Tools
Thrust to weight
Total static thrust divided by all-up weight. The arithmetic is simple; the number you feed it is the part worth thinking about.
Thrust to weight is the single number that decides whether a build feels sluggish, responsive or unmanageable. Below about 2:1 a quad struggles to accelerate out of a dive. Well above 4:1 it has authority to spare, but hover sits so low on the stick that fine control gets harder rather than easier.
Where the thrust figure comes from, and why it flatters
Motor manufacturers publish thrust measured on a static test stand: the motor is bolted down, given a fixed voltage from a bench supply, spun up with one specific prop, and the load cell reads the peak. Nothing about that resembles your build.
Three things pull the real figure down. Your battery sags under load, so the motor never sees the bench voltage at full throttle. Your prop is probably not the one on the chart, and thrust changes sharply with pitch. And a moving aircraft has air already flowing through the disc, which reduces the thrust a prop produces compared with the same prop held still.
None of that makes the number useless. It makes it a comparative number. Two builds calculated the same way can be ranked against each other honestly. A single build's calculated ratio should not be read as a measurement.
- Formula(motors x thrust) / all-up weight
- Thrust unitsgrams, at full throttle
- Weight unitsgrams, including battery
What all-up weight has to include
Everything that leaves the ground. Frame, stack, motors, props, battery, camera, video transmitter, antennas, receiver, printed mounts, the strap, and an action camera if you fly one. The battery is usually the largest single item and the one people forget when they quote a build's weight, which is why published weights and flown weights differ so much.
This is also why the battery is a thrust-to-weight decision rather than only a flight-time one. A larger pack buys endurance and spends ratio. See batteries for the trade, and flight time for the other half of it.
Hover throttle
The tool reports hover throttle as the reciprocal of the ratio, which assumes thrust rises linearly with stick position. It does not: thrust goes roughly with the square of motor RPM, so real hover sits higher than the arithmetic suggests. Read it as a floor, not a prediction.
Choosing motors from a target ratio
Working backwards is often more useful than working forwards. Decide the ratio you want, multiply by your estimated all-up weight, divide by the motor count, and you have the thrust each motor needs to produce with the prop you intend to fly. That figure, not a KV number, is what to look for on a manufacturer's chart. How stator size and KV get you there is on the motors and ESCs page.
The frame constrains this before the motors do, because it fixes the prop size and therefore the thrust available per motor. That is the start of the chain described on frames, and the reason first build puts the frame decision first.