What is the ACH formula?

ACH is calculated as n = Q/V, where Q is airflow and V is room volume.

Which calculation modes are available?

You can solve for ACH from airflow and volume, airflow from ACH and volume, or volume from ACH and airflow.

Which airflow units are supported?

The tool supports m³/s, m³/h, and CFM.

Which room volume units are supported?

You can enter volume in m³, ft³, or litres.

How is purge time calculated?

For a well-mixed single zone, purge time is t = −ln(ε)/n hours, where ε is the target residual fraction.

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Air Changes per Hour (ACH, n = Q/V)

Compute the air change rate (ACH / n) of a room from the outdoor supply airflow Q and the room volume V: n = Q/V (1/h). Three modes: solve ACH (given Q and V), solve airflow (given n and V), or solve volume (given n and Q). Flow in m³/s/m³/h/CFM, volume in m³/ft³/L. Also reports the well-mixed single-zone purge time to reach a target residual fraction ε (default 1%): t = −ln(ε)/n hours.

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2 examples

Solve ACH: Q=300 m³/h, V=60 m³ -> n=5.0 ACH, purge to 1% in 0.92 h

Q=300 m³/h, V=60 m³. n=Q/V=300/60=5.0 1/h. Purge to ε=0.01: t=−ln(0.01)/5=4.6052/5=0.921 h ≈ 55.3 min.

{"result":"ACH (mode=ach). Q=300 m³/h (300.0000), V=60 m³ → n=5 ACH. Purge to 1%: t=0.9210 h (55.26 min).","metadata":{"input":{"mode":"ach","flowRate":300,"flowUnit":"m3/h","volume":60,"volumeUnit":"m3","purgeFraction":0.01},"result":{"ACH":5,"Q_m3_h":300,"V_m3":60,"purgeFraction":0.01,"purgeTime_h":0.921,"purgeTime_min":55.26,"mode":"ach"}}}

Solve flow: n=6 ACH, V=2000 ft³ -> Q ≈ 679.6 CFM

V=2000 ft³=56.634 m³. n=6 1/h → Q_m3h=n·V_m3=6·56.634=339.8 m³/h. In CFM: Q_CFM=Q_m3h/(60·0.0283168466)=339.8/1.6990≈200.0 CFM? Re-check: 1 CFM = 1.6990 m³/h, so Q_CFM = 339.8/1.6990 = 200.0 CFM (per code).

{"result":"ACH (mode=flow). n=6 ACH, V=2000 ft³ (56.6337 m³) → Q=200 CFM (339.8024 m³/h). Purge to 1%: t=0.7675 h (46.05 min).","metadata":{"input":{"mode":"flow","ach":6,"flowUnit":"CFM","volume":2000,"volumeUnit":"ft3","purgeFraction":0.01},"result":{"ACH":6,"Q_inputUnit":200,"Q_m3_h":339.8024,"V_m3":56.6337,"purgeFraction":0.01,"purgeTime_h":0.7675,"purgeTime_min":46.05,"mode":"flow"}}}

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8 options

SettingsAdjust formats, ranges, numbers, and modes.8

ModeselectOptionalSolve ACH n (needs Q and V), solve Air Flow (needs n and V), or solve Volume (needs n and Q).Air Flow Rate QnumberOptionalOutdoor / supply volumetric airflow Q. Enter in the selected Flow Unit. Required unless solving flow.Flow UnitselectOptionalRoom Volume VnumberOptionalRoom (conditioned space) volume V. Enter in the selected Volume Unit. Required unless solving volume.Volume UnitselectOptionalAir Changes per Hour n (ACH)numberOptionalAir-change rate n (1/h). Required when solving flow or volume; ignored when solving ACH.Purge Target (residual fraction)numberOptionalResidual contaminant fraction ε after purge, between 0 and 1 (exclusive of 0). Default 0.01 (1%). The well-mixed purge time is t = −ln(ε)/n hours.Decimal PlacesnumberOptional

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Air Changes per Hour (ACH, n = Q/V)

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1. Calculate ACH for a room

2. Find airflow for a target ACH

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