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How to estimate drone flight time and battery size

By Keayvan Keramati · ~7 min read

"How long will it fly?" is the question every drone project comes back to. You can get a reliable first estimate from the same physics chain every time: thrust → power → energy → capacity → weight. Here's how each step works, and two traps that catch people out.

Step 1 — Hover power

To hover, the rotors must throw air downward. Momentum (actuator-disk) theory gives the least power needed, for N rotors each of disc area A = πD²/4, carrying weight W, with an overall efficiency η (rotor figure-of-merit × motor × ESC, usually ~0.5–0.6):

Two things fall straight out of this: power grows as weight^1.5 (heavier hurts disproportionately), and it shrinks with disc area, so a bigger propeller hovers far more efficiently. That's the single biggest efficiency lever on a multirotor.

Step 2 — Energy and flight time

A battery stores energy E = capacity × voltage. You can only safely use a fraction (the depth of discharge, DoD ≈ 80%). Flight time is simply usable energy divided by power:

For forward flight, add range = t × cruise speed, and use cruise power P = T·V/η instead of the hover expression.

Step 3 — The battery-mass optimum

Here's the counter-intuitive part. Adding battery adds energy — but it also adds weight, which raises hover power (remember weight^1.5). So endurance doesn't rise forever; it peaks. For a multirotor in hover, the optimum is a battery mass of about twice the dry (airframe + payload) mass:

Beyond that point, extra cells cost more in power than they add in energy. Knowing this stops you from endlessly "adding one more pack".

Step 4 — Size the battery (and don't forget current)

Working backwards from a target flight time t (seconds), the pack must satisfy both an energy requirement and a current requirement:

Long, gentle flights are energy-limited; short, high-power ones are current-limited — the pack has to deliver the amps within its C-rating or it sags and overheats. Sizing only for energy and ignoring current is a classic mistake.

Worked example

4 motors × 12 N, rotor 0.30 m, 6S (22.2 V), η = 0.55, target 6 minutes (360 s), DoD 80%, 180 Wh/kg cells:

• Hover power ≈ 736 W → current ≈ 33 A
• Required capacity ≈ 4150 mAh (energy-limited)
• Battery weight ≈ 0.51 kg

This is a preliminary momentum-theory model with a lumped efficiency. For final numbers, use measured motor/prop bench data and real cell discharge curves.