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

Room Dimensions

Air Purifier Specifications

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 × 60) ÷ 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.

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

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: multiply room volume (m³) by your target ACH, then divide by 60. For example, a 40 m³ room targeting 5 ACH needs CADR = (40 × 5) / 60 = 3.3 m³/min. Use this calculator to find the exact CADR for your room and target ACH.
What is the ACH formula?+
ACH = (CADR × 60) ÷ Room Volume. Where CADR is in m³/h and Room Volume is in m³. 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

DIY Corsi-Rosenthal Box Guide →HEPA vs MERV Filters Explained →Compare DIY vs Commercial Purifiers →Air Quality Near Me →

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