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PURIFIER ROOM VOLUME CLEAN AIR {{ visualCadrMarker }}
Presets fill realistic dimensions; they are not product coverage claims.
Changing the unit converts the value without changing the physical length.
Length and width establish floor area before ceiling height adds volume.
Use the average height of the measured room volume.
Choose a starting target, then tune ACH directly below.
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Use 1–10 ACH; higher targets need proportionally more CADR.
Changing the unit converts the value without changing the clean-air flow.
Compare one purifier or a combination of up to eight matching units.
units
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Use 0% for the strict room-volume formula or add up to 60% buying margin.
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Leave at 100% for the entered label rating; lower only when you have a reason to derate it.
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Use 0% for the measured closed room or add up to 40% planning allowance.
Choose zero, one, or two decimal places without changing the calculated result.
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Fit checks

Candidate fit metrics

Compare the adjusted room target with the candidate after derating.
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How to use this fit

The label comparison is only as realistic as the selected room, fan speed, and placement assumptions.
Room target · {{ formatCadr(analysis.target_cfm) }}

Base room-volume requirement plus the selected buying cushion and open-plan allowance.

Effective candidate · {{ formatCadr(analysis.effective_candidate_cfm) }}

Combined label CADR after the usable-CADR factor.

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CADR estimates particle removal from air reaching the purifier. It does not rate gas or odor removal, certify room mixing, or replace source control and ventilation.

ACH coverage map

Compare this room's CADR demand across ACH targets with the selected candidate's effective CADR.

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Clean air delivery rate (CADR) expresses how much particle-filtered air an air cleaner supplies. A higher number can serve a larger room or turn over the air more often, but only while the unit runs at the fan speed represented by its rating and air can circulate through the room.

Room volume matters because a 300-square-foot room with a high ceiling contains more air than the same floor area with an 8-foot ceiling. Air changes per hour (ACH) connects that volume to a cleaning target: 5 ACH means clean-air flow equal to five room volumes each hour. It is a flow comparison, not a promise that every particle is removed five times.

FactorWhy it changes the choiceCommon mistake
Room volumeLarger volume needs more CADR for the same ACH.Using floor area while ignoring a high ceiling or connected space.
Particle CADRSmoke, dust, and pollen ratings can differ by particle size.Assuming one CADR also measures gas or odor removal.
Fan speedPublished CADR is commonly measured at the highest setting.Buying to the label limit, then running only a quiet low speed.
Placement and mixingBlocked intake, closed doors, and remote corners reduce useful circulation.Treating combined label flow as perfectly mixed delivery everywhere.
Filter conditionLoading and overdue replacement can reduce airflow.Expecting new-filter performance indefinitely.

CADR applies to particle removal. Gases and volatile organic compounds require suitable sorbent media, such as enough activated carbon, and do not have the same widely used consumer rating. Source control and ventilation with clean outdoor air remain the primary ways to address many indoor pollutants; filtration supplements them.

Health boundary: Sizing arithmetic cannot certify a health outcome, remove every pollutant, fix a moisture or mold source, or make an unsafe space safe. Follow public-health guidance during smoke events and seek qualified help for persistent indoor-air problems.

How to Use This Tool:

Measure the whole air volume the purifier will actually serve, including connected areas that remain open.

  1. Enter the room Length, Width, and average Ceiling height. Presets supply sample dimensions, not product coverage claims.
  2. Choose a Cleaning goal, then set Target air changes from 1 to 10 ACH. Higher ACH raises required CADR in direct proportion.
  3. Enter one purifier's particle CADR rating and the number of matching units. For fine particles, smoke CADR or the lowest relevant published particle CADR is the conservative comparison.
  4. Add a Quiet-speed cushion when buying above the strict formula target. Use Usable CADR factor to derate the candidate for lower speed or constrained operation, and Open-plan allowance for connected volume not fully captured by the room dimensions.
  5. Compare Target CADR with Effective candidate CADR, then check delivered ACH and the minimum matching unit count.

Interpreting Results:

Formula CADR is the clean-air flow needed for the measured room volume and chosen ACH. Target CADR raises that requirement with the selected open-plan allowance and quiet-speed cushion. Effective candidate CADR derates the combined label rating by the usable factor.

  • A positive Fit margin means the effective candidate exceeds the adjusted target. A negative margin means it falls short.
  • Delivered ACH uses effective candidate CADR and measured room volume. It does not subtract the quiet-speed cushion or open-plan allowance a second time.
  • Coverage area assumes the entered ceiling height and target ACH but excludes the two purchasing allowances. Use the adjusted target for the more conservative decision.
  • Units needed rounds up because a fraction of a matching purifier cannot be installed. Placement can still make several units perform differently from one combined number.

Technical Details:

Room measurements are normalized to metres and converted to cubic feet for the CFM calculation. CADR entered in cubic metres per hour is divided by 1.69901082 to obtain cubic feet per minute (CFM). Full precision is retained; display precision changes formatting only.

Formula Core

Room volume and target ACH establish the base particle-cleaning flow:

Qbase=V×ACH60

The two planning allowances multiply the base requirement separately:

Qtarget=Qbase×(1+O100)×(1+H100)

The candidate side adds matching label ratings and then applies the usable factor:

Qeffective=Qlabel×N×E100

Delivered ACH equals effective CADR × 60 ÷ room volume. Fit margin is (effective CADR − target CADR) ÷ target CADR × 100%. The required unit count divides target CADR by one unit's effective CADR and rounds up to the next whole unit.

Rule Core

StatusExact ruleReading
Target onlyEffective candidate CADR ≤ 0No positive candidate flow is available for comparison.
More CADR neededFit margin < −15%Candidate is materially below the adjusted target.
Borderline fitFit margin ≥ −15% and < 0%Candidate is close but remains below target.
Candidate clears targetFit margin ≥ 0% and ≤ 35%Candidate meets the target with moderate headroom.
Large CADR headroomFit margin > 35%Candidate substantially exceeds the adjusted target.

The descriptive product class is a planning label based on target CADR, not an AHAM or EPA certification.

Planning classTarget CADR in CFM
Compact bedroom class< 90
Medium room class≥ 90 and < 180
Large room class≥ 180 and < 320
High-output room class≥ 320 and < 500
Multiple-unit or commercial class≥ 500

The accepted planning ranges are 1–10 ACH, 1–8 matching purifiers, 0–60% quiet-speed cushion, 40–100% usable CADR factor, and 0–40% open-plan allowance.

Limitations:

The calculation assumes a well-mixed room and steady equivalent clean-air flow. Real concentration changes also depend on particle sources, outdoor air, deposition, leakage, doors, occupancy, operating time, and purifier placement.

  • Use a particle CADR that matches the pollutant size of concern. Smoke CADR is the conservative label comparison for smaller particles.
  • CADR does not rate gas or odor removal. Check the product's gas-filter media and evidence separately.
  • A lower fan speed may deliver less than the highest-speed label CADR. Derate only from measurements or a justified operating assumption.
  • Filtration does not replace source control, clean-air ventilation, moisture repair, filter replacement, or medical and public-health guidance.
  • Avoid devices that intentionally produce ozone; ozone is a lung irritant.

Worked Examples:

Bedroom with quiet-speed headroom

A 4.0 × 3.2 × 2.4 m room has 30.72 m³ of air. At 4.8 ACH, the base requirement is about 147.46 m³/h. Adding a 15% quiet-speed cushion raises the target to about 169.57 m³/h. A 300 m³/h candidate at a 100% usable factor has large modeled headroom, but the practical decision still depends on noise, placement, filter condition, and the relevant particle CADR.

References: