Commercial Ventilation Calculator — Fresh Air Rates

Approved Document F Volume 2 (2021) sets the whole-building minimum for offices as the greater of 10 l/s per person or 1 l/s per m² of floor area (whichever is higher). The 10 l/s per person figure also matches the CIBSE Guide A office recommendation. For a typical open-plan office at 10m² per person and a 2.7m ceiling, 10 l/s per person works out at only about 1.2–1.5 air changes per hour, because the space is sparsely occupied — ACH is not a useful target for offices, the per-person rate governs. The Workplace (Health, Safety and Welfare) Regulations 1992 require "effective and suitable" ventilation. Note that a full design to BS EN 16798-1:2019 sums the per-person and per-m² components and so gives a higher rate than this regulatory minimum.

A densely occupied restaurant dining area typically works out at roughly 8-12 ACH because of its high occupancy, not because ACH is the design target. The fresh-air rate is set per person (10 l/s per person) with an indicative building allowance of about 1.5 l/s per m²; the design takes the greater of the two. At a typical restaurant density of 0.7 persons per m² (one person per 1.4m²), the per-person rate dominates and produces that high effective ACH. The kitchen area requires separate, much higher extraction — see our Cooker Hood Extraction Calculator.

Gyms and fitness studios are usually designed at around 12 l/s per person — higher than the office 10 l/s because metabolic rate and CO2 output rise sharply during exercise — with the greater of the per-person rate or an indicative building allowance (this tool uses ~1.5 l/s per m²) taken as the minimum. A typical occupancy density is around 0.15 persons per m² (roughly 6.5m² per person). Temperature control is also important — gym spaces are commonly held cooler, around 14-16 degrees C, through a combination of ventilation and cooling.

Where the headcount is not yet known, designers use a typical occupancy density for the room type. Indicative figures are open-plan offices at about 0.1 persons/m² (10m² per person), restaurants at about 0.7 persons/m² (1.4m² per person), and classrooms at about 0.5 persons/m² (2m² per person). These are design conventions, not figures from a single standard, so confirm them against your project brief. For Building Control submissions the maximum intended occupancy should be used. This calculator applies these typical densities as defaults but lets you override the headcount or density directly.

Person-based rates (l/s per person) account for the CO2 and bioeffluents produced by occupants. Area-based rates (l/s per m²) account for building-related pollutants like VOCs from furniture, carpet, and finishes. Approved Document F Volume 2 (2021) sets the office whole-building minimum as the greater of the two — that is the method this calculator uses, so for sparsely occupied spaces the area rate can govern and for densely occupied spaces the person rate dominates. The current European design standard, BS EN 16798-1:2019, instead adds the two components together (qtot = n·qp + A·qB), which gives a higher design rate than the greater-of-two regulatory minimum.