How long will my pack last? This calculator answers that for any RC plane, multirotor, or FPV drone by combining battery capacity with your average current draw, while reserving a safe portion of the pack you should never discharge into.
How it works
Run time is simply usable charge divided by current, with units lined up:
usable Ah = (capacity mAh / 1000) × usable factor
time (h) = usable Ah / average current A
time (min) = time (h) × 60The usable factor (default 0.8) keeps a reserve so you never run a LiPo flat, which would damage the cells and risk a dead-stick landing.
Worked examples
FPV freestyle quad, 5-inch build
A 1500 mAh 4S pack flying at an aggressive average of 20 A:
- Usable capacity: (1500 / 1000) × 0.80 = 1.20 Ah
- Flight time: 1.20 / 20 = 0.060 h × 60 = 3.6 minutes
This is a realistic number for hard freestyle flying. Many pilots get 3–4 minutes before the telemetry alarm fires.
Long-range fixed-wing cruiser
A 4000 mAh pack cruising at an efficient average of 6 A:
- Usable capacity: (4000 / 1000) × 0.80 = 3.20 Ah
- Flight time: 3.20 / 6 = 0.533 h × 60 = 32 minutes
With a GPS return-to-home feature and a conservative 75% usable factor for extra margin, that drops to around 30 minutes — still a strong cross-country run.
Choosing the right usable factor
The default of 0.80 (80%) reflects the standard advice to land when the pack reaches roughly 3.5 V per cell resting voltage, preserving 20% of capacity in reserve. In practice the right factor depends on your situation:
| Situation | Suggested factor | Why |
|---|---|---|
| New pack, well-known chemistry | 0.80 | Standard safe reserve |
| Cold weather (below 10°C) | 0.70 | Cold reduces usable capacity measurably |
| Older or cycled pack (100+ cycles) | 0.70 | Capacity fade reduces usable charge |
| Long-range or over-water flight | 0.65–0.70 | Extra buffer for unexpected headwind or detour |
| Indoor park flyer, close supervision | 0.85 | Low risk of stranding; can push slightly harder |
Why bigger is not always better
A heavier battery raises the thrust load, which pushes up current draw. For a multirotor, every gram of battery added must be lifted by the motors. Beyond a certain pack size, the added weight draws enough extra current that the net flight-time gain flattens and eventually reverses. The optimal pack for a given frame is usually found by testing two or three capacities and comparing measured flight time, not by simply fitting the largest pack that physically fits.
Getting your average current right
Throttle position is a poor proxy for current because current scales roughly with the square of throttle (more precisely, with the square of thrust). The most reliable method:
- Fit an inline watt-meter between the battery and ESC (or use a power analyser on the ESC output leads).
- Fly a representative mission — freestyle, cruising, or a mix.
- Read the average or mean current from the logger after landing.
For a first estimate without a watt-meter, use the motor manufacturer’s data sheet: find the current at the throttle percentage you typically fly. Add 15–20% for realistic variation. This gives a useful upper-bound estimate before you have real telemetry.
LiPo health and what changes the result over time
A fresh pack delivers close to its rated capacity. With each charge cycle, the internal resistance rises and usable capacity gradually falls. A pack showing visible puffing, a resting voltage that drops quickly to 3.7 V per cell, or cells that diverge in voltage mid-flight are past their safe working life and should be retired. Continuing to fly an aging pack makes the calculator’s estimate optimistic — the real run time will fall short of the prediction.