How the calculation works
The steady-state CO₂ concentration in a classroom is the outdoor CO₂ (~420 ppm) plus the CO₂ produced by occupants divided by the ventilation rate.
A typical secondary-school student produces about 4.5 L/h of CO₂ at rest; an adult, about 5 L/h. A primary student, ~3 L/h.
For a 30-student class with 1 teacher, total CO₂ production is roughly: (30 × 4.5) + 5 = 140 L/h, or 2,330 mL/min.
Divide that by the ventilation rate in L/min to get the steady-state CO₂ in ppm above outdoor. A class with 5 L/s/person (a reasonable winter target) gets 150 L/s = 9,000 L/min of fresh air — giving 2,330/9,000 = 260 ppm above outdoor, so ~680 ppm at steady state. Well within DfE limits.
A class with only 3 L/s/person — common in poorly-ventilated UK schools — sees 1,000+ ppm steady state, often peaking higher.
A typical secondary-school student produces about 4.5 L/h of CO₂ at rest; an adult, about 5 L/h. A primary student, ~3 L/h.
For a 30-student class with 1 teacher, total CO₂ production is roughly: (30 × 4.5) + 5 = 140 L/h, or 2,330 mL/min.
Divide that by the ventilation rate in L/min to get the steady-state CO₂ in ppm above outdoor. A class with 5 L/s/person (a reasonable winter target) gets 150 L/s = 9,000 L/min of fresh air — giving 2,330/9,000 = 260 ppm above outdoor, so ~680 ppm at steady state. Well within DfE limits.
A class with only 3 L/s/person — common in poorly-ventilated UK schools — sees 1,000+ ppm steady state, often peaking higher.
Try the calculator
Interactive calculator coming soon — for the moment, use the rule of thumb below.
Quick estimate:
- 5 L/s/person ventilation → 600–800 ppm steady state ✓
- 3 L/s/person → 1,000–1,400 ppm ⚠
- 1 L/s/person (effectively closed-up room) → 2,500+ ppm ✗
The DfE Building Bulletin 101 requires 8 L/s/person fresh air supply in new schools, with the system designed so that ad-hoc occupancy still keeps CO₂ below 1,500 ppm. Many older schools — particularly Victorian and 1960s buildings — operate at 2–3 L/s/person in winter when windows are closed.
Quick estimate:
- 5 L/s/person ventilation → 600–800 ppm steady state ✓
- 3 L/s/person → 1,000–1,400 ppm ⚠
- 1 L/s/person (effectively closed-up room) → 2,500+ ppm ✗
The DfE Building Bulletin 101 requires 8 L/s/person fresh air supply in new schools, with the system designed so that ad-hoc occupancy still keeps CO₂ below 1,500 ppm. Many older schools — particularly Victorian and 1960s buildings — operate at 2–3 L/s/person in winter when windows are closed.
What to do with the answer
If your calculator output is above 1,500 ppm at full occupancy, the room needs one of:
1. Cross-flow ventilation (windows on opposing walls, opened for trickle airflow)
2. Mechanical extract (typically 6–8 air changes per hour)
3. MVHR (mechanical ventilation with heat recovery) — best for winter performance
4. Reduced occupancy until the building service is upgraded
A continuous CO₂ monitor measures what is actually happening — the calculator is the design tool, the monitor is the verification.
1. Cross-flow ventilation (windows on opposing walls, opened for trickle airflow)
2. Mechanical extract (typically 6–8 air changes per hour)
3. MVHR (mechanical ventilation with heat recovery) — best for winter performance
4. Reduced occupancy until the building service is upgraded
A continuous CO₂ monitor measures what is actually happening — the calculator is the design tool, the monitor is the verification.