How the estimates are calculated

CADR: clean-air delivery rate

CADR is the volume of fully clean air delivered per unit time: the airflow through the box multiplied by how much of the targeted particulate it removes on a single pass. It is the standard portable-air-cleaner metric defined by ANSI/AHAM AC-1.

For PC-fan builds the operating-point airflow comes from the fan/filter intersection (see Total airflow); it is then reduced by a build-efficiency factor of 0.85 for leakage, fan guards, and fan-to-fan interaction, and multiplied by the MERV-13 single-pass efficiency (see Filter efficiency). For the box-fan "filter cube" build, airflow comes from an empirical power-law fit through reference four-filter cubes (capped at the unit's no-filter shroud airflow) and the single-pass capture is a conservative whole-cube figure (0.50 for media ≥30 mm deep, otherwise 0.43). The cube model's depth and shroud gains match the EPA's DIY chamber study.

You can also measure your finished box's CADR with a phone barometer, working backward from the pressure drop across the filters instead of predicting it from the fan.

Sources: AHAM AC-1 (CADR) · box-fan airflow from NIOSH bench tests (Derk et al. 2023); cube single-pass capture calibrated to UC Davis (Dal Porto et al. 2022) and HouseFresh in-room reviews.

Infection-risk reduction

A purifier delivers its CADR as equivalent clean air. In a well-mixed room the steady-state airborne-pathogen concentration is inversely proportional to the total clean-air supply rate, so adding the unit changes that rate from the room's baseline ventilation B to B + CADR. The relative reduction in long-range airborne dose (and, for a fixed exposure, infection risk) is:

Working in air changes per hour cancels the room volume, so the only extra input is the room's own ventilation, which you set with the Room ventilation field in Advanced (default 1 ACH; roughly 0.3 for a sealed room, 1 for a typical home, 3+ if well ventilated). This is the Wells-Riley result: raising the clean-air supply Q lowers risk in proportion to 1/Q.

Sources: Rudnick & Milton, 2003 (rebreathed-fraction / Wells-Riley) · dilution-based Wells-Riley expansion (well-mixed limits).

Total airflow: the operating point

A fan and a filter meet at one operating point: the pressure where the fan can still push as much air as the filter will pass. Each fan model carries a normalised pressure/flow (PQ) curve scaled to its rated free-air flow Q₀ and maximum static pressure P₀. The filter's resistance is a quadratic in face velocity V (FPM), the standard media-resistance form:

The tool solves by bisection for the ΔP where N · fan(Q) = filter(Q), then converts that airflow to CFM/m³h. Effective pleated-media area is derived from the filter's outer dimensions and grows with depth (up to 1.8× for deep media). When the honeycomb grill is enabled, its restriction is added in series with each fan as an Idel'chik thin-perforated-plate loss (a deliberately conservative model, so it never over-states airflow). Reported airflow is after the 0.85 build-efficiency factor.

Sources: the MERV-13 resistance coefficients, the effective-area formula, and the 120/140 mm fan curve shapes are reverse-engineered from Nukit and community data built on 3M Filtrete lab data and Arctic P12/P14 PQ curves; the solver reproduces that sheet's worked example to ~0.1%. Each fan's free-air flow Q₀ and static pressure P₀ come from independent Cybenetics LW-9266 bench measurements where available, rather than manufacturer marketing specs. The fan-curve / system-resistance intersection is standard fan engineering; the grill loss uses I. E. Idel'chik, Handbook of Hydraulic Resistance (thin-perforated-plate coefficient).

Face velocity

The air speed through the filter media: airflow divided by the effective media area, taken from the resistance curve at the operating ΔP and converted from ft/min to m/s (× 0.00508).

It matters because a filter's rated efficiency is measured at a reference face velocity (around 2.5 m/s in ASHRAE 52.2 testing). The large media area of a multi-filter box runs far below that, which tends to improve capture and keep pressure drop (and noise) low.

System pressure drop

The static pressure the fans work against at the operating point, where fan output equals the filter (plus grill) resistance, converted from inH₂O to pascals (× 249.09). Higher pressure drop means a clogged or undersized filter relative to the fans; it pushes the operating point to lower airflow.

Filter efficiency

The single-pass fraction of particles the media captures. ANSI/ASHRAE 52.2 reports MERV efficiency in three particle-size bands: E1 (0.3–1 µm), E2 (1–3 µm), and E3 (3–10 µm). This tool uses the published per-band values for a MERV-13 furnace filter (62% / 87% / 95%) and averages them to about 81% as the single headline figure; the per-band numbers are shown in the breakdown. The box-fan cube pushes air through the media much faster, so it uses a conservative whole-system single-pass figure instead (0.43 for 1" media, 0.50 for ≥2"), consistent with the 41–58% effective efficiency measured at UC Davis and calibrated so the cube's CADR matches independent room reviews.

A packaged particle filter (such as the IKEA STARKVIND, an EPA12 unit rated at about 99.5% of PM2.5) is not a furnace pleat, so it is modeled with its own rated single-pass efficiency and a plain frontal-area resistance, both calibrated so a measured reference box reproduces its published CADR. The Performance panel shows that filter's own grade (for example EPA12) on the efficiency tile.

Sources: ANSI/ASHRAE 52.2 (MERV / E1–E3 bands); per-band values from the filter manufacturer's published MERV-13 data; cube efficiency from Dal Porto et al. 2022 (UC Davis).

Power & current

PC fans run on 12 V: total current is the per-fan rated current × the number of fans, and power is P = 12 V × I. The box fan runs on mains: the figure is its rated wattage at the selected speed, and current is I = P / 120 V. Per-fan specs come from each model's datasheet, or from the values you enter for a Custom fan.

Cost to run (24/7)

Energy use at the build's maximum-power draw, run continuously, priced at your electricity rate:

It assumes the fans run flat-out around the clock, an upper bound. Running slower, or only when needed, costs proportionally less. The rate shown by default is the approximate US residential average; set your own from a recent bill in Advanced.

Air changes per hour (ACH)

How many times per hour the build can process a volume of clean air equal to the whole room:

Room volume comes from the width, length, and ceiling height you enter. Public-health guidance for infection control targets at least 5 ACH of clean air in occupied spaces (US CDC), and ASHRAE Standard 241 frames the same idea as an "equivalent clean airflow" rate. A purifier's CADR counts toward that target alongside any fresh-air ventilation.

Sources: US CDC, ventilation (5+ ACH) · ASHRAE Standard 241.

Noise

For PC-fan builds, each fan's spec dBA is first mapped to a realistic in-enclosure level: Arctic fans use a measured linear fit (0.461 × spec + 26.4) and other brands use spec + a 3.4 dB enclosure offset. The fans are then summed as independent sources (+10·log₁₀(N)) and corrected from the 3 ft spec distance to 1 m (≈ −0.78 dB). The box-fan build uses the whole unit's measured dBA at the chosen speed, distance-corrected. These offsets are calibrated against HouseFresh in-room measurements, so the "real-world" figure is usually higher than the bare spec.

Sources: incoherent-source addition (standard acoustics, L = 10·log₁₀ Σ 10^(Lᵢ/10)); calibration against HouseFresh measurements.