TAHA Engineering Toolsby TAHA Knowledge Lab

HVAC & Building Services

Air Changes per Hour (ACH) Calculator

Air changes per hour from room size and airflow — or the airflow needed for a target ACH.

Inputs

Calculate from
Room drawn as a box with length L, width W and height H. Supply air enters through the ceiling and extract air leaves through the side wall.supply airextract airL — lengthW — widthH
Room of length L, width W and height H. Air changes per hour = airflow per hour ÷ room volume (L × W × H). Diagram not to scale.

Supply or extract airflow of outdoor (or treated) air into the room.

Result

Air changes per hour
2ACH
Room volume
60
Time for one air change
30
Working
ACH = airflow × 3600 ÷ volume = 0.033333 m³/s × 3600 ÷ 60 m³ = 2 per hour

The airflow replaces the air in the room about 2 ACH — one air change every 30 min.

Educational estimate — check the assumptions below.

About this calculator

Air changes per hour (ACH) tells you how many times in one hour a volume of air equal to the room’s volume is supplied to, or extracted from, the room. It is a simple way to compare ventilation rates of rooms of different sizes and is used, for example, to size exhaust fans or check a ventilation design.

Required ventilation rates depend on the room’s use, its occupants and the code or standard that applies (for example ASHRAE 62.1 / 62.2 or national building regulations). Many standards specify airflow per person or per floor area rather than ACH, so this calculator does not suggest a value.

Formula

Find ACH

ACH = V̇ × 3600 ÷ Vroom    (V̇ in m³/s, Vroom in m³)

ACH = m³/h ÷ m³     ACH = cfm × 60 ÷ ft³

Vroom = length × width × height. One air change takes 60 ÷ ACH minutes.

Airflow for a target ACH

V̇ (m³/h) = ACH × Vroom (m³)     cfm = ACH × ft³ ÷ 60

The airflow needed to give a target number of air changes in a room of volume Vroom.

Worked example

  • A room 5 m × 4 m × 3 m has a volume of 60 m³. With 120 m³/h of ventilation: ACH = 120 ÷ 60 = 2 air changes per hour (one every 30 minutes).
  • For 6 ACH in the same room: 6 × 60 = 360 m³/h = 100 L/s.
  • Inch-pound: a 20 ft × 15 ft × 10 ft room (3000 ft³) with 300 cfm → 300 × 60 ÷ 3000 = 6 ACH.

Assumptions and limitations

  • ACH is a volume ratio. It assumes the airflow is distributed through the whole room; with poor air mixing, parts of the room are ventilated much less.
  • Use the outdoor or treated airflow that actually enters the room. Recirculated air in a fan-coil or split unit does not ventilate the room.
  • Furniture and equipment are not subtracted from the room volume, as is usual practice.
  • This calculator does not state a required ACH. Use the standard, code or design brief that applies to your building.

References

  • ASHRAE Standard 62.1 (commercial) and 62.2 (residential): ventilation and acceptable indoor air quality — for required ventilation rates.
  • Definition: air change rate = ventilation airflow ÷ room volume.