🌫 Fog Machine Calculator
🌫 Fog Machine Calculator
Size the haze for a room: how long the machine takes to reach a target visibility, how often to re-burst, how much fluid an evening eats, and when the air clears again.
Haze concentration through one run and the clear-down afterwards. The dashed line is the target that gives your chosen visibility.
Haze plan
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The model, the visibility formula, and what fog does to a room
Well-mixed box. The room is treated as one well-mixed volume. Fluid is emitted at a constant rate, and fresh air replaces the room at the air-change rate you enter. Concentration follows a first-order rise towards a steady value, and decays exponentially once the machine stops. Real rooms have dead corners, warm ceilings and draughts, so treat the numbers as a plan rather than a measurement.
Fluid to mass. Fog fluid is mostly water and glycol at about 1 g/ml, so millilitres per minute become grams per minute. Dividing the emission rate by the room's air-change flow gives the steady concentration in grams per cubic metre. That is why the same machine produces a thin haze in a hall and a pea-souper in a front room.
Visibility. The page uses a Koschmieder-style relation, visibility ≈ 4.6 ÷ (0.5 × concentration + 0.002), with the small constant standing in for clear-air extinction. It is calibrated to look reasonable for theatrical haze, not to match a meteorologist's transmissometer. If the manufacturer quotes a coverage figure, use theirs — it beats any formula.
Working the room. Fog is heavier than air and pools low, so a machine on the floor fills a room from the ankles up; a fan helps mix it, but it also drags in fresh air and cuts the hang time. Glycol leaves a slippery film on hard floors, stairs and dance floors, and it can set off optical smoke detectors. Use a detector-safe fluid where the venue allows it, isolate alarms with the venue's permission, keep the nozzle away from faces, and ventilate before the room is used by anyone with a respiratory condition.
Fluid and fog type. Water-based glycol fluid is the norm indoors; quick-dissipating fluid trades hang time for a cleaner room, and heavy fluid pools low for floor-level effects. Cold fog from dry ice or liquid nitrogen is a different effect with different hazards — never handle it without proper training. Whatever the fluid, keep the machine level, let it warm up fully before the first burst, and never run it dry: the pump is cooled by the fluid it is moving.
Last reviewed: 2026-10-20. A planning estimate for a theatrical effect; follow the fluid's safety data sheet and the venue's fire and ventilation rules.