What is the minimum ventilation rate for a UK office?

CIBSE Guide A recommends 10 l/s per person for general offices, with a minimum area-based rate of 1.0 l/s per m². For a typical open-plan office at 10m² per person, this equates to approximately 4-6 air changes per hour. The Workplace (Health, Safety and Welfare) Regulations 1992 require "effective and suitable" ventilation, and CIBSE Guide A provides the specific rates to achieve this.

How many air changes does a restaurant need?

A restaurant dining area typically requires 8-12 ACH per CIBSE Guide A. The recommended fresh air rate is 10 l/s per person plus 1.5 l/s per m² of floor area. With typical restaurant occupancy of 0.7 persons per m² (one person per 1.4m²), this results in a high ventilation rate. The kitchen area requires separate, much higher extraction — see our Cooker Hood Extraction Calculator.

What ventilation does a gym or fitness studio need?

Gyms and fitness studios require 12 l/s per person (higher than offices due to increased metabolic rates during exercise) plus 2.0 l/s per m² of floor area per CIBSE Guide A. Typical occupancy density is 0.15 persons per m² (approximately 6.5m² per person). Temperature control is also critical — CIBSE recommends 14-16 degrees C for gym spaces, achieved through a combination of ventilation and cooling.

How do I calculate occupancy for ventilation design?

CIBSE Guide A Table 1.5 provides typical occupancy densities for different room types. For example: open-plan offices at 0.1 persons/m² (10m² per person), restaurants at 0.7 persons/m² (1.4m² per person), and classrooms at 0.5 persons/m² (2m² per person). For Building Control submissions, the maximum legal occupancy should be used. Our calculator uses CIBSE typical densities as defaults but allows manual override.

What is the difference between person-based and area-based ventilation rates?

Person-based rates (l/s per person) account for the CO2 and bioeffluents produced by occupants and are the primary design method for occupied spaces. Area-based rates (l/s per m²) account for building-related pollutants like VOCs from furniture, carpet, and finishes. CIBSE Guide A recommends using the higher of the two calculated rates. For sparsely occupied spaces, the area-based rate often governs; for densely occupied spaces, the person-based rate dominates.

Commercial Ventilation Calculator — Fresh Air Rates

How We Calculate This

This calculator determines the fresh air ventilation rate for commercial spaces using the outdoor air supply rates from CIBSE Guide A Table 1.5. It calculates both person-based and area-based ventilation rates and uses the higher value.

Design method

Total ventilation = MAX(Person rate x Occupancy, Area rate x Floor area)

The person-based rate accounts for CO2 and body odour generated by occupants. The area-based rate accounts for emissions from the building fabric and furnishings. The higher of the two ensures adequate air quality under all conditions.

Occupancy densities

Where the actual occupancy is not known, CIBSE Guide A typical densities are used. These range from 0.04 persons/m\u00B2 for hotel bedrooms to 0.8 persons/m\u00B2 for lecture halls. The calculator allows manual override for specific projects.

Duct sizing

The main supply duct diameter is calculated from the total ventilation rate and design velocity (default 5 m/s for commercial systems). Lower velocities reduce noise but require larger ducts.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.