Air changes per hour (ACH) is one of the most fundamental concepts in ventilation design. It tells you how many times the air in a room is completely replaced in one hour. Whether you're designing hospital ventilation, office HVAC, or kitchen extract, ACH is where you start.
What is ACH?
ACH measures how many times the total volume of air in a space is replaced per hour. An ACH of 6 means the entire room volume of air is replaced 6 times every hour.
How to calculate ACH
The formula: ACH = (Q × 3600) / V
Where: Q = flow rate (m³/s), V = room volume (m³)
Or in simpler units: ACH = Qm3h / V where Qm3h = flow rate in m³/hr.
Worked example: Room: 10 m × 8 m × 3 m = 240 m³. Supply airflow: 400 L/s = 1.44 m³/s. ACH = (1.44 × 3600) / 240 = 5184 / 240 = 21.6 ACH.
Quick calc: Use our free ACH Calculator to get instant results — enter flow rate and room dimensions, or work backwards from a target ACH to find the required flow rate.
Working backwards — flow rate from ACH
Sometimes you know the required ACH and need to find the flow rate: Q (m³/s) = (ACH × V) / 3600
Example: Required 6 ACH in a 150 m³ office. Q = (6 × 150) / 3600 = 0.25 m³/s = 250 L/s.
Typical ACH rates by room type
| Space type | Typical ACH | Source / notes |
|---|---|---|
| Office (general) | 4–6 | CIBSE Guide A |
| Classroom | 5–8 | BB101 / CIBSE Guide A |
| Hospital ward | 6 | HTM 03-01 |
| Operating theatre | 15–25 | HTM 03-01 |
| Kitchen (commercial) | 15–30 | CIBSE Guide A / DW172 |
| Laboratory | 6–15 | Depends on hazard class |
| Toilet / WC | 6–10 | Part F / CIBSE Guide A |
| Plant room | 5–10 | CIBSE Guide A |
| Car park (enclosed) | 6–10 | Approved Document B, CIBSE Guide B |
| Retail | 3–6 | CIBSE Guide A |
| Server room | 10–15+ | Based on cooling load |
| Residential (living areas) | 0.5–1.0 | Part F |
Source: Typical values — always verify against current editions of CIBSE Guide A, Guide B, HTM 03-01, and relevant Building Regulations.
ACH vs L/s per person
ACH and L/s per person are two different ways to specify ventilation. ACH is volume-based and works well for extract systems, plant rooms, and car parks. L/s per person is occupancy-based and is more appropriate for offices, classrooms, and meeting rooms. Most projects use both — ACH for extract and L/s per person for fresh air supply. The Standards Picker can help you figure out which approach your project needs.
Common mistakes
Confusing supply ACH with extract ACH is more common than you'd think. They're not always the same, especially in pressurised or depressurised spaces. A hospital isolation room, for example, will have different supply and extract ACH rates by design to maintain negative or positive pressure. Once you know your flow rate, you can size the ductwork to deliver it.
Using ACH where L/s per person is more appropriate (or vice versa) can lead to under- or over-ventilated spaces. If occupancy varies significantly, L/s per person gives a more accurate ventilation rate. If the room has a fixed process load (like a kitchen or lab), ACH is usually the right metric.
Forgetting infiltration is another common issue. In naturally ventilated buildings, infiltration through the building fabric contributes to the total air change rate and must be accounted for in your calculations. This is particularly relevant for residential buildings where Part F rates are low and infiltration can be a significant proportion of total ventilation.
Related free tools
Our free ACH Calculator handles both directions — ACH from flow rate, or flow rate from target ACH. Enter room dimensions or volume and get instant results.
Open ACH Calculator → Try MEP Desk free →