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Power bank phone charges calculator.
Enter the power bank and phone battery ratings and see how many full phone charges you really get.
Inputs
Charges from one power bank
Enter the numbers above.
How many times can a power bank charge a phone?
Compare energy, not mAh. A power bank’s mAh is counted at the cell voltage (about 3.7 V), and some energy is lost as heat while it charges the phone, so you never get the full number. Charges = power bank watt-hours x efficiency / phone watt-hours, where watt-hours = mAh x volts / 1,000. Public Safety Canada suggests a battery pack that can charge critical devices multiple times as part of power-outage backup.
Worked example. A 20,000 mAh power bank at 3.7 V holds 74 Wh. At 70 percent efficiency 51.8 Wh reaches the phone. A 4,000 mAh phone battery at 3.85 V is 15.4 Wh, so 51.8 / 15.4 = 3.4 full charges. At 1.5 charges a day that is about 2.2 days.
The 70 percent efficiency and the 3.7 V and 3.85 V figures are common rules of thumb, not published standards; check the voltage printed on your power bank and phone and adjust. Cold weather, fast charging and using the phone while it charges all lower the result. Turn on power-saving settings and limit cellphone use when you cannot charge, as Public Safety Canada advises.
Frequently asked questions
How many times can a 10,000 mAh power bank charge a phone?
With the default 70 percent efficiency, 3.7 V and a 4,000 mAh phone at 3.85 V, a 10,000 mAh bank gives about 1.7 full charges. Change the phone size and efficiency to match your devices.
Why is the number lower than capacity divided by phone mAh?
Because the power bank is rated at its cell voltage and loses energy as heat when it charges. The tool converts both batteries to watt-hours and applies an efficiency.
What efficiency should I use?
Around 70 percent is a common rule of thumb for planning, not a published standard. Use a lower number for cold weather or fast charging.
What should I do to make a phone last during an outage?
Public Safety Canada says to limit cellphone use if you cannot charge, turn on power-saving settings, and keep a battery pack that can charge critical devices more than once.
Sources
Checked
- Public Safety Canada, Get Prepared, "How to prepare before a power outage" (page dated 12 March 2026), opened 8 October 2026. a battery pack that can charge critical devices multiple times, limit cellphone use, power saving settings
- Public Safety Canada, Get Prepared, "Emergency kits", opened 8 October 2026. chargers for mobile devices in the emergency kit
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Scope
Efficiency, cell voltage and phone voltage are assumptions you can edit, not published standards. Never rely on one device for medical equipment; plan that separately with your medical provider.