ACH Calculator UK — Air Changes Per Hour Calculator

Calculate air changes per hour (ACH) for any room in your home, school, or office. Enter your room dimensions and air purifier CADR rating to see whether you meet UK indoor air quality standards.

UK guidance: 4–6 ACH for homes · 5–6 ACH for schools and offices · 6–12 ACH for healthcare

Quick answer

Air changes per hour (ACH) is how many times the full volume of air in a room is filtered each hour. Divide your purifier's clean-air delivery rate (CADR, in m³/h) by the room volume (in m³). Example: a 40 m³ bedroom with a 200 m³/h purifier gives 200 ÷ 40 = 5.0 ACH — meeting the UK home target of 4–6. Enter your own room and purifier below to check yours.

Room Dimensions

Length (m)

Width (m)

Height (m)

Air Purifier Specifications

CADR per Unit

Number of Units

Target ACH

Understanding Air Changes Per Hour (ACH)

What is ACH?

Air Changes per Hour (ACH) measures how many times the full volume of air in a room is replaced or filtered within an hour. A higher ACH means pollutants, allergens, and airborne particles are removed faster.

The ACH Formula

ACH = CADR ÷ Room Volume

Where CADR is in m³/h and room volume is in m³. If your CADR is in CFM, multiply by 1.699 to convert to m³/h first. Example: a 200 m³/h purifier in a 40 m³ room = 200 ÷ 40 = 5.0 ACH.

UK ACH Guidelines

Bedrooms / Living rooms

4–6 ACH

General household standard

Schools / Offices

5–6 ACH

Ventilation regulations

Healthcare / High-risk

6–12 ACH

SAGE / NHS guidance

CADR needed by room size (for 5 ACH)

Based on a standard 2.4 m ceiling. Find your room area, then match a purifier (or number of DIY boxes) that meets or beats the CADR shown.

Room	Floor area	Volume	CADR for 5 ACH
Small bedroom / nursery	10 m²	24 m³	120 m³/h
Double bedroom	15 m²	36 m³	180 m³/h
Living room	20 m²	48 m³	240 m³/h
Home office	25 m²	60 m³	300 m³/h
Open-plan / large lounge	35 m²	84 m³	420 m³/h
Classroom	50 m²	120 m³	600 m³/h

A 5-fan Corsi-Rosenthal box (~800–1,200 m³/h) covers any single room here on its own; larger or open-plan spaces may need two.

What is CADR?

CADR (Clean Air Delivery Rate) is the volume of filtered air an air purifier delivers per hour. It is measured in m³/h or CFM. A higher CADR means more air is cleaned faster. Check your purifier's manual or product listing for its CADR rating.

Frequently Asked Questions

What is an ACH calculator?+

An ACH (Air Changes per Hour) calculator works out how many times the full volume of air in a room is filtered each hour by your air purifier. You enter your room dimensions (length, width, height) and the CADR (Clean Air Delivery Rate) of your purifier, and the calculator returns the ACH value.

How many air changes per hour do I need UK?+

For general use in UK homes, 4–6 ACH is recommended. Schools and offices should target 5–6 ACH. Healthcare settings and spaces occupied by vulnerable people (asthma, elderly) should target 6–12 ACH. SAGE guidance during COVID-19 recommended a minimum of 6 ACH for high-risk settings.

What CADR do I need for my room?+

To calculate required CADR (in m³/h): multiply room volume (m³) by your target ACH. For example, a 40 m³ room targeting 5 ACH needs CADR = 40 × 5 = 200 m³/h. Use this calculator to find the exact CADR for your room and target ACH.

What is the ACH formula?+

ACH = CADR ÷ Room Volume. Where CADR is in m³/h and Room Volume is in m³. For example, a 200 m³/h purifier in a 40 m³ room gives 200 ÷ 40 = 5.0 ACH. If your CADR is given in CFM (cubic feet per minute), convert it first: 1 CFM = 1.699 m³/h.

Is a DIY Corsi-Rosenthal Box good enough to meet ACH targets?+

Yes. A standard 5-fan Corsi-Rosenthal Box using MERV-13 filters produces a CADR of 800–1,200 m³/h depending on fan speed — significantly more than most commercial purifiers at a fraction of the cost. For a typical UK classroom (200 m³), one box can achieve 4–6 ACH.

Related Tools & Guides

MERV 13 vs HEPA: Full UK Comparison Guide →DIY Corsi-Rosenthal Box Guide →MERV 13 vs HEPA Filters Explained →Compare DIY vs Commercial Purifiers →Air Quality Near Me →

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