Subsonic margin
A suppressor silences the muzzle blast. It cannot do anything about a bullet that breaks the sound barrier — that crack is made downrange, well past the can. This works out whether your load actually stays subsonic at the temperature you are shooting in, and the temperature at which it stops.
Your load
Muzzle velocity comes from the box or, far better, your own chronograph — box figures are measured in the manufacturer's test barrel, which is rarely the length of yours. Temperature sensitivity varies by powder; around 1 fps per °F is a common working figure, but chronograph on a cold day and a hot one if it matters to you.
Margin
Why altitude isn't an input
The speed of sound in air depends on absolute temperature and nothing else that matters here: c = 1116.45 × √((T + 459.67) / 518.67) fps. Pressure and density cancel out of the equation. Altitude only changes the speed of sound to the extent that it changes the temperature, which you are already entering directly.
Plenty of calculators ask for altitude here. They shouldn't.
The bit almost everyone gets backwards
“Subsonic ammo goes supersonic on a hot day” is only half true, and the missing half decides the answer. The speed of sound rises with temperature too — at c / (2 × T), which works out to about 1.08 fps per °F around room temperature.
So the question is not whether your load speeds up in the heat. It is whether your load speeds up faster than the air does. A temperature-insensitive powder gaining 0.4 fps per °F is losing that race, and its subsonic margin gets wider as the day warms. A sensitive powder gaining 2 fps per °F is winning it, and will eventually crack.
The crossover temperature is solved by bisection because both lines move. If the tool says your load never crosses, that is not a bug — it means your powder is less temperature-sensitive than the atmosphere, and your risk is a cold morning with a slow-burning lot, not a hot afternoon.
Not modelled
- Lot-to-lot velocity variation, which on cheap subsonic ammunition can be wider than the margin you are trying to protect.
- Barrel length. A shorter barrel than the test barrel means lower velocity, which works in your favour here and against you everywhere else.
- Suppressor back pressure, which slightly increases velocity on some hosts.
- Humidity. It raises the speed of sound very slightly — in your favour.
If the margin is under about 30 fps, treat the load as unreliable rather than subsonic. Ammunition does not read its own box.