0.8 charge | About 1.6 charges | About 3.2 charges |
| 6,000mAh | About 0.7 charge | About 1.3 charges | About 2.7 charges |
A result such as 1.6 charges means one full recharge plus roughly 60% of another under the stated assumptions. In everyday use, you may receive less if you use the phone while charging, start with a partially charged power bank, or rely on wireless charging.
How to estimate power bank phone charges
Use this basic power bank recharge calculator:
Estimated charges = power bank capacity × usable-efficiency factor ÷ phone battery capacity
For the reference table, the efficiency factor is 0.80. To calculate a range, use 0.75 for a cautious estimate and 0.85 for a favorable one.
- Find the power bank’s advertised capacity, such as 10,000mAh.
- Multiply that number by your chosen efficiency factor. At 80%, 10,000mAh becomes 8,000mAh of estimated usable capacity.
- Find your phone’s battery capacity, such as 4,500mAh.
- Divide the usable power bank capacity by the phone capacity: 8,000 ÷ 4,500 = about 1.8 charges.
- Round the result sensibly. Treat it as an estimate of energy available, not a promise of a particular number of 0% to 100% charging cycles.
This method works best when comparing conventional wired charging. For a phone-specific buying decision, see the compatibility guide for iPhone, Samsung Galaxy, and Google Pixel models, since charging speed, ports, and cable requirements also affect convenience.
Inputs and assumptions
Power bank capacity: The mAh figure printed on a power bank describes its internal battery capacity under the maker’s stated testing conditions. It is useful for comparing products in the same category, but it does not directly represent the amount delivered through the USB port.
Phone battery capacity: Check the phone’s technical specifications, its original documentation, or a reliable device information screen. Battery capacity is usually listed in mAh. If your phone has a battery health feature, remember that reduced health can mean less stored energy than when the phone was new.
Efficiency: The 80% assumption balances common conversion losses without pretending that every setup performs identically. A wired connection with a good cable may perform closer to the favorable end of the range, while heat, simultaneous phone use, an aging power bank, or low-quality accessories can push the result lower.
Charging method: Wireless charging generally introduces additional losses compared with a direct wired connection. A magnetic wireless power bank can therefore provide fewer effective charges than the same-capacity wired model. For MagSafe-compatible phones, compare battery capacity with magnetic strength, charging behavior, and portability rather than looking at mAh alone. Our guide to MagSafe power banks for iPhone covers those trade-offs.
Rated output: Wattage mainly affects how quickly the phone charges, not the theoretical amount of energy stored. A fast charging power bank may finish sooner, but it does not automatically provide more total charges. The phone, cable, and power bank must support compatible charging standards. See this USB-C PD, GaN, wattage, and cable guide before choosing accessories.
Worked examples
Example 1: 10,000mAh power bank and a 4,500mAh phone
Start with 10,000mAh and apply the 80% estimate: 10,000 × 0.80 = 8,000mAh. Divide 8,000 by 4,500 to get approximately 1.8 charges. A cautious 75% calculation gives about 1.7 charges, while an 85% calculation gives about 1.9. In practical terms, expect one complete recharge and most of a second, rather than exactly two full charges.
Example 2: 20,000mAh power bank and a 5,000mAh phone
At 80% efficiency, the estimated usable capacity is 16,000mAh. Dividing by 5,000mAh produces about 3.2 charges. This is why a 20,000mAh high-capacity portable charger is often a better choice for a multi-day trip or for charging more than one device, although its additional weight and size may matter.
Example 3: 5,000mAh power bank and a 3,000mAh phone
The estimated usable capacity is 4,000mAh. Dividing by 3,000mAh gives about 1.3 charges. This type of power bank can be convenient for an emergency top-up, but it may not provide a complete recharge if the phone is being used heavily during charging.
For a deeper capacity comparison, read 10,000mAh vs 20,000mAh power banks. The right choice depends on whether you value a lighter everyday carry or more reserve energy.
When to recalculate
Recalculate your expected runtime whenever you change phones, switch from wired to wireless charging, replace an old power bank, or begin charging multiple devices. A new phone may have a different battery capacity, and an older power bank may no longer perform as it did when new.
It is also worth revisiting the estimate when your real experience differs noticeably from the table. Record the phone’s starting percentage, the power bank’s starting charge, the charging method, and whether the phone was in use. Comparing those details over two or three charging sessions gives you a more useful personal efficiency estimate than the label alone.
Before travel, check the power bank’s stated capacity and plan how many devices need charging. For packing guidance, use our power bank travel rules guide, and keep the manufacturer’s capacity information available if it is printed on the device or packaging. Finally, store and charge the power bank sensibly; our guide to power bank care and battery health explains practical ways to reduce avoidable wear.
The quickest repeatable method is simple: check both battery capacities, multiply the power bank’s capacity by 0.80, divide by the phone’s capacity, and then adjust expectations downward for heat, wireless charging, phone use, or an aging battery. Use that calculation whenever you compare a new portable charger or move to a different phone.
