CADR Calculator for Air Purifiers | indoorair.guide

Size an air purifier to your room. The CADR Calculator computes the minimum Clean Air Delivery Rate you need based on floor area, ceiling height, occupant profile, and how quietly you want the unit to run. Defaults are calibrated for high-pollution Indian conditions.

• Inputs: room length × width × height, occupant profile, target fan-speed mode
• Outputs required CADR in m³/h, ft³/min (cfm), and matching rated CADR
• 5× ACH baseline for healthy adults; 6× for sensitive occupants; 8× for high-pollution cities
• Noise-adjusted sizing — rated CADR is scaled up for sleep, living, or office modes
• 20–30% buffer guidance for outdoor AQI spikes and envelope leakage
• Pairs with the ACH Calculator to back-check a purifier you already own

The 5 ACH baseline

Clean Air Delivery Rate (CADR) is the standardised volume of clean air a purifier delivers per unit time, measured against smoke, dust, and pollen by AHAM AC-1 or a comparable test. The calculator converts your room volume into a required CADR using air changes per hour as the bridge: required CADR = room volume × target ACH.

A 5 ACH target is the working baseline for healthy adults in Indian metros. It keeps indoor PM2.5 within WHO interim targets when outdoor AQI is moderate-to-high. The calculator raises that to 6 ACH for sensitive occupants — children under 5, elderly, pregnant women, asthma and COPD patients — and to 8 ACH for sustained high-pollution conditions such as Delhi-NCR October through February.

Why a noise adjustment matters

Manufacturer-rated CADR is measured at the highest fan speed, which is rarely the speed you actually run. At bedroom-friendly noise (≤30 dB) a typical purifier delivers about 45% of its rated CADR; medium-speed living-room operation delivers about 60–75%. The calculator inverts that ratio so the rated CADR it recommends will still meet your ACH target at the noise mode you select — sleep, living, office, or max.

The result is two numbers: the required CADR (what your room actually needs) and the rated CADR (the headline number to look for on a spec sheet). Buying to required CADR alone is a common mistake — the unit will hit the target on max only, and most people never run their purifier on max.

Buffers and edge cases

A 20–30% buffer over the calculated CADR is standard practice for Indian conditions. It absorbs three real-world losses: rated CADR measured in a sealed chamber versus a real room with door and window leakage, filter loading reducing airflow over the replacement cycle, and short-duration outdoor AQI spikes from traffic, cooking, or stubble burning.

Open-plan layouts (kitchen merged with living room) should be sized to the full combined volume, not the room the purifier sits in. Tall ceilings — common in older flats and bungalows — need explicit height input rather than the 9 ft default. For very small rooms below 100 sq ft, oversizing makes little practical difference and lets you run the purifier on a lower, quieter setting.