# Carbon filter match calculator

> Free carbon filter calculator: match a grow-room carbon filter to your fan's real airflow, check contact time through the carbon bed and get the filter-before-fan layout right.

Interactive: https://genetixbay.com/knowledge/tools/carbon-filter.html

Buy a filter rated for at least the air you actually push through it, with the same flange size as your fan. Bigger is fine: slower air through more carbon smells better and lasts longer. Hang it so the fan pulls through it: filter, then fan, then out.

## How it works

### Match the airflow

Filter rating ÷ the airflow you run should be 1.0–1.5. Under 1.0, smell bleeds through and the carbon wears out sooner.

### Contact time

Contact time = carbon bed volume ÷ airflow. A 10 × 16 inch filter with a 1.5-inch bed holds about 10.5 L of carbon; at 330 CFM air spends about 0.07 s in it. Slower air or more carbon means more time to grab the smell.

### Layout

Filter → fan → duct → outside. Pulling through the filter keeps the whole run under negative pressure, so a leak sucks in rather than leaking smell. Use a pre-filter sleeve to keep dust off the carbon.

## Constants

| Constant | Value | Unit | What it is | Evidence |
|---|---|---|---|---|
| FILTER_MATCH_LO | 1 | × fan CFM | Filter rated airflow should be at least the airflow you actually run | Grower practice |
| FILTER_MATCH_HI | 1.5 | × fan CFM | Up to ~1.5× is better: slower air through the bed means longer contact and longer carbon life | Grower practice |
| M3H_PER_CFM | 1.699 | m³/h per CFM | Airflow conversion | Established |

## Sources

1. ASHRAE. 2019. ASHRAE Handbook: HVAC Applications, chapter 'Control of gaseous indoor air contaminants' (sorbent bed depth and residence time govern removal). https://www.ashrae.org/technical-resources/ashrae-handbook
2. NIST. Guide for the Use of the International System of Units (SP 811), Appendix B: conversion factors (1 W = 3.412 142 BTU/h; 1 ft³ = 0.028 316 8 m³; 1 US gal = 3.785 412 L). https://www.nist.gov/pml/special-publication-811