Required CADR by Particle Type
Full Results Table
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How to Use This Calculator
Pick a preset room type to load typical Canadian dimensions, or enter your own length, width, and ceiling height. The calculator computes room volume automatically and shows it in cubic metres.
Select 4 ACH for standard use, 5 ACH to match the 4.8 ACH that AHAM uses for its suggested room size, or 6 ACH and up during wildfire smoke events. Higher ACH means a larger or faster-running purifier.
The use case adjusts the minimum recommended CADR rating for the particle type that matters most for your situation. Smoke mode emphasizes smoke CADR; allergy mode emphasizes pollen CADR.
The results show required smoke, dust, and pollen CADR in both m3/min and CFM so you can match against product spec sheets. AHAM-certified purifiers list all three CADR values on the packaging.
Air Purifier Sizing for Canadian Homes and Offices
Air purifiers have moved from a niche purchase to a common household appliance across much of Canada, driven partly by worsening wildfire smoke seasons and partly by growing awareness of how much time Canadians spend indoors. Getting the sizing right matters. An undersized unit running continuously at full speed barely keeps up and wears out quickly. An oversized unit set to a lower fan speed can achieve the same air changes with less noise and longer filter life.
What CADR Actually Means
Clean Air Delivery Rate is the volume of particle-free air a purifier produces per unit time at its highest fan speed. The Association of Home Appliance Manufacturers (AHAM) certifies purifiers under ANSI/AHAM AC-1 and publishes CADR in cubic feet per minute for smoke (0.09 to 1 micron), dust (0.5 to 3 micron), and pollen (5 to 11 micron) test particles. A purifier with a smoke CADR of 300 CFM (8.5 m3/min) delivers the equivalent of 300 cubic feet of particle-free air every minute. That does not clean a room instantly: particle concentration falls exponentially over time rather than dropping to zero. The PM2.5 calculator models that decay curve in detail.
The ACH Rule of Thumb
AHAM's seal lists a suggested room size based on 4.8 air changes per hour. Its 2/3 rule is the quick version: a smoke CADR in CFM of at least two-thirds of the floor area in square feet, for ceilings up to 8 feet (2.4 m). This calculator works from actual room volume, so a taller ceiling raises the required CADR automatically. For the 4.6 x 3.7 m master bedroom preset (about 183 ft2), the 2/3 rule gives about 122 CFM, and the calculator at 5 ACH gives 121 CFM. In a well-mixed room with no new particle sources, concentration falls as e raised to the power of minus ACH times hours, so 4 ACH leaves about 2% of the starting particles after one hour and 5 ACH leaves about 0.7%. Real rooms mix imperfectly, so treat those as best-case figures. For wildfire smoke, AHAM recommends a smoke CADR equal to the room's floor area in square feet, which works out to about 7.5 ACH at 2.4 m ceilings. The 6 ACH preset sits below that, so enter 7.5 in the custom ACH field to match AHAM's figure. The ACH calculator covers the same air-change arithmetic for mechanical ventilation, which helps when you want to compare a purifier against the room's existing air exchange.
Wildfire Smoke in Canada
Wildfire smoke reaches communities across Canada every fire season, often far from the fires themselves, and the Air Quality Health Index (AQHI) is the place to check local conditions. Health Canada's wildfire smoke guidance names fine particles (PM2.5) as the main health risk and says there is no known safe level of exposure for some of the pollutants in smoke. Its advice for indoor air is to keep windows and doors closed as much as possible, use the best filter your ventilation system can handle, use a certified portable air cleaner that filters fine particles, and limit exhaust fan use. A purifier sized for 6 ACH or more, running continuously with the windows closed, gives you the best chance of holding indoor PM2.5 down while outdoor levels stay high. How far indoor levels fall depends on how leaky the room is, because smoke keeps seeping in. See the IAQ calculator for a composite view of indoor air quality.
Placement and Room Mixing
CADR ratings assume a well-mixed room. In practice, a purifier in the corner of a large open-plan space won't mix the whole room effectively. Placement near the centre of the room or near the dominant source of particles works better. For bedrooms, placing the unit between the bed and the door catches particles entering the room. For open-plan living areas over 50 m2, two smaller units at opposite ends can circulate air better than one large unit in a corner.
Portable vs. Whole-Home Systems
Portable purifiers handle one room. Whole-home systems connected to the HVAC ductwork treat all the air circulating through the system. The tradeoff is that whole-home systems only run when the air handler is on, and they depend on the duct system distributing air well. In Canadian homes where the gas furnace runs for heating and the fan idles the rest of the time, a portable unit in the bedroom can maintain better overnight air quality than a duct-mounted system alone. To cover both, upgrade the furnace filter to the highest rating the blower can handle, often MERV 13 (see the air filtration calculator), and add a portable HEPA unit in the bedroom.
Filter Replacement and Running Costs
Most HEPA purifiers need filter replacement every 6 to 12 months depending on use intensity, and annual filter costs typically run $40 to $150 for a mid-sized unit. Energy use at typical fan speeds is 20 to 80 W. A 50 W unit running around the clock uses about 438 kWh a year, which costs roughly $44 to $74 at electricity rates of $0.10 to $0.17/kWh. The annual energy cost calculator runs the same arithmetic for any wattage and electricity rate. Pre-filters that capture larger particles extend HEPA filter life and cost $10 to $30 annually.
Frequently Asked Questions
Multiply the room volume in cubic metres by the target ACH, then divide by 60 to get m3/min CADR. A 30 m2 bedroom with 2.4 m ceilings has a volume of 72 m3 and needs a smoke CADR of about 4.8 m3/min (170 CFM) to achieve 4 ACH. AHAM sizes its suggested room sizes at 4.8 ACH, so a target of 4 to 5 ACH lines up with it. Use this calculator to get exact values for your room and use case. Check the PM2.5 calculator to see how that CADR translates to indoor concentration reduction.
Smoke particles (0.09 to 1 micron in the AHAM test) are the hardest to capture, so smoke CADR is the most demanding rating. Dust (0.5 to 3 micron) is somewhat easier. Pollen (5 to 11 micron in the AHAM test) is captured efficiently by almost any filter media. A purifier with a high smoke CADR handles all three well. The smoke CADR is the most useful single number for sizing when PM2.5 or wildfire smoke is your concern. For allergy relief from tree and grass pollen, pollen CADR is the relevant metric, though it's almost always higher than smoke CADR on the same unit.
AHAM sizes its suggested room sizes at 4.8 ACH, so 4 to 5 ACH is a sensible target for bedrooms and living rooms. For wildfire smoke, AHAM recommends a smoke CADR equal to the room's floor area in square feet, which works out to about 7.5 ACH at 2.4 m ceilings. The 6 ACH preset is a lower starting point, and the custom ACH field lets you enter 7.5 or higher. Use 3 ACH only for basic, low-pollution rooms. Higher ceilings mean more air volume, so enter your actual ceiling height instead of assuming 2.4 m.
They serve different functions. An HVAC filter cleans air circulating through the whole system and protects equipment. A portable purifier cleans one room independently of whether the HVAC is running. In Canadian homes where the furnace runs intermittently, a portable HEPA purifier in the bedroom maintains cleaner overnight air even when the system is idle. For whole-building protection, HVAC filtration upgrades are more effective. The best approach combines both: the highest-rated furnace filter the blower can handle, often MERV 13 (see the air filtration calculator), plus a portable unit for targeted room-level control.
Related Air Quality Tools
Model indoor PM2.5 from filtration and infiltration
Multi-stage HVAC filter efficiency
HEPA sizing and face velocity
Composite indoor air quality score
Filter efficiency by MERV rating
AQHI from raw pollutant readings