Atmosphere and air density

These are two different numbers and mixing them is the most common atmosphere mistake. Station pressure is the absolute pressure where you are standing — that is what a Kestrel or a barometer in absolute mode shows, and at 8,000 ft it might read 22 inHg. A sea-level or altimeter setting is that same air corrected back down to sea level so pilots and forecasters can compare readings; it will sit near 29.9 inHg no matter what your altitude is.

In this calculator, pick the reading you actually have. A station reading is used exactly as entered and your altitude is ignored for pressure purposes. A sea-level reading is reduced to your altitude first. If you enter a station reading but label it sea level, the altitude correction is applied a second time and the solver thinks the air is far thinner than it is.

• Altitude — only used to reduce a sea-level pressure reading. Higher means thinner air, less drag, less drop.
• Temperature — hot air is thinner. It also warms your powder, which raises muzzle velocity; the calculator does not model that, so re-chronograph in extreme heat or cold.
• Pressure — the dominant term. A 1 inHg error is worth roughly 3% in density.
• Humidity — water vapour is lighter than dry air, so humid air is very slightly thinner. The effect is real but small; it rarely moves a mid-range solution by a click.

Density is computed from your station pressure, temperature and vapour pressure, then compared with the ICAO standard sea-level value. The ratio scales the effective ballistic coefficient: thin air behaves like a higher BC. This is the standard density-correction approach, and it is accurate as long as your inputs are.

Leave the atmosphere box unticked and the solve runs against the standard atmosphere — a reasonable default for a first look, and the condition most published BCs were measured in. Set it on the trajectory workspace when you know your real conditions.