Every drone startup has answered the question "how long does it fly?" with a number that later needed a footnote. This guide gives you the arithmetic to produce numbers that survive contact with a customer site — and to smell other people's footnotes from across the room.
The one formula
Endurance (min) = E_usable / P_avg × 60
E_usable = capacity (Ah) × voltage (V) × usable fraction (~0.8)
P_avg = hover/cruise power + avionics + payload powerEverything interesting hides inside P_avg.
Hover power from first principles
Momentum theory gives ideal hover power: P = T^1.5 / √(2ρA) — thrust to the 1.5, over air density and total disk area. Real rotors add losses, captured by a figure of merit (FM ≈ 0.55–0.7 for small props, higher for large well-made ones), and the drivetrain adds motor+ESC efficiency (~80–90% combined at a good operating point, per the motor guide).
Three design truths fall straight out of the exponent and the √A:
- Mass is punished at the power 1.5. +10% mass → ~+15% hover power. Guard your mass budget.
- Disk area is rewarded at the square root. Double area → ~30% less hover power. Hence big slow props.
- Air density counts. Hot-and-high operations (ρ down ~10% at 1000 m on a warm day) cost ~5% power. Quote endurance at conditions.
Worked example: 12 kg industrial quad
- MTOW 12 kg, four 22-inch props → disk area 0.98 m², sea level ρ = 1.225 kg/m³.
- Ideal hover power: (12 × 9.81)^1.5 / √(2 × 1.225 × 0.98) ≈ 1277 / 1.549 ≈ 824 W.
- With FM 0.65 and drivetrain 85%: 824 / (0.65 × 0.85) ≈ 1492 W electrical — that's ~8 g/W, matching good thrust-table data for this class.
- Add avionics + compute + payload: ~60 W → P_avg ≈ 1550 W.
- Battery: 12S (44.4 V nominal) × 22 Ah = 977 Wh; usable 80% → 781 Wh.
- Endurance ≈ 781 / 1550 × 60 ≈ 30 minutes. Mostly-hover mission, rated payload, sane reserve — a number you can put in a datasheet and still like next quarter.
The battery mass spiral and optimal fraction
Add battery and MTOW rises, so hover power rises faster than linearly. Writing battery mass fraction as f_b, endurance is proportional to f_b / (total mass)^0.5 at fixed disk area — a curve that peaks. For electric multirotors the optimum lands around f_b ≈ 35–45%; beyond it, watt-hours arrive pre-spent on carrying themselves. If your endurance target requires f_b > 45%, the honest answers are: bigger props, less payload, a different platform, or a different promise to the customer.
Fixed-wing and VTOL notes
Fixed-wing cruise power = weight × speed / (L/D × prop efficiency × drivetrain). With L/D ≈ 12 and η ≈ 0.6, a 12 kg aircraft at 18 m/s needs roughly 12×9.81×18/(12×0.6×0.85) ≈ 350 W — a quarter of the quad's hover power, which is the whole fixed-wing argument in one line. Hybrid VTOLs pay hover power only during take-off, transition and landing (budget 2–4 minutes at ~4× cruise power), then fly on the wing; their killer detail is the lift-system dead mass riding along all cruise.
De-fluffing a datasheet (yours or theirs)
| Claim smells like | Ask |
|---|---|
| "55 min flight time" | At what payload? (Usually zero.) |
| "up to" | New pack, no wind, no reserve, 20 °C. Knock off 25–35%. |
| "with optional HD battery" | At what MTOW, and is it still under the 25 kg cliff? |
| Winter endurance unstated | Cold packs sag; unheated LiPo below 5 °C can cost 20%+ (see battery guide). |
Quote endurance as payload @ minutes with reserve, e.g. "2.5 kg @ 32 min incl. 20% reserve, ISA sea level". Customers burned by "up to" marketing treat honest specs as a competitive feature — and your operations team gets to plan missions that finish with charge in the pack.
Frequently asked questions
How do I calculate drone flight time?
Flight time (min) = usable battery energy (Wh) ÷ average power draw (W) × 60. Usable energy is capacity × voltage × ~0.8; average power comes from hover efficiency (g/W) at your take-off mass, plus avionics and payload power.
Why doesn't adding a bigger battery keep increasing flight time?
Because battery mass increases total weight, and hover power grows with weight to the power 1.5. Each added watt-hour also costs power to carry. Past an optimal battery fraction — typically 35–45% of take-off mass for multirotors — endurance decreases.
What is a realistic flight time for an electric multirotor?
With current battery technology: 20–35 minutes for compact camera drones, 35–55 minutes for efficiency-optimised industrial multirotors with large propellers, at rated payload. Claims well beyond that at payload usually assume no payload, no reserve, or no wind.