Anonymous air purifier in measured room

Recalculate air purifier ACH from CADR before you buy: UK rules

Air changes per hour, or ACH, tells you how many times a purifier filters the equivalent volume of a room’s air in one hour. The practical takeaway matters more than the definition: calculate ACH yourself from the unit’s CADR and your actual room volume, because a manufacturer’s headline ACH figure is usually measured for a different space than yours. Guidance from GOV.UK and the Department for Education, alongside Simco Direct’s own sizing tools, can help you get the number right before you buy.

TL;DR:

  • ACH should be recalculated for your specific room size; a high manufacturer-rated ACH may not match real-world performance.
  • Filter quality matters; prioritize HEPA H13 filters and check noise levels at the airflow setting needed to achieve your desired ACH.
  • ACH alone does not account for ventilation or gaseous pollutants; proper ventilation remains essential to address CO2, humidity, and gas-based contaminants.
  • Use proven tools and guidelines to determine the actual CADR and ACH needed, and verify filter condition and placement for optimal air cleaning.
  • For shared spaces, higher ACH targets are recommended, but increased noise and occupancy levels can affect the purifier’s effectiveness and comfort.
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Table of Contents

What ACH is and why it matters for indoor air cleaning

ACH stands for air changes per hour. If a room holds 30 cubic metres of air and a purifier moves 30 cubic metres of filtered air through it every hour, that’s 1 ACH. Run the same purifier in a room twice the size and the ACH halves, even though nothing about the machine has changed.

That single fact explains why ACH is a relationship, not a fixed property of a device. A purifier rated at “5 ACH” on a manufacturer’s test room might deliver 2 ACH or less in your living room, depending on how much bigger or smaller your space is than theirs. The number only means something once it’s tied to a specific volume.

ACH also says nothing about how thoroughly the air is mixed. A purifier tucked in a corner behind furniture can hit its rated ACH on paper while barely touching air on the other side of the room, because real rooms don’t mix air as evenly as a lab chamber does.

  • ACH measures how many times an equivalent volume of room air passes through the filter each hour.
  • It scales directly with room size: double the volume, and ACH halves for the same airflow.
  • A high ACH figure from a spec sheet is only meaningful once you recalculate it for your own room.
  • Air mixing, furniture placement and door position all affect how evenly that filtered air actually reaches you.

Used correctly, ACH is one of the most useful ways to judge whether a purifier is properly matched to a room. Used as a marketing number lifted straight off a box, it tells you very little.

How to calculate ACH from CADR and room size

The formula is straightforward once you have two figures: the purifier’s Clean Air Delivery Rate, or CADR, and your room’s volume.

Illustration showing CADR and room volume

ACH = (CADR in m³/h × 60) ÷ room volume in m³

If your CADR is already in cubic metres per hour, drop the “× 60” and just divide by room volume. Some manufacturers list CADR in litres per second or cubic feet per minute, so convert first: multiply litres per second by 3.6 to get m³/h, or multiply cubic feet per minute by 1.699 to get m³/h. Mixing units at this stage is the most common source of a wrong answer.

  1. Measure your room: length x width x height, in metres, to get volume in cubic metres.
  2. Find the purifier’s CADR figure and convert it to m³/h if it isn’t already.
  3. Divide the hourly CADR by the room volume to get ACH.
  4. Cross-check the result against the manufacturer’s stated ACH: a big mismatch usually means their test room is a different size to yours.

Here’s a worked example. Say a bedroom measures 4m by 3.5m with a 2.4m ceiling, giving a volume of 33.6 cubic metres. A purifier with a CADR of 250 m³/h delivers ACH of (250 ÷ 33.6), which comes to roughly 7.4 ACH in that room. Put the same purifier in a 60 cubic metre open-plan living room and the ACH drops to about 4.2, using the same maths.

A purifier’s CADR of 180 m³/h supports up to 8 occupants in a standard classroom-sized space, according to Department for Education procurement figures, which gives a useful anchor to scale the same calculation to larger rooms.

What counts as a good ACH for different rooms

There’s no single “correct” ACH, because the right target depends on the room’s use, how many people are in it and what you’re trying to control.

  • Bedrooms and living rooms: domestic guidance and building regulations point to roughly 0.4 to 1 ACH for background ventilation in these spaces, which is a baseline for general air exchange rather than a purifier-specific target, as set out in Approved Document F.
  • Purge ventilation: the same guidance recommends rapid dilution ventilation, such as opening windows fully, capable of around 4 ACH for quickly clearing pollutants after cooking or a one-off event.
  • Classrooms and shared spaces: the Department for Education’s air cleaning unit specification lists example CADR values of 180, 360 and 720 m³/h for spaces serving up to 8, 16 and 32 occupants respectively, alongside a mandatory HEPA H13 filter requirement.

This Department for Education figures sit well above typical domestic ventilation targets because classrooms carry far more occupants relative to floor area, and because the specification also enforces sound power limits at the required airflow, so higher CADR doesn’t necessarily mean a noisier unit.

Occupancy and activity level change the picture too. A room full of people breathing, talking and moving generates more particles and CO2 than an empty one, so the same ACH that feels adequate overnight might fall short during a busy gathering. Chasing a very high ACH also tends to mean running the purifier on a louder fan setting, which is where noise and comfort start to compete with air-cleaning ambition.

Why ACH alone doesn’t tell the full story

ACH describes how much filtered air is recirculated through a room. It says nothing about fresh air coming in from outside, and that gap matters more than most buyers expect.

A HEPA purifier captures particles very effectively, but it cannot add oxygen, remove carbon dioxide or reduce excess humidity. It also does nothing for combustion pollutants such as nitrogen dioxide from a gas hob, since these are gases rather than particles a filter traps.

  • Carbon dioxide build-up in a closed room, from occupants breathing, is not addressed by any purifier regardless of its ACH.
  • Humidity and damp-related issues need ventilation or dehumidification, not filtration.
  • Gaseous pollutants from cooking, heating or combustion pass straight through most HEPA-only units.

GOV.UK’s summary of ventilation guidance, based on SAGE’s Environmental Modelling Group advice, states that air cleaners can be useful where ventilation cannot be improved but should not replace it, and specifically recommends avoiding ozone-producing devices given the drawbacks that technology carries. UKHSA guidance echoes this, advising that ventilation stays the primary control for indoor air quality, with air cleaners treated as a supplementary measure rather than a substitute for opening a window or running an extractor fan.

Pro Tip: If a room feels stuffy even with a purifier running, check for a ventilation problem first: a rising CO2 reading is a better clue than air quality apps that only track particles.

What to check beyond the headline ACH figure

A high ACH on the box is only useful once you’ve recalculated it for your own room, but a handful of other details decide whether the purifier is actually pleasant to live with.

  • Recalculate, don’t trust the label: convert the stated CADR to ACH for your specific room volume before comparing two models, since a bigger CADR in a bigger test room can still mean a lower real-world ACH than a smaller unit tested in a smaller room.
  • Filter class: look for HEPA H13 or better where higher performance matters, and be wary of ionisers or technologies that generate ozone as a by-product.
  • Noise at the flow rate you’ll actually use: a unit’s quietest decibel rating is often measured on its lowest, least effective setting, so check the rating at the fan speed needed to hit your target ACH.
  • Maintenance and running cost: filters need periodic replacement, and airflow drops as they clog, so factor filter availability and cost into the decision rather than the purchase price alone.
  • Controls: auto mode, timers and a sensor-driven fan speed help keep effective ACH up without you having to manage the unit manually.

Pro Tip: A quieter purifier left running continuously on a moderate setting typically delivers a better real-world ACH than a louder unit that gets switched down, or off, because it’s disruptive.

Simco Direct’s guide to air purifier CADR walks through the conversion in more detail, and the filter replacement guidance covers how to spot when performance has started to drop.

Working out CADR and ACH before you buy

Getting the sizing right before purchase saves the hassle of returning a unit that never quite keeps up with the room. Simco Direct’s room size calculator takes your room dimensions and works out the CADR you need in a few minutes, and the placement advice guide covers where to position a unit so filtered air actually reaches the space you spend time in, rather than circulating near a wall.

There is an air treatment range supported by fast UK delivery and a customer-friendly returns policy, which matters when a purifier that looked right on paper turns out to be undersized once it’s running in the actual room.

Balancing ACH with real-world constraints

Size by CADR against your actual room volume, not the number on the box. A quieter unit run continuously beats a louder one that gets turned down out of frustration, since effective ACH depends on the machine actually running. Check CO2 levels before assuming a stuffy room needs more filtration rather than more ventilation, and keep filters clean: a clogged filter quietly erodes the ACH you calculated on day one.

— Carl

Where to start shopping for an air purifier

Once you’ve worked out the ACH your room needs, the next step is comparing units on their actual CADR and filter specification rather than a headline number. Simco Direct’s air treatment range covers purifiers alongside the rest of its washroom, heating and bathroom catalogue, all backed by fast UK delivery and a 30-day returns policy if a unit doesn’t suit the space once it’s installed.

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  • Compare CADR and filter class on each product page before choosing between similarly priced units.
  • Recalculate ACH for your own room volume rather than relying on the manufacturer’s test figure.
  • Contact the support team if you want help matching a unit to a specific room size.

Browse the current range and check availability at Simcodirect.

Sources

FAQ

What is ACH on an air purifier?

ACH, or air changes per hour, is the number of times a purifier filters a volume of air equal to the room’s volume in one hour. It’s calculated by dividing the unit’s CADR by the room’s volume, so the same purifier gives a different ACH in every room it’s placed in.

What is a good ACH for an air purifier?

There’s no single figure that suits every room, since the right target depends on room use and occupancy. Domestic guidance points to roughly 0.4 to 1 ACH for general background ventilation in living rooms and bedrooms, under Approved Document F, while shared spaces with more occupants, such as classrooms, warrant a higher rate.

What does ACH mean for ventilation?

In a ventilation context, ACH describes how often the air in a space is replaced or recirculated each hour, whether by natural airflow, mechanical ventilation or a filtration device. It’s a rate relative to room volume, not a fixed property of any single system.

What is a good ACH for a house?

Approved Document F and related domestic guidance suggest around 0.4 to 1 ACH for everyday living spaces, with purge ventilation, such as fully opening windows, aiming for about 4 ACH to clear pollutants quickly after cooking or similar activity. A purifier can help maintain filtered air between those events, but it works alongside ventilation rather than instead of it.

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