CFM and m³/h — sizing ductwork across unit systems
1 CFM = 1.699 m³/h = 0.472 L/s. Simple ratios that hide a bigger issue: ventilation codes across regions target very different air-change rates.
Fan datasheets, air handlers, VAV boxes and register throws are all sized by volumetric airflow. The US uses cubic feet per minute (CFM). Europe uses cubic metres per hour (m³/h) or litres per second (L/s). ASHRAE and EN both specify minimum ventilation rates — but in different units and with different assumptions.
The exact factors
1 CFM = 1.69901 m³/h = 0.47195 L/s (exact) > 1 m³/h = 0.5886 CFM = 0.2778 L/s > 1 L/s = 2.119 CFM = 3.6 m³/h
Codes
ASHRAE 62.1: minimum 5 CFM/person + 0.06 CFM/ft² for offices (about 7.5 L/s + 0.3 L/s·m²). EN 16798: 10 L/s/person for good IAQ (Category II). These translate to roughly comparable per-person rates but the per-area component differs materially — a European engineer sizing to EN often exceeds ASHRAE minimums.
Air changes per hour
ACH = fresh air rate / room volume. A 40 m² office at 3 m ceiling = 120 m³. Ventilating at 60 L/s = 216 m³/h gives 1.8 ACH. ASHRAE typically results in 0.5–1.5 ACH for offices, 4–6 for restaurants, 15+ for laboratories. Same physical rate, three different equivalent expressions (CFM, m³/h, ACH) — engineers switch depending on what the datasheet lists.
Worked example — a bathroom exhaust fan
US code (IRC): 50 CFM intermittent or 20 CFM continuous for a residential bath. 50 CFM = 85 m³/h = 23.6 L/s. UK Building Regs Part F: 15 L/s for a bath = 54 m³/h = 31.8 CFM. UK spec is 37% lower than US intermittent — reflecting different assumed run times and moisture-load assumptions.
The five-second recap
Multiply CFM by 1.7 for m³/h. Multiply CFM by 0.47 for L/s. Ventilation codes agree on per-person rates but diverge on per-area rates.
Try the converters mentioned in this article
FAQ
What is a 'ton' of air conditioning?+
12,000 BTU/hr of cooling = 3.517 kW. Historical: the heat needed to melt 1 short ton of ice in 24 hours. Airflow of a 1-ton system is typically ~400 CFM (680 m³/h).
Why is L/s common in Europe?+
EN 16798 chose L/s because per-person ventilation rates land in comfortable single digits. m³/h is more common on equipment datasheets.
How does duct velocity compare?+
Typical supply duct velocity: 4–8 m/s (800–1,600 fpm). Higher = noisier. Sizing charts convert between airflow and duct diameter given a target velocity.
This article was written by The Turbo Unit Converter engineering desk (HVAC engineering) and last reviewed on 2026-06-30 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.