Air changes per hour calculator

What air changes per hour actually measures

Air changes per hour, or ACH, is the number of times the total air volume in a room is replaced with new air over the course of one hour. At 5 ACH, a 1,200 cubic foot bedroom has the equivalent of its entire air volume swapped out every 12 minutes. The formula is straightforward: ACH equals the CFM coming into the room multiplied by 60 (minutes per hour) divided by the room volume in cubic feet. Reverse the formula to find the CFM you need to hit a target ACH.

ACH matters across three distinct HVAC use cases. The first is ventilation design, where engineers size fresh-air intake to dilute indoor pollutants per ASHRAE Standards 62.1 (commercial) and 62.2 (residential). The second is infection control, where the CDC and ASHRAE 170 set minimum ACH for medical and high-risk spaces (operating rooms run 20+ ACH, isolation rooms 12 ACH). The third is residential load calculation, where the infiltration ACH feeds directly into Manual J heating and cooling load math.

The four calculation modes and when to use each

The calculator handles four distinct scenarios. Pick the mode that matches what you actually have on hand:

• Basic mode: You know the CFM (from a fan rating, balancing report, or supply register measurement) and the room dimensions. Returns ACH. Or you know the target ACH you want and need the CFM to deliver it.
• ASHRAE 62.1 mode: Commercial space ventilation rate procedure. Pick a space type from the standard's Table 6-1 (office, classroom, restaurant dining, retail, gym, lobby), enter floor area and (optionally) actual occupant count. Returns required outdoor air CFM and resulting ACH.
• ASHRAE 62.2 mode: Residential whole-house ventilation. Enter conditioned floor area and bedroom count. Returns required continuous ventilation CFM per the formula Qfan = 0.03 × floor area + 7.5 × (bedrooms + 1).
• Blower-door mode: You have an ACH50 result from a blower-door test. Returns natural ACH (the actual leakage rate under typical operating pressure) using the Sherman-Grimsrud N-factor by climate zone and stories.

ASHRAE 62.1 commercial: the Ventilation Rate Procedure

Commercial ventilation in any US jurisdiction with adopted building code is governed by ASHRAE Standard 62.1. The Ventilation Rate Procedure (VRP) uses two components: a person rate (Rp, in CFM per person) and an area rate (Ra, in CFM per square foot). Total ventilation in cubic feet per minute equals Rp × number of people + Ra × floor area. Most online ACH calculators only show one component or the other. The calculator on this page implements both, because the difference matters in real occupancy variance.

A few common space types from Table 6-1 to calibrate your expectations:

• Office space: Rp = 5 cfm/person, Ra = 0.06 cfm/ft². A 2,000 ft² office with 10 people needs 50 + 120 = 170 CFM of outdoor air.
• Classroom (K-12): Rp = 10 cfm/person, Ra = 0.12 cfm/ft². A 900 ft² classroom with 25 students needs 250 + 108 = 358 CFM.
• Restaurant dining: Rp = 7.5 cfm/person, Ra = 0.18 cfm/ft². The high Ra reflects cooking aerosols and cleaning chemicals as much as occupants.
• Gym / exercise area: Rp = 20 cfm/person, Ra = 0.06 cfm/ft². Very high Rp because elevated breathing rates increase CO2 production by 3 to 5x.

If your occupant count varies (conference room used sometimes, sometimes empty), pick the design population for the worst-case scenario the space is built for. Or use a demand-controlled ventilation strategy where CO2 sensors modulate outdoor air. That is worth a separate engineering discussion the calculator does not replace.

ASHRAE 62.2 residential: whole-house ventilation requirements

Residential ventilation requirements changed materially over the past decade because new homes are airtight enough that pre-2010 default leakage assumptions no longer hold. ASHRAE 62.2 specifies whole-house ventilation as a continuous fan rate using the formula:

Qfan = 0.03 × A + 7.5 × (Nbr + 1)

where A is conditioned floor area in square feet and Nbr is bedroom count. A 2,000 ft² 3-bedroom home needs 0.03 × 2000 + 7.5 × 4 = 60 + 30 = 90 CFM of continuous outdoor air. Most balanced or supply-only fans hit this with 80 to 120 watts of power draw, which adds up to about $10 to $20 per year in electricity.

Beyond the whole-house number, ASHRAE 62.2 calls for local exhaust at moisture sources. Bathrooms need 50 CFM intermittent (during use) or 20 CFM continuous. Kitchens need 100 CFM at the hood or 5 ACH whichever is larger. These are exhaust requirements, not supply, and they run in addition to whole-house ventilation. A properly designed system uses an HRV or ERV to deliver the whole-house air, and bath fans or range hoods for local exhaust. Do not use the bath fan as the whole-house ventilator. It tends to short-cycle and run loudly.

Blower-door ACH50 to natural ACH: the conversion most people get wrong

A blower-door test measures air leakage at a standardized 50 Pascal pressure difference between inside and outside. The result, ACH50, is much higher than the leakage rate under normal operating conditions. Converting between them uses the Sherman-Grimsrud N-factor, which depends on climate zone (windier zones lower N), number of stories (taller buildings lower N from stack effect), and shielding from surrounding terrain.

Typical N-factor values from LBL Report 23665, simplified:

• Climate zone 1-2 (mild south): N = 22-25 for a 1-story home
• Climate zone 3-4 (mid-Atlantic, Pacific NW): N = 18-20
• Climate zone 5-6 (Midwest, inland NE): N = 16-17
• Climate zone 7-8 (northern Midwest, mountain): N = 13-14

So a 1-story home in climate zone 4 with a 5.0 ACH50 result converts to about 5.0 / 18 = 0.28 natural ACH. A 2-story home in climate zone 6 with the same 5.0 ACH50 result converts to 5.0 / 15 = 0.33 ACH (more stack effect makes the same leaks pull more air). These natural ACH values feed directly into the infiltration component of any Manual J load calculation. The heat loss calculator on this site accepts ACH50 directly and does this conversion internally.

ACH recommendations by space type

The "right" ACH depends on the space. The calculator shows a recommended band when you pick a space type in basic mode. Common targets, drawn from ASHRAE 62.1, 62.2, 170, and CDC NIOSH guidance:

• Residential bedroom or living room: 0.35 to 1.0 ACH (whole-house ventilation equivalent; too high is wasted heating/cooling energy)
• Residential bathroom: 6 to 8 ACH when in use (50 CFM intermittent or 20 CFM continuous)
• Residential kitchen: 5 to 15 ACH (100 CFM range hood minimum)
• Office space: 4 to 10 ACH (CDC's "Aim for 5 ACH" guidance for occupied indoor space)
• Classroom: 4 to 8 ACH
• Restaurant dining: 8 to 12 ACH
• Gym / exercise space: 6 to 8 ACH
• Medical exam room: 6 to 8 ACH (ASHRAE 170 outpatient)
• Operating room: 20 to 25 ACH (ASHRAE 170 surgery)

Connecting ACH to duct sizing, CFM, and load calculations

ACH does not exist in isolation. Once you know what CFM the space needs, that airflow has to come through ductwork sized to deliver it at acceptable velocity and static pressure. Use the CFM calculator to verify room-by-room airflow targets, then the duct sizing calculator (Manual D) to pick duct diameters, the duct velocity calculator to keep noise below acceptable thresholds, and the static pressure calculator to confirm the air handler can actually push the design CFM through the system.

For residential load calculation, the infiltration ACH calculated here is one of the most influential inputs to total heating and cooling load. The heat loss calculator and the full Manual J calculator both accept natural ACH as the infiltration input. Getting this number right (especially via blower-door test instead of rule-of-thumb defaults) can swing total load by 15 to 25 percent in either direction.