A 20,000 mAh power bank usually delivers about 2–4 full phone charges, depending on efficiency and your device’s battery size.
Here’s the short path to a solid answer. A 20,000 mAh pack stores energy in 3.6–3.7 V cells; during DC-DC conversion to 5 V or USB-PD voltages, some energy is lost as heat. Usable capacity often lands near 60–70% of the printed figure. With that, many recent phones in the 3,300–5,000 mAh range see roughly two to four full charges.
How Many Charges From A 20,000 mAh Bank: Real-World Math
Use this quick formula:
Estimated Full Charges = (Rated Capacity × Efficiency) ÷ Device Battery (mAh)
If we assume 65% efficiency, a 20,000 mAh unit yields ~13,000 mAh of usable charge. Divide by your phone’s battery number to get a realistic count.
Quick Reference Table (65% Efficiency Assumed)
The table below shows estimated full charges for popular phone sizes. It’s a guide, not a promise—your cable, port setting, and charge rate all move the needle.
| Phone Battery (mAh) | Example Devices | Estimated Full Charges* |
|---|---|---|
| ~3,300–3,400 | iPhone 15 (~3,349 mAh); iPhone 15 Pro (~3,274 mAh) | ~3.8–4.0 |
| ~4,000 | Galaxy S24 (4,000 mAh) | ~3.2 |
| ~4,400–4,500 | iPhone 15 Pro Max (~4,422 mAh) | ~2.9 |
| ~4,900–5,000 | Galaxy S24+ (4,900 mAh); S24 Ultra (5,000 mAh) | ~2.6–2.7 |
| ~10,000 | Small tablet/handhelds (varies) | ~1.3 |
*Assumes 20,000 × 0.65 = ~13,000 mAh usable; round to the nearest tenth for readability.
Why Printed mAh Isn’t What You Actually Get
Power banks list mAh at cell voltage (~3.6–3.7 V). Your phone charges closer to 5 V or higher PD steps. The boost converter raises voltage and sheds heat, so you never see 100% transfer. That’s why the “real” output feels smaller than the label.
Efficiency Range You Can Expect
Most well-built packs land near 60–70% usable capacity in day-to-day use. Quality cells, a good converter, and reasonable charge speeds push you toward the top of that range. Pushing wattage hard for speed bumps losses upward.
How To Calculate Your Own Charge Count
Grab your device’s battery number and run the math with either 60% (conservative) or 70% (optimistic) efficiency. Here are two quick scenarios for a 20,000 mAh pack:
Conservative Pass (60%)
- Usable: 20,000 × 0.60 = 12,000 mAh
- 3,350 mAh phone → ~3.6 charges
- 4,000 mAh phone → ~3.0 charges
- 5,000 mAh phone → ~2.4 charges
Optimistic Pass (70%)
- Usable: 20,000 × 0.70 = 14,000 mAh
- 3,350 mAh phone → ~4.2 charges
- 4,000 mAh phone → ~3.5 charges
- 5,000 mAh phone → ~2.8 charges
Real Device Numbers To Anchor Your Math
Recent iPhone models reported via regulatory filings sit around the mid-3,000s for the base/Pro models and low-4,000s for the Pro Max. Samsung’s S24 series ranges from 4,000 mAh up to 5,000 mAh on Ultra. Two handy references:
- iPhone 15 capacities from Chinese filings (mAh + Wh figures).
- Samsung’s S24 battery page with official mAh values.
Watt-Hours Beat mAh For Apples-To-Apples Comparisons
mAh is tied to voltage. Watt-hours (Wh) compare energy directly: Wh = (mAh × V) ÷ 1000. Many power banks list Wh on the label for airline rules. If yours shows 74 Wh, that’s roughly “20,000 mAh at 3.7 V” in another outfit.
Why Wh Matters
Two packs can both say 20,000 mAh but use slightly different cell voltages. Wh normalizes that difference. It also helps when you’re sizing gear for laptops and tablets where 9 V, 12 V, 15 V, or 20 V PD steps come into play.
Fast Charging And Why It Changes The Outcome
USB Power Delivery can raise voltage and push higher wattage, which shortens charge time but trims efficiency. Heat grows, conversion losses rise, and the phone may throttle near the top of the charge curve. If your goal is the most full charges per pack, moderate power levels usually win.
Curious about PD profiles and voltage steps? The standards body’s page on USB Power Delivery outlines power levels up to 240 W.
Factors That Move Your Charge Count Up Or Down
Small details add up. Here are the levers you can pull to get more complete refills from a 20,000 mAh unit.
| Factor | Effect On Charges | What To Do |
|---|---|---|
| Cable Quality & Length | Thin or long cables waste energy as heat. | Use short, well-made USB-C cables rated for your wattage. |
| Charge Speed | Higher wattage raises conversion loss. | Pick a mid-range PD step unless you truly need the speed. |
| Phone State | Screen-on and background tasks sip power during charging. | Switch to airplane mode or lock the screen while topping up. |
| Ambient Temperature | Hot or cold conditions reduce efficiency. | Charge at room temperature and keep gear shaded. |
| End-Of-Charge Taper | Phones slow near 80–100%, stretching time and loss. | Top to ~80–90% for better energy return when on the go. |
| Pass-Through Use | Charging the bank and phone together adds overhead. | Fill the bank first, then charge devices. |
Worked Examples You Can Trust
Compact iPhone Case
Assume ~3,350 mAh. With 65% efficiency, a 20,000 mAh pack offers ~13,000 mAh. That’s about 13,000 ÷ 3,350 ≈ 3.9. In plain terms: three to four full refills, with some extra for trickle usage during the day.
Large Android Case
At ~5,000 mAh, the count is 13,000 ÷ 5,000 ≈ 2.6. Expect two full refills and a healthy partial.
Tablet Mini Case
Smaller tablets often land near 7,000–10,000 mAh. Using our 13,000 mAh usable figure: 13,000 ÷ 7,500 ≈ 1.7 and 13,000 ÷ 10,000 ≈ 1.3. That’s one full plus change, or about one and a half if the battery is on the smaller side.
Why Your Count Drops When You Chase Maximum Speed
Boosting from 5 V to 9–20 V gives you fast refills, but conversion steps aren’t free. The pack’s converter and the phone’s charge circuit both work harder. Heat grows, and little bits of loss creep in through the whole cycle. If you care about how many full charges you can squeeze from a pack, base your daily routine around moderate PD levels and reserve the highest tiers for tight schedules.
How To Pick The Right Pack For Your Mix Of Devices
Match Output To Your Heaviest Device
If you carry a laptop or a tablet that needs 20 V PD, make sure your bank advertises that step and the wattage your device expects. A phone-only kit can stick to 20–30 W outputs and stay light.
Check The Wh Label
Airlines use Wh for rules and it’s the cleanest way to compare across brands. A number around 72–74 Wh matches the typical 20,000 mAh class at 3.6–3.7 V. Packs with similar Wh will behave similarly, even if the mAh looks a little different.
Look For Smart Details
- Two USB-C ports if you charge phone and earbuds together.
- Clear percentage display beats vague four-LED bars for planning.
- Low-current mode for watches and buds reduces hiccups.
Care Tips That Keep Capacity Closer To The Label
- Store near half charge if you won’t use the pack for a while.
- Avoid baking it in a car or freezing it outdoors.
- Don’t run to 0% every time; shallow cycles are easier on cells.
- Use certified cables and skip no-name adapters.
Putting It All Together
A 20,000 mAh bank is a sweet middle ground for most phone users. With real-world losses, plan on two to four complete refills, shaped by your device’s battery size, your charge speed, and the little habits that add up during a day. If you want faster top-ups, you trade a bit of efficiency; if you slow things down, you gain a little extra range. Once you know your phone’s mAh, the formula at the top gives you a dependable answer every time.