A 10,000 mAh bank gives about 1.5–2.5 phone charges; estimate by bank Wh ÷ phone Wh × ~60–85% efficiency.
Wondering how far a pocket charger will take your phone? You can get a solid estimate with two numbers: the bank’s energy in watt-hours and your phone’s battery energy. Add a realistic efficiency range and you’ll know whether that slim brick covers a day trip or a full weekend.
Power Bank Charge Calculator: How Many Phone Recharges?
Here’s the short path. Convert the bank’s labeled capacity into watt-hours (Wh), do the same for the phone, then apply real-world efficiency. The core math looks like this:
Step-By-Step Formula
Estimated full charges ≈ (Bank Wh × efficiency) ÷ Phone Wh
Most banks use cells with a nominal 3.7 V. So, Bank Wh ≈ (mAh × 3.7) ÷ 1000. For the phone, use the same 3.7 V baseline unless the spec lists Wh directly. Efficiency spans a range because some energy is lost in voltage conversion, heat, and cable resistance. A reasonable planning range is 60–85%.
Worked Examples You Can Copy
- 10,000 mAh bank → about 37 Wh. With a 4,000 mAh phone (~14.8 Wh), you get about (37 × 0.75) ÷ 14.8 ≈ 1.9 full charges.
- 20,000 mAh bank → about 74 Wh. With a 5,000 mAh phone (~18.5 Wh), you get about (74 × 0.75) ÷ 18.5 ≈ 3.0 full charges.
- 5,000 mAh bank → about 18.5 Wh. With a 3,000 mAh phone (~11.1 Wh), you get about (18.5 × 0.7) ÷ 11.1 ≈ 1.2 full charges.
Quick Reference Table For Common Bank Sizes
The table below uses a mid-range 75% efficiency and a 4,000 mAh phone (~14.8 Wh). It’s a broad starting point; your cables, speeds, and habits can nudge the result up or down.
| Bank Size (mAh) | Approx Delivered Energy (Wh @75%) | Estimated Full Charges (4,000 mAh Phone) |
|---|---|---|
| 5,000 | 13.9 | 0.9 |
| 10,000 | 27.8 | 1.9 |
| 15,000 | 41.6 | 2.8 |
| 20,000 | 55.5 | 3.7 |
| 26,800 | 74.2 | 5.0 |
Why The Labeled Capacity Doesn’t Equal What You Get
Two things shrink the real output. First, the cells sit at ~3.7 V but many phones expect higher USB output levels, so the bank’s converter boosts voltage and wastes some energy as heat. Second, your phone’s internal charger also wastes a slice while stepping that power down to the battery. Stack those together and only part of the bank’s stored energy turns into battery percentage on the screen.
What Efficiency Range Should You Expect?
Manufacturers and labs often see delivered energy land around 60–85% of the raw stored number. Slower standard-voltage charging tends to sit near the top end. High-watt fast modes draw more power, raise heat, and usually shave a few points.
Does Fast Charging Change The Math?
Fast protocols move more watts by raising voltage, current, or both. You finish earlier, but conversion and thermal losses eat into the bank’s usable energy. If the goal is trip length, not speed, a modest 10–18 W setting often gets more total phone charges from the same pack.
How To Find Your Phone’s Battery Energy
Phone makers list capacity in mAh more often than Wh. To get energy, multiply the mAh by 3.7 V and divide by 1000. Some spec sheets state Wh directly, which saves the step. Newer models with 4,500–5,500 mAh packs will need more energy per fill than smaller phones from a few years back.
Realistic Scenarios By Phone Size
- Small phones (3,000–3,500 mAh): a 10,000 mAh bank often nets about 2–2.5 fills at 70–80% efficiency.
- Mid phones (4,000–4,500 mAh): the same bank nets about 1.6–2.1 fills.
- Big phones (5,000–5,500 mAh): expect about 1.3–1.8 fills.
Where To Place Trustworthy Numbers
Two sources help you ground the math. First, the aviation safety pages from the FAA explain the watt-hour formula (Wh = Ah × V), which is the same conversion behind every capacity chart. Second, the USB-IF pages outline USB Power Delivery power levels that affect heat and conversion loss. Both give you the right yardsticks for estimating how many recharges you’ll get from a given pack.
How Cable, Ports, And Habits Change The Result
Cable Gauge And Length
Thin, long, or worn cables waste power. A short, decent-gauge cable keeps voltage sag in check, which cuts heat and keeps the charger in its happy zone.
Charging Speed Choices
Many packs let you cap the watts by picking a lower-power port. That slower top-up can squeeze extra percentage out of the pack, trading minutes for miles.
Phone Use While Charging
Screen-on time, maps, and games pull watts as you charge. That live draw steals energy that could have gone into the battery, trimming the total number of fills.
Step-By-Step: Build Your Own Estimate
1) Get The Bank’s Energy
Find the mAh on the label. Multiply by 3.7 and divide by 1000. Example: 10,000 mAh → ~37 Wh.
2) Get The Phone’s Energy
Check the spec sheet or a trusted database. If you only see mAh, multiply by 3.7 and divide by 1000. Example: 4,500 mAh → ~16.7 Wh.
3) Pick An Efficiency
Use a band that matches your setup. Slow USB-A or low-watt USB-C: 75–85%. Mixed speeds: 70–80%. Heavy fast charge: 60–70%.
4) Do One Division
Estimated full charges ≈ (Bank Wh × chosen efficiency) ÷ Phone Wh.
5) Sanity-Check With A Real Run
Charge from 20% to full twice and note how much the bank drops. That quick field test pins your personal efficiency without lab gear.
Efficiency And Losses At A Glance
The ranges below reflect common setups so you can sense where your result lands and what tweaks help.
| Loss Source | Typical Range | What Helps |
|---|---|---|
| DC-DC Conversion | 5–20% | Lower watt modes, quality pack |
| Cable Resistance | 1–8% | Shorter, thicker cable |
| Phone Charging Overhead | 5–15% | Cooler temps, steady speed |
| Heat From Fast Modes | 3–10% | Use moderate charge power |
Travel Tips For Picking The Right Capacity
Match the bank to the trip, not just the spec sheet. A 5,000 mAh unit handles a night out. Ten thousand covers a long day. Twenty thousand and up suits long weekends or two phones. For flights, many packs show the Wh on the case, which maps to aviation rules and helps you sail through screening with confidence.
Frequently Missed Factors
Temperature Swings
Cold days lower output and slow the phone’s intake. Keep both in a bag or pocket to reduce the hit.
Mixed Ports And Standards
A USB-C PD port can output many voltage steps. If your phone only accepts certain steps, the charger falls back, which can shift efficiency and total fills.
Standby Drain
LEDs, screens, and idle circuits sip energy while the pack sits in a bag. Top it off before you head out for a long day.
Trip Scenarios And Picks
Use the math to steer buying choices, then adjust for the way you spend a day.
Commute And Office Days
If maps and music run for hours, a 10,000 mAh pack keeps you covered with room for a co-worker’s phone in a pinch. Two ports help when a headset or watch needs a sip.
Road Trips And Air Travel
Pick 20,000 mAh or more when screens stay on for navigation and streaming. That size carries two large phones or a phone plus earbuds and a camera. Many packs show the Wh rating on the shell, which makes airline checks straightforward.
Festivals And Camping
Plan for daylight photos, short clips, and poor signal that hammers the radio. A 26,800 mAh pack paired with a stout cable keeps your phone and a friend’s device alive across a long weekend.
Small Tweaks That Stretch Capacity
- Top up earlier. Charging from 20–80% runs cooler and wastes less energy than dead-to-full sprints.
- Kill zombie apps. Navigation, camera, or a stuck game can hide in the background and draw watts while you think you’re idle.
- Use airplane mode in dead zones. When signal is poor, the modem hunts and burns energy. Toggle it during long stretches without coverage.
- Keep the pack cool. Heat raises resistance. A shaded pocket beats a hot dashboard every time.
Math Notes And Assumptions
Phone and bank cells use a nominal 3.7 V for capacity math, even when the charger negotiates higher USB-C levels on the cable. The negotiation just shifts where the voltage conversion happens. The energy number in Wh still rules the estimate. When a spec lists Wh directly, you can skip any conversion and plug that number into the formula above.
Some makers publish efficiency claims under ideal benches. Your day rarely mirrors that. Expect a wider band on a hot day, with fast charge engaged, screen on, and a long cable. Cool temps, low screen time, and a short cable move you closer to the top end of the range. That’s why a quick at-home test is so handy.
Wrap-Up: What To Do Next
Grab your bank’s label, convert to Wh, pick an efficiency, and divide by your phone’s Wh. That’s your real answer. With a better cable and a moderate charge speed, you’ll squeeze the most time away from a wall socket.