Twist Rate Calculator

Twist rate describes how quickly the rifling in a barrel causes a bullet to spin, expressed as 1 turn in X inches — for example, 1:10 means the bullet completes one full rotation every 10 inches of barrel travel. A faster twist (lower X number) spins the bullet faster, which is needed to stabilize longer, heavier bullets.

The Greenhill formula estimates twist rate as T = (C × D²) / L × √(SG / 10.9), where C is a constant (150 for velocities under 2800 fps, 180 above), D is bullet diameter in inches, L is bullet length in inches, and SG is the bullet's specific gravity. This formula was developed in 1879 and works well for traditional lead-core bullets.

The Miller twist rule is a more modern formula that also accounts for muzzle velocity, bullet weight, and atmospheric conditions. It computes a gyroscopic stability factor (Sg) directly, making it more accurate for high-velocity cartridges and non-lead-core bullets. An Sg of 1.5 or higher is generally considered stable.

The gyroscopic stability factor (Sg) quantifies how stable a bullet will be in flight. An Sg below 1.0 means the bullet is under-stabilized and will tumble. Values between 1.0–1.5 are marginal, 1.5–2.0 is considered stable, and above 2.0 is over-stabilized, which can reduce accuracy. A target of roughly 1.5 is the common recommendation.

Higher altitude and higher temperature both reduce air density. Less dense air requires less gyroscopic spin to stabilize a bullet, so a bullet that is marginally stable at sea level may become adequately stable at higher elevations. The Miller rule incorporates correction factors for both temperature and altitude to give a more accurate Sg at your shooting conditions.

A twist rate that is too slow (under-stabilized, Sg < 1.0) will cause the bullet to tumble in flight, drastically reducing accuracy and range. A twist rate that is too fast (over-stabilized, Sg > 2.0) can cause the bullet to resist following a ballistic arc and may lead to keyholing at longer ranges. For most applications, an Sg between 1.4 and 2.0 is ideal.

Most traditional jacketed lead-core bullets have a specific gravity of approximately 10.9, which is the default. Solid copper or brass bullets have a lower specific gravity of around 8.9. Tungsten-core bullets have a higher specific gravity around 17.5. Using the correct specific gravity is important for an accurate Sg calculation.

Yes — this calculator works for any rifle caliber from small varmint cartridges (.17 cal) up to large bore rifles (.50 cal and beyond). Simply enter the actual bullet diameter, length, weight, and your barrel's twist rate. The Greenhill and Miller formulas are universal and apply across calibers.