Air changes per hour calculator
Enter room length, width and height in metres, then the airflow in L/s, m³/h or CFM. Use the airflow identified by the design brief. For an irregular room, calculate its total volume first and use dimensions that produce that volume.
Illustrative inputs. This calculation does not select equipment or establish a safe operating limit.
The result is a nominal air-change rate. It does not show whether air reaches every occupied part of the room or whether a fume cupboard contains a particular chemical process. Our fume hood design guide connects room ventilation with extraction-system planning.
The ACH formula and unit conversions
ACH = hourly airflow ÷ room volume. For Q in L/s and V in m³, ACH = Q × 3.6 ÷ V. For Q in m³/h, divide directly by V. One cubic foot is approximately 0.0283168 m³, so airflow in CFM becomes m³/h when multiplied by approximately 1.699.
Keep the units visible in your notes. Dividing L/s directly by m³ produces a per-second rate, not ACH. An airflow labelled as a fan's free-air capacity is also different from a measured installed airflow against the resistance of the actual system.
For a room 6 m long, 4 m wide and 3 m high, volume is 72 m³. An illustrative 120 L/s becomes 432 m³/h, giving 432 ÷ 72 = 6 ACH. This is a calculation example, not a recommended rate for a laboratory.
Laboratory air changes per hour: interpreting the result
A useful ventilation brief identifies the work, chemicals, occupancy, heat loads, room layout and required controls. ACH summarises one part of that brief. A larger number alone does not prove that contaminants are captured where they arise, or that a particular cupboard is working correctly.
WorkSafe describes local exhaust ventilation as capture at the source. That is a different task from dilution by general room ventilation. Record the cupboard or process extraction requirement alongside the room airflow rather than substituting one for the other.
For design or alteration work, have the responsible ventilation professional establish the required rates and verification method for the activities in that room. A general calculator cannot set that project-specific requirement.
Laboratory ventilation requirements and pressure balance
Supply and extract figures may differ deliberately. A room that extracts more than its mechanical supply needs replacement air through a designed path. Door position, transfer openings and adjoining spaces influence how that air moves. Review the pressure relationship using the negative-pressure laboratory guide.
Do not add supply ACH and extract ACH as though they were two independent changes of the same room air. State which airflow the calculation uses and whether it is outdoor air, total supply or extract. Recirculated supply and outdoor-air flow answer different questions.
Prepare a useful measurement record
Record the room dimensions, airflow source, units, equipment operating state and date. Identify which cupboards were running, sash positions, doors and the condition of filters or controls. This lets the designer compare like conditions and investigate a difference between design and measured figures.
After a layout or process change, review the system rather than retaining an old ACH figure by habit. Moving a supply outlet can create draughts around a cupboard even when total airflow stays similar. Our cupboard placement guide covers the surrounding layout considerations.
Your questions
What is a good ACH value for a laboratory?
The appropriate value depends on the work, hazards and ventilation design. Ask the responsible designer to establish the rate and containment requirements for your laboratory.
Does six ACH mean all the air is replaced every ten minutes?
It means the hourly airflow equals six room volumes. Actual mixing and contaminant removal depend on the airflow pattern, so it is not a guarantee that every parcel of air leaves in ten minutes.