Divide the bank’s usable watt-hours by your phone’s watt-hours; a 10,000 mAh bank often gives around 1.8–2.5 full charges.
The goal here is simple: figure out how many full top-ups you’ll get before your pack runs dry. The fastest way is to use watt-hours (Wh), not just milliamp-hours (mAh). Below you’ll find the quick formula, a broad reference table, and clear steps to adjust for your own phone, cable, and habits. No fluff—just the math that matches what you see in the real world.
Quick Formula And What “Usable” Means
Most packs use cells with a nominal voltage around 3.6–3.7 V. Labels show capacity in mAh at that cell voltage, while your phone charges near 5 V (or higher with USB-C PD). Energy is what matters, so convert to watt-hours:
Wh = (mAh × V) ÷ 1000
Next, account for conversion losses inside the pack and cable. A practical budget is 75–85% efficiency. Some brands quote similar figures and explain why the rated mAh never fully reaches your phone.
Broad Estimates: Phone Sizes Vs Common Power Banks
This table gives ballpark full-charge counts for popular bank sizes at two realistic efficiency points. If your phone’s battery is new and you use a good cable, you’ll land near the right column. Older batteries or warm conditions push you toward the left.
| Phone Battery (mAh) | 10,000 mAh Bank (~75% eff.) |
10,000 mAh Bank (~85% eff.) |
|---|---|---|
| 3,000 | ~2.2 charges | ~2.5 charges |
| 3,500 | ~1.9 charges | ~2.2 charges |
| 4,000 | ~1.7 charges | ~2.0 charges |
| 4,500 | ~1.5 charges | ~1.8 charges |
| 5,000 | ~1.4 charges | ~1.6 charges |
| 5,500 | ~1.2 charges | ~1.5 charges |
Where do these numbers come from? A 10,000 mAh pack stores about 10,000 mAh × 3.7 V ≈ 37 Wh inside. Delivering to 5 V and dealing with heat and electronics trims that. With 75–85% efficiency, usable energy is roughly 27.8–31.5 Wh. A phone with a 4,000 mAh battery at ~3.85 V holds about 15.4 Wh. Divide usable by phone Wh to get the counts shown.
How Many Power-Bank Recharges You’ll Get — Step-By-Step
1) Find The Bank’s Stored Energy
Use the pack’s printed mAh and the cell voltage (3.6–3.7 V is typical). Compute: Bank Wh = (mAh × 3.7) ÷ 1000. If the bank already lists Wh, use that.
2) Estimate The Usable Portion
Multiply by a real-world efficiency. A safe middle ground is 0.8 (80%). Fast outputs (PD at higher wattage) and older cells tend to shave a little more.
3) Turn Your Phone’s Capacity Into Wh
Phones list mAh; their nominal voltage is near 3.85 V. Compute: Phone Wh = (mAh × 3.85) ÷ 1000.
4) Divide To Get Full-Charge Counts
Full charges = Usable Bank Wh ÷ Phone Wh. The result is an average across the last 10–20% tail where charging slows down. In practice you’ll see similar totals after a few complete cycles.
Worked Examples You Can Copy
10,000 mAh Bank → Modern Midrange Phone (4,500 mAh)
Bank Wh: 10,000 × 3.7 ÷ 1000 = 37 Wh. Usable (80%): 29.6 Wh. Phone Wh: 4,500 × 3.85 ÷ 1000 ≈ 17.3 Wh. Full charges: 29.6 ÷ 17.3 ≈ 1.7.
20,000 mAh Bank → Big Battery Phone (5,000 mAh)
Bank Wh: 20,000 × 3.7 ÷ 1000 = 74 Wh. Usable (80%): 59.2 Wh. Phone Wh: 5,000 × 3.85 ÷ 1000 ≈ 19.3 Wh. Full charges: 59.2 ÷ 19.3 ≈ 3.1.
Pocket Bank (5,000 mAh) → Small Phone (3,000 mAh)
Bank Wh: 5,000 × 3.7 ÷ 1000 = 18.5 Wh. Usable (80%): 14.8 Wh. Phone Wh: 3,000 × 3.85 ÷ 1000 ≈ 11.6 Wh. Full charges: 14.8 ÷ 11.6 ≈ 1.3.
Why Label mAh Doesn’t Equal Output mAh
Labels are printed at the cell voltage inside the case. When the bank steps up to 5 V (or 9/12/20 V under PD), the math changes. The energy stays the same, but the current and voltage swap around, and some energy turns into heat. A well-built pack still loses a slice. Many well-known brands explain this exact point and cite typical efficiency ranges. See the linked brand note mid-way down this page for the plain-English breakdown.
Where Real-World Losses Creep In
Cable Quality And Length
Thin or long cables run warm and waste power. Use a short, certified cable that matches your charging standard.
Charging Speed And Heat
High-wattage PD can raise temperatures. Warm cells waste more energy. If you only need a top-up on the go, a mid-level profile (say, 9 V) can be a good balance.
Battery Age In The Phone
As cycle count climbs, a phone holds less energy. That sounds bad, but it means each “full charge” from a pack can inch upward since the phone’s tank shrank. The flip side is shorter phone runtime per charge.
Background Drain
Screen-on use during charging means the pack is both filling and fueling. Counts drop when you watch video or tether while charging.
Trust The Math: Two Solid References
The mAh↔Wh conversion and the idea of treating energy—not just current—are standard practice in battery work. For a plain formula refresher, see Battery University on Wh math. For a brand’s take on why output is lower than the label, see Anker’s explanation of conversion loss.
Common Bank Sizes And What To Expect
This quick grid uses the same 75–85% efficiency range as the first table, but adds a larger bank for road trips and multi-day events. Pick the row close to your phone.
| Phone Battery (mAh) | 20,000 mAh Bank | 30,000 mAh Bank |
|---|---|---|
| 3,000 | ~3.5–4.2 charges | ~5.2–6.3 charges |
| 3,500 | ~3.0–3.6 charges | ~4.5–5.4 charges |
| 4,000 | ~2.7–3.3 charges | ~4.0–4.9 charges |
| 4,500 | ~2.4–3.0 charges | ~3.6–4.4 charges |
| 5,000 | ~2.2–2.7 charges | ~3.2–3.9 charges |
| 5,500 | ~2.0–2.5 charges | ~2.9–3.6 charges |
Shortcuts If You Don’t Want To Calculate
Use The “Half-Minus-A-Bit” Rule
Double your phone’s mAh. That’s about the bank mAh you need for two full charges with headroom. A 5,000 mAh phone? Look at a 12,000–15,000 mAh pack.
Check The Wh Number On The Label
Some banks print Wh right on the shell. Compare that straight to your phone’s Wh (mAh × 3.85 ÷ 1000). If the bank has at least 2× your phone’s Wh, expect two solid full charges.
Mind Airline Limits
For flights, packs up to 100 Wh usually go in carry-on under common rules. Many 20,000 mAh banks sit near 74 Wh, which stays under the line. Bigger bricks may need approval.
Picking A Pack That Matches Your Day
Commuter And Casual Use
A slim 5,000–10,000 mAh pack is perfect for one to two phone fills. Add a short cable to keep losses down.
Heavy Screen Time Or Hotspots
Reach for 15,000–20,000 mAh. That covers long streaming sessions, maps, and tethering without hitting zero before you’re home.
Weekend Away Or Backpacking
Think in watt-hours and ports, not just mAh. A 74 Wh bank with two outputs covers a phone, earbuds, and a watch. Pair with a cable that supports your fastest safe profile.
Cable, Port, And Phone Settings That Affect Counts
Use A Quality USB-C Cable
Certified cables keep resistance low. That means less heat and more of the bank’s energy reaching the phone.
Turn Down Screen Brightness While Charging
Background drain steals from your total. Lower brightness or lock the screen during the bulk of the charge.
Avoid Leaving The Pack In A Hot Car
Heat raises internal resistance and slashes efficiency. Store the pack in shade and cool spots when you can.
What If Your Results Don’t Match The Table?
If your counts are lower, the usual suspects are a long cable, heat, or a pack with aging cells. Try a shorter cable first. Charge when the phone is near 20–40% rather than 1–5% if you can—charging is gentler and losses are a bit lower. If the pack is a few years old, expect a drop in storage as well.
FAQ-Style Nuggets Without The Clutter
Does Fast Charging Reduce Total Counts?
It can trim them slightly because higher power often runs warmer. The hit is small on modern hardware. If you’re chasing every milliamp, use a mid-level profile when convenient.
Do Two Phones At Once Hurt Efficiency?
Yes, splitting current means the converter works harder. Expect a small drop in total energy delivered. If you care about every extra 10%, charge one device at a time.
Should I Care More About Wh Than mAh?
Yes. Wh is the straight measure of energy, so it compares across voltages cleanly. mAh only makes sense when you also know the voltage.
Final Takeaway You Can Use Today
Grab your pack’s mAh, convert to Wh, shave 15–25% for losses, convert your phone’s mAh to Wh, then divide. That one line gives you a repeatable answer across banks, phones, and cables. Keep a short cable, avoid heat, and your counts will land near the top of the ranges you saw above.