What is MTOW?
MTOW stands for Maximum Take-Off Weight — the total mass of the aircraft at the moment it leaves the ground. It includes everything: the airframe, battery, motors, ESCs, wiring, payload and any munition or sensor package. MTOW is the single number every other part of the design is sized from. Get it wrong and the thrust, power and motor figures downstream are all wrong with it.
How this calculator works
The required total thrust is your MTOW multiplied by the thrust-to-weight ratio. Dividing by the number of motors gives the maximum thrust each motor must produce. At a steady hover the total thrust equals the aircraft weight exactly, so each motor only works at MTOW ÷ motors — and the gap between that and its maximum is your control headroom.
Thrust-to-weight ratio
A ratio above 1 is needed just to leave the ground. In practice 2:1 is the floor for a controllable multirotor, 2.5–3:1 gives comfortable margin for heavy-lift work, and agile or loitering platforms run higher for acceleration and speed. A useful side effect: a 2.5:1 build hovers at roughly 40% throttle, which is the efficient, recoverable place you want to be.
Hover power, peak power and flight time
Hover efficiency is measured in grams of thrust per watt (g/W). Large slow props on heavy-lift builds reach 10–15 g/W; small fast props sit nearer 3–5 g/W. Hover power per motor is simply hover thrust divided by that efficiency. Peak power is far higher than peak thrust would suggest, because power rises with roughly the 1.5 power of thrust — a motor pushing 2.5× its hover thrust draws close to 4× the power. Enter a battery energy in watt-hours and the calculator estimates hover endurance, allowing for usable capacity.
Choosing motor KV, voltage and prop
Heavy thrust per motor means big props, which means low KV (slow, torquey) and higher voltage to keep current and heat manageable. Small light builds want the opposite. The suggested band here pairs a prop size, KV range and cell count to your per-motor thrust as a sensible place to start a design conversation — not a final specification.
Exsabre designs and builds sovereign UK brushless motors for military and commercial drone applications — replacing Chinese-built propulsion with UK-designed, UK-wound alternatives. If you're sizing motors for a UAS programme and want a non-China supply route, this tool is a starting point; talk to us about the rest.