Air Changes Per Hour (ACH) Calculator — Free Online Calculator
Air changes per hour (ACH) is the ratio of total ventilation airflow to room volume, normalized to a one-hour period. It is the standard metric for comparing ventilation intensity across different room sizes — from residential bedrooms to hospital operating rooms. Enter room volume and airflow to calculate ACH, or enter target ACH to find the required CFM, with automatic ASHRAE 62.1 and ASHRAE 170 compliance checking by space type.
Enter room dimensions & airflow
Air changes per hour
Room volume
2,700 ft³
ASHRAE minimum
4 ACH
Non-compliant
See the breakdown
ACH = (CFM × 60) ÷ room volume (ft³). ASHRAE minimums shown are representative; always verify the applicable code edition and space classification with the authority having jurisdiction (AHJ).
ASHRAE minimum ACH by space type (representative values)
Always consult the adopted code edition and authority having jurisdiction (AHJ) for project-specific requirements.
| Space | Code reference | Min ACH |
|---|---|---|
| Residential bedroom | ASHRAE 62.2 (0.35 ACH) | 0.35 |
| General office | ASHRAE 62.1 (per-person + area) | ~4 typical |
| Conference room | ASHRAE 62.1 | 4–6 |
| Restroom | ASHRAE 62.1 | 10 |
| Commercial kitchen | ASHRAE 62.1 / IMC | 12–20 |
| Hospital patient room | ASHRAE 170 | 2 (+ 2 OA) |
| Operating room | ASHRAE 170 | 20 (+ 4 OA) |
| Isolation (negative) | ASHRAE 170 | 12 (+ 2 OA) |
| Parking garage | IMC 406 | 1.5 |
The ACH formula, explained
Air changes per hour is a dimensionless index that describes how many times per hour the total air volume of a space is exchanged. It standardises ventilation comparisons across different room sizes: a 500 CFM system in a 5,000 cubic foot room delivers the same 6 ACH as a 50 CFM system in a 500 cubic foot room. ASHRAE prescribes minimum ACH values by space type to protect indoor air quality — ASHRAE 62.1 for commercial spaces, ASHRAE 62.2 for residential, and ASHRAE 170 for healthcare environments.
ACH vs. CFM per person
ACH is a volume-based metric useful for dilution ventilation. ASHRAE 62.1 Table 6-1 specifies both a per-person component (CFM/person) and an area component (CFM/sq ft) — the per-person method ensures adequate outdoor air regardless of room size.
Total vs. outside air ACH
A space may have 10 total ACH (recirculated + outside air) but only 2 outside air ACH. ASHRAE 170 healthcare requirements specify outside air ACH separately from total ACH. Confirm whether a code requirement refers to total supply air or outside air only.
Residential 0.35 ACH
ASHRAE 62.2 sets a minimum of 0.35 ACH or 15 CFM, whichever is greater, for residential whole-building mechanical ventilation. This is a much lower rate than commercial standards because dwellings rely on envelope infiltration for a portion of fresh air.
Healthcare isolation rooms
ASHRAE 170 requires negative-pressure isolation rooms to maintain 12+ total ACH (2 outside air ACH minimum) with 100% exhaust to prevent cross-contamination. Positive-pressure protective environment rooms require 12 ACH with HEPA-filtered supply.
Worked examples
Three real-world scenarios showing compliant ventilation, an ASHRAE 170 sizing calculation, and a non-compliant restroom exhaust fan.
Office break room — 15×12×9 ft, 150 CFM supply
ACH = (150 × 60) / 1,620 = 9,000 / 1,620 = 5.6 ACH
Result: General office ASHRAE 62.1 typical ≈ 4 ACH — Compliant.
Hospital patient room — 14×12×9 ft, what CFM for 2 ACH minimum?
Q (outside air) = 2 × 1,512 / 60 = 50.4 CFM minimum OA
Q (total supply, 4 ACH) = 4 × 1,512 / 60 = 100.8 CFM total supply
Result: ASHRAE 170 requires 2 outside air ACH for patient rooms; total supply is typically 4 ACH. The outside air CFM and total supply CFM are sized independently.
Commercial restroom — 8×6×9 ft, 70 CFM exhaust fan
ACH = (70 × 60) / 432 = 4,200 / 432 = 9.7 ACH
Result: ASHRAE 62.1 restroom minimum = 10 ACH — Non-compliant (just below). Increase exhaust fan to 73 CFM: Q = 10 × 432 / 60 = 72 CFM.
Frequently asked questions
Common questions about ACH, ASHRAE ventilation requirements, and code compliance.
What is ACH (air changes per hour)?
ACH is the number of times per hour the total air volume of a room is replaced by ventilation supply air. It equals airflow in CFM times 60 (to convert to per-hour), divided by the room volume in cubic feet: ACH = (CFM × 60) / volume. Higher ACH means more dilution ventilation — critical for spaces with high pollutant loads, infection risk, or odour levels.
What ACH is required for an office?
ASHRAE 62.1 for commercial buildings specifies ventilation by people and floor area — typically resulting in 4–6 total ACH for a standard office depending on occupancy density. The code formula: outdoor air = (occupants × 5 CFM/person) + (floor area × 0.06 CFM/sq ft).
What ACH is required for a hospital operating room?
ASHRAE 170 (ventilation for healthcare facilities) requires a minimum of 20 total ACH for operating rooms, of which at least 4 ACH must be outdoor air. Air must be supplied from ceiling HEPA diffusers in a unidirectional downward pattern, with exhaust from low side-wall returns.
What is a good ACH for a residential home?
ASHRAE 62.2 sets a minimum of 0.35 ACH (or 15 CFM, whichever is greater) for residential whole-house mechanical ventilation. Modern tight construction often requires mechanical ventilation — HRV or ERV — to meet this minimum. Well-sealed passive houses typically need 0.3–0.5 ACH of mechanical ventilation.
How many CFM do I need for a target ACH?
Rearrange the ACH formula: CFM = ACH × room volume (ft³) / 60. For example, a 10×10×9 ft room (900 ft³) needing 10 ACH requires: 10 × 900 / 60 = 150 CFM.
Is higher ACH always better?
Not necessarily. Very high ACH in a heating or cooling climate increases the ventilation energy load — you must condition all that outside air. ASHRAE standards set minimums for health and code compliance, not targets to maximise. For energy efficiency, use demand-controlled ventilation (DCV) to modulate ACH based on actual occupancy and CO₂ sensors.
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