ACH Calculator for Air Purifiers | indoorair.guide

Calculate the air changes per hour (ACH) a purifier can deliver in your room. Enter floor area, ceiling height, and the purifier’s rated CADR to see whether the unit meets the 5 ACH target recommended for moderate-to-high pollution conditions in Indian cities.

• Room volume from floor area and ceiling height (defaults to 9–10 ft)
• ACH from CADR — accepts m³/h, ft³/min, and cfm
• Flags rooms where a purifier is undersized for 5 ACH
• Includes 20–30% buffer guidance for Delhi-level pollution loading
• Pairs with the CADR Calculator to size a purifier from scratch

Why 5 ACH for India

Air changes per hour measures how many times a purifier can theoretically clean the entire volume of air in a room in one hour. The widely-quoted 2 ACH "minimum" comes from low-pollution Western indoor environments and is inadequate for Indian conditions. Outdoor PM2.5 in most Indian metros routinely exceeds 60 µg/m³ during winter and pre-monsoon months, and door, window, and envelope leakage continuously reintroduces outdoor particles into the indoor space.

A 5 ACH target — recommended by AHAM 2/3 sizing and reinforced in independent reviews for South Asian cities — keeps indoor PM2.5 within reach of the WHO interim target even when outdoor AQI peaks. For sensitive occupants (asthma, COPD, children under 5, elderly) or rooms with infiltration-heavy construction, 6–8 ACH is more appropriate.

How the calculation works

The tool computes room volume as floor area × ceiling height. ACH is then (CADR × 60) ÷ volume when CADR is in ft³/min, or CADR ÷ volume when CADR is in m³/h. Manufacturers usually publish "smoke" or "particle" CADR — that is the figure to use, not the dust or pollen CADR which test against larger particles.

Rated CADR is measured at the purifier’s highest fan speed. Real-world delivered CADR at sleep-friendly speeds is typically 40–60% of rated, so a unit that just meets 5 ACH on paper will under-deliver at night. Applying a 20–30% buffer when sizing — or running the purifier on medium-high during peak pollution hours — closes that gap.

When to use this tool

Use the ACH Calculator to back-check a purifier you already own or are considering buying. If the calculated ACH falls below 5, either move the unit to a smaller room, add a second purifier, or step up to a higher-CADR model. Use the CADR Calculator instead when you are starting from a room and need to derive the minimum purifier capacity.