Indoor CO₂ & Ventilation Rate Calculator
Steady-state indoor CO₂ concentration from occupancy, activity and outdoor-air ventilation rate, the outdoor air needed per person to hold a CO₂ target (800, 1,000 or 1,200 ppm), ASHRAE 62.1 minimums by space type, and how long a room takes to reach equilibrium.
How it works
C_indoor = C_outdoor + G ÷ Q (steady state), with G = CO₂ generation (L/min) and Q = outdoor air (L/min; 1 CFM = 28.3 L/min); Q needed = G ÷ (C_target − C_outdoor) × 10⁶; time constant = volume ÷ Q
People exhale carbon dioxide at a fairly predictable rate — about 0.3 litres a minute seated, several times that when exercising — so the CO₂ level in an occupied room is a direct proxy for how much outdoor air is diluting everything else the occupants release. At steady state the excess over outdoor CO₂ equals generation divided by outdoor-air flow: 15 CFM per seated adult holds about 1,000 ppm, the level most guidance treats as the ceiling for classrooms and offices, while 5 CFM per person drifts past 2,000 ppm and people notice drowsiness and headaches. Classrooms are the classic problem: many small bodies, little ventilation. The room’s volume sets only how fast the level climbs — a big hall takes hours to reach equilibrium, a bedroom with the door shut a few tens of minutes — not where it ends up.
Worked example
A classroom of 24 children and one teacher generates about 5.6 L of CO₂ a minute. With 300 CFM (8,500 L/min) of outdoor air the room settles at about 1,080 ppm — just over the 1,000 ppm target, which would need 340 CFM (13.6 CFM per person). In a 200 m³ room the time constant is 24 minutes, so levels plateau within about an hour.
Frequently asked questions
Can I use CO₂ readings to estimate ventilation?
Yes — rearrange: Q = G ÷ (C_measured − C_outdoor). A room of known occupancy that plateaus at 1,600 ppm with 420 outside is getting roughly 8 CFM per seated person.
Is CO₂ itself harmful at these levels?
Not acutely below ~5,000 ppm (the 8-hour occupational limit), but 1,000–2,000 ppm is associated with reduced cognitive performance and indicates that bio-effluents, viruses and other pollutants are also poorly diluted.
Related calculators
Building-physics estimate using steady-state formulas and typical material values; real assemblies, workmanship, air leakage and climate data can move results significantly — confirm with a qualified designer for code compliance.