One litre per second is exactly 3.6 cubic metres per hour. The 3.6 is the same number that converts metres per second to kilometres per hour, and for exactly the same reason: 3,600 seconds per hour divided by 1,000 litres per cubic metre.
About the units
Litres per second is the ventilation unit of the Nordic countries and parts of Europe, where air flow requirements in building regulations are written in it. Cubic metres per hour is the convention in Germany, the Netherlands and much of the rest of Europe, and it dominates equipment catalogues. The two describe identical flows, and a ventilation system designed against a Swedish requirement in l/s but specified from a German catalogue in m³/h needs the conversion applied consistently at every step.
The exact factor
The factor is exactly 3.6. Reference values from building ventilation: a bathroom extract is typically 10 to 15 l/s, which is 36 to 54 m³/h; a kitchen hood 25 to 75 l/s or 90 to 270 m³/h; and a whole dwelling might need 0.35 l/s per square metre of floor area, which for 100 m² is 35 l/s or 126 m³/h. That last figure is a regulatory minimum in Swedish building rules and is the number every residential ventilation design starts from.
Where you meet this conversion
Ventilation design and commissioning throughout Europe. Air handling unit performance, duct sizing, diffuser selection and measured commissioning values all appear in one unit or the other. Water supply engineering uses litres per second for mains and cubic metres per hour for pumps. Wastewater and stormwater design uses l/s for peak flows and m³/h for treatment capacity. Anyone working across national standards converts constantly, and the 3.6 is the single most used number in the field.