Airflow Units (CFM ⇄ m³/h ⇄ L/s)
The three dialects of volumetric airflow, converted exactly.
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The engineering
Same air, three accounting systems: American ductwork and fan curves speak CFM, European AHU datasheets speak m³/h, and ventilation codes increasingly speak L/s per person. The factors are exact because the foot has been defined as exactly 0.3048 m since 1959 — no rounding drama, just cubes.
The classic 3 a.m. mistake is m³/h versus m³/min on Asian-market fan specs — a factor of 60 that will size your duct like a drinking straw or a subway tunnel. When a number looks off by 'about 60' or 'about 1.7', it's this card.
Where this math comes from
The cubic foot per minute is a child of Victorian mine and building ventilation — engineers like John Theophilus Desaguliers (who measured air per person a century earlier, circa 1727) and the mine-safety inspectors of the 1800s needed air quantified, and the foot and the minute were what Britain had. The metric side descends from the French Revolution's meter (1795) and the SI formalization of 1960.
Neither side ever surrendered: ASHRAE tables carry both to this day, and the 1959 international yard-and-pound agreement at least made the conversion exact. Global HVAC engineering is now permanently bilingual, and this card is the phrasebook.
- 1727J. T. DesaguliersEarly quantified ventilation — air per occupant (circa).
- 1795French RepublicThe metre — the metric flow units' ancestor.
- 1959International yard & pound agreement1 ft = 0.3048 m exactly; the conversion becomes exact.
- 1960CGPMSI adopted — m³/h and L/s standardized.
See the full timeline of the math behind every calculator →
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