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Enter bullet data and set the twist rate above.
Every ammo box shows the bullet weight. Few show the bullet length, but length has the larger effect on stability. The PreciseLoad bullet database lists measured dimensions for popular bullet brands like Hornady, Berger, Nosler and thousands of others. An account is necessary, but it is free. Find your bullet there, then enter the exact length above.
Enter the bullet weight, diameter, and length. Then adjust the twist slider until the stability factor (Sg) is between 1.5 and 2.5. Long, heavy bullets need a faster twist (a smaller number, for example 1:7). Common examples: 1:7–1:8 for 69–77 gr .223, 1:8 for 140 gr 6.5 Creedmoor, 1:10 for 175 gr .308, and 1:7 or faster for heavy, long aspect subsonic 300 Blackout.
Sg compares the moment that keeps the bullet stable with the moment that turns it over. Below 1.0, the bullet tumbles. From 1.0 to 1.5, stability is low and the ballistic coefficient decreases. From 1.5 to 2.5, stability is good. Above 2.5, the spin is more than necessary.
The Miller Twist rule provides accurate results for supersonic projectiles, usually to a few percent. Solid-copper, flat-base, and unusual shaped subsonic projectiles can show larger errors. This calculator also applies Miller's corrections for temperature, pressure, altitude, and muzzle velocity.
The standard Miller formula is for supersonic flight. For long bullets below Mach 1, for example 200+ gr 300 Blackout loads, it shows too much stability. This calculator applies a subsonic correction automatically. This is why long aspect subsonic bullets frequently need a twist of 1:7 or faster.
The ballistic coefficient has no effect on gyroscopic stability, so the Miller formula does not use it. Calculators that ask for a BC use it for one extra output: the effective BC after the drag from coning at low stability. That penalty depends only on Sg. Below 1.5, a bullet loses about 3% of its BC for each 0.1 of Sg (Bryan Litz's rule of thumb). At 1.5 or above, the bullet gets its full BC. This calculator gives the same Sg with one less input.