Rocket Static Stability Calculator

The classic rule of thumb is 1 to 2 calibers, where one caliber equals the maximum body tube diameter. The center of pressure should sit behind the center of gravity, and the tool marks this band as stable.

Subtract the CG distance from the nose from the CP distance from the nose to get the CG to CP gap, then divide by the body diameter. All three inputs must be in the same unit, so this tool uses millimetres throughout.

It means the center of pressure is ahead of the center of gravity, so the rocket is unstable and will tumble or loop instead of flying straight. Add nose weight to move the CG forward until the margin turns positive.

Yes. Above about 2 calibers a rocket is overstable and tends to weathercock, turning sharply into the wind and losing altitude. Moving the CG back a little, or reducing fin size, brings the margin down.

Balance the fully loaded rocket, with the motor and recovery gear installed, on a ruler edge or hang it from a loop of string. The point where it balances level is the CG. Measure its distance from the nose tip.

The CP depends on the fin and body shape and is hard to measure by hand. Most builders read it from design software such as OpenRocket or a Barrowman calculation, then enter that distance from the nose here.

Using one common reference, the nose tip, keeps the subtraction simple and sign correct. When both are measured from the nose, a CP with the larger value sits behind the CG, which is the stable case and gives a positive margin.

No. Static margin only covers straight line stability from the shape and balance. Thrust to weight, launch rod length, wind and a clean recovery deployment all matter too, so treat a good margin as necessary but not sufficient.

No. The calculation runs in your browser, so your rocket measurements stay on your device and nothing is sent to a server.