Your “Real-World” Capacity
Power Bank Calculator
Discover how many usable milliamp-hours your power bank truly delivers — not just the number printed on the box.
Authority Formula
Per-Device Estimate
Efficiency Aware
Free Tool
This site uses affiliate links, which may earn us commissions from purchases made at no cost to you.
That “20,000 mAh” label on your power bank? It’s not the whole story. Power banks store energy at their internal cell voltage (typically 3.7 V), but your phone charges at 5 V. On top of that, every conversion loses heat — so the real, usable capacity you actually receive is always lower than the marketed figure. Use the Authority Formula below to cut through the marketing numbers and find out exactly what you’re working with.
The Authority Formula
Actual Capacity =
Marked mAh × 3.7 × Efficiency Rating
Device Voltage
Example: A 20,000 mAh bank at 85% efficiency charging a phone at 5 V
→ 20,000 × 3.7 × 0.85 ÷ 5 = 12,580 mAh of usable power.
Real-World Capacity Calculator
85%
—
mAh of Usable Capacity
Real-world output delivered to your device
Full Breakdown
—
Wh Stored (cell)
—
Wh Delivered
—
Efficiency Used
—
mAh Lost to Heat
Efficiency Meter
Estimated Charges for Your Device
| Scenario | Charges (full) | Notes |
|---|
How the Formula Works
-
1Marked mAh → Watt-hours: Multiply the printed mAh by the internal cell voltage (3.7 V) to convert to the energy unit airlines, manufacturers, and physicists all agree on — Wh.
-
2Apply efficiency loss: Every DC-to-DC voltage conversion generates heat. A good bank might be 85–90% efficient; a cheap one can be as low as 75%. Multiplying by the efficiency rating removes those losses.
-
3Convert back to mAh at device voltage: Dividing the remaining Wh by your device’s charging voltage (typically 5 V) gives the real mAh your device actually receives — the number that matters.
-
4Divide by device battery capacity: Finally, divide the real mAh output by your phone’s battery size to see how many full charges you’ll get. Expect real-world results to be roughly 60–75% of the headline figure.
Heads up:
Efficiency ratings vary widely between brands and are rarely published. If you don’t have the spec sheet, use 85% as a reliable middle-ground estimate for reputable brands. For no-name or very cheap banks, drop to 78–80%. For premium GaN or Anker models, you can push to 90–92%.
Check our Fly with Power Guide for more tips.
Frequently Asked Questions
Manufacturers measure cell capacity at 3.7 V, but your phone charges at 5 V. The converter circuit that steps up the voltage is never 100% efficient — it loses energy as heat. So even a perfect bank loses ~26% just from the voltage conversion before any circuit inefficiency is counted.
Use 85% as the default for any mid-range branded bank. For budget banks without a brand name, drop to 78–80%. For high-end models (Anker 737, Baseus GaN series, etc.) you can use 88–92%. If your bank’s spec sheet lists a conversion efficiency figure, use that directly.
Yes — higher device voltage means fewer usable mAh because the formula divides by device voltage. However, the energy (Wh) delivered is the same. Fast charging doesn’t give you more energy; it just pushes that energy faster. Change the “Device Voltage” field to 9 V or 12 V to see the difference in mAh terms.
The box calculates charges using the raw mAh number at 3.7 V without accounting for the voltage step-up or conversion losses. Our calculator applies the real physics. Additionally, real charging stops at ~95% full (battery management systems prevent 100% to protect longevity), reducing each charge slightly further.
Yes. Lithium cells degrade over charge cycles. After ~300 cycles a cell might retain only 80% of its original capacity, and after ~500 cycles it may be down to 70%. For an older power bank, reduce the “Marked mAh” figure by 10–30% to get a more accurate real-world result.
Disclaimer: Links on this site are affiliate links. We may earn a commission from purchases made through these links.