What is an air change per hour (ACH)?

An air change per hour (ACH) represents the number of times the entire volume of air in a room is replaced in one hour. For example, 6 ACH in a 50 m³ room means 300 m³ of fresh air is supplied every hour. Higher ACH values indicate more ventilation, which is needed in spaces with higher occupancy, heat, or moisture loads.

How many air changes does a UK kitchen need?

A domestic kitchen typically requires 10-15 ACH per CIBSE Guide A recommendations. Commercial kitchens need significantly more — 20-30 ACH — due to higher heat, steam, and grease-laden air. Approved Document F 2021 specifies minimum intermittent extract of 30 L/s (adjacent to hob) or 60 L/s (elsewhere), or 13 L/s continuous. Cooker hoods: 60 L/s ducted, 30 L/s recirculating.

What ACH is required for an office?

CIBSE Guide A recommends 6-10 air changes per hour for offices, depending on occupancy density and internal heat gains. The minimum fresh air supply per person is typically 10 L/s per person per CIBSE Guide A Table 1.5. Open-plan offices with higher occupancy may need the upper end of this range.

How do I convert ACH to litres per second?

Multiply the room volume in m³ by the ACH value to get m³/h, then divide by 3.6 to convert to L/s. For example: a 60 m³ room at 6 ACH = 360 m³/h = 100 L/s. This calculator performs this conversion automatically.

What is the difference between ACH and fresh air rate?

ACH describes total air volume replacement including recirculated air, while fresh air rate (outdoor air) is the portion of supply air drawn from outside. Building Regulations specify minimum fresh air rates per person (typically 10 L/s), whereas ACH covers the total air movement needed for comfort and contaminant control. Both must be satisfied.

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Air Change Rate Calculator — ACH to Flow Rate

Calculate required ventilation flow rates from air changes per hour (ACH) for any room type. Uses CIBSE Guide A and Approved Document F recommended rates.

Room UseDetermines recommended ACH per CIBSE Guide A

Room Length

Room Width

Ceiling Height

How We Calculate This

This calculator converts air changes per hour (ACH) requirements into practical flow rates in litres per second (L/s) and cubic metres per hour (m³/h) for ventilation system design.

Calculation method

Flow rate (m³/h) = Room volume (m³) x ACH. The result is then converted to L/s by dividing by 3.6. ACH recommendations come from CIBSE Guide A and are based on the room use category.