Air Change Rate Calculator — ACH to Flow Rate

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.

For dwellings the legal requirement is an extract RATE, not an ACH: Approved Document F 2021 Table 1.1 sets minimum intermittent extract of 30 L/s adjacent to the hob (ducted cooker hood over hob) or 60 L/s elsewhere in the kitchen (no hood or recirculating hood), or 13 L/s continuous (Table 1.2). A 10-15 ACH band is only a rough volume-equivalent for a domestic kitchen. Commercial kitchens are typically designed to a much higher 20-30 ACH due to greater heat, steam, and grease-laden air.

A typical design range for offices is 4-8 air changes per hour, depending on occupancy density and internal heat gains — this calculator applies 6 ACH by default and lets you override it. In practice UK office ventilation is more often sized on a fresh-air basis of about 10 L/s per person rather than a fixed ACH, so check both against your occupancy. Open-plan offices with higher heat gains tend toward the upper end of the range.

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.

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.