Twist Rate Calculator (Greenhill Formula)
Calculation Result:
Recommended Twist: 1 in "
How to Calculate Twist Rate
The twist rate of a rifle barrel is defined as the distance the bullet travels down the barrel to complete one full revolution. For example, a "1 in 7" twist means the bullet rotates 360 degrees every 7 inches of travel. Calculating the correct twist rate is essential for bullet stability; if the twist is too slow, the bullet will "tumble" in flight, leading to poor accuracy.
The Greenhill Formula
The most common method for calculating the required twist rate is the Greenhill Formula. Developed in 1879 by Sir Alfred George Greenhill, this formula provides a reliable estimate for lead-core bullets at standard velocities.
Twist = (C * D²) / L
- C = 150 (Standard constant for velocities up to 2,800 fps)
- C = 180 (Constant for velocities exceeding 2,800 fps)
- D = Bullet Diameter in inches
- L = Bullet Length in inches
Why Bullet Length Matters More Than Weight
A common misconception is that bullet weight determines the required twist rate. In reality, it is the length of the bullet that dictates stability requirements. Because longer bullets (which are usually heavier) have more surface area for air to push against, they require a faster spin (a lower twist rate number) to remain gyroscopically stable.
Example Calculation
If you are shooting a .224 caliber bullet that is 0.8 inches long at a velocity of 2,900 fps:
- Since velocity is over 2,800 fps, we use 180 as our constant.
- Square the diameter: 0.224 * 0.224 = 0.050176
- Multiply by the constant: 180 * 0.050176 = 9.03168
- Divide by bullet length: 9.03168 / 0.8 = 11.28
The result suggests a 1:11″ twist rate or faster (such as 1:9 or 1:7) would be ideal to stabilize that specific bullet.