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About this tool
Size a kitchen or bathroom exhaust fan from room volume and air changes per hour, with duct diameter and fan body size.
This calculator sizes a bathroom or kitchen extractor fan using the air-change method — airflow equals room volume multiplied by the air changes per hour required for that room type — and then checks the answer against the Home Ventilating Institute rules and the ASHRAE 62.2 local-exhaust minimums of 50 CFM for a bathroom and 100 CFM for a vented range hood. It adds a duct-resistance allowance of about 2% per metre of run and 5% per 90-degree bend, then returns the fan rating, fan body size and round duct diameter at a 4.5 m/s design velocity. Useful for anyone specifying an extractor for a renovation and choosing between a 100 mm and a 150 mm unit.
Open Exhaust Fan Size Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Reports the air-change result and the HVI or code minimum side by side and tells you which one governs.
A long run with several bends can need 15-25% more fan than the bare room calculation suggests.
Enter metres or feet and read the answer in CFM and m³/h, with fan and duct sizes in mm and inches.
Multiply the room volume by 8 air changes per hour, then take at least 50 CFM. A 2.4 × 1.8 × 2.7 m bathroom is 11.7 m³, which at 8 ACH is 93 m³/h or about 55 CFM — so a 50 CFM fan is marginal and a 100 mm unit with a short duct is the practical choice.
Multiply m³/h by 0.5886 to get CFM, or multiply CFM by 1.699 to get m³/h. A fan advertised at 100 CFM moves about 170 m³/h, which is why European and North American spec sheets for the same fan look so different.
Yes, substantially. Allow roughly 2% of airflow per metre of straight duct and 5% for each 90-degree bend, so a 6 m run with three bends needs about 27% more rated airflow than the room calculation alone. Flexible duct is worse than rigid, and a crushed or sagging flex run can halve delivery.
About 15 ACH for a domestic kitchen, against 8 ACH for a bathroom and 6 ACH for a garage or general room. ASHRAE 62.2 also sets a floor of 100 CFM for an intermittently used vented range hood, which governs in small kitchens where 15 ACH alone would give a lower figure.
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