Ventilation calculation → Calculate your natural ventilation requirements in 3 steps

A guide on ventilation calculation – work out ventilation requirements for new buildings. 

Ensuring your buildings have adequate and effective ventilation is essential for compliance reasons as well as for the safety and comfort of the building’s occupants. Since COVID-19, architects and construction companies are expected to pay special attention to the airflow in all buildings, from offices to commercial properties, and from homes to factories. 


In this article, we’ll look at:

1. Ventilation requirements 

– Non-residential

– Other settings (whisky storage facilities)

2. How to calculate your ventilation requirements

– Formula

3. Types of ventilation

4. Types of ventilation

5. Specifying your project

6. Natural ventilation products


Ventilation Requirements

Adequate ventilation will ensure a steady supply of fresh air into a building which has numerous benefits.

Benefits of proper ventilation:

– Moderating temperatures
– Moderating humidity
– Replenishing oxygen
– Managing and minimising the build up of moisture, odours, bacteria, dust, carbon dioxide, smoke or other contaminants that can accumulate over time
– Creating air movement to improve the comfort of the occupants
– It will also aid in reducing the transmission of viruses like Covid-19

3 steps to ventilation calculation:

1. Classification

– You need to understand the classification of your building to determine the standard of air quality you need to maintain, and the volume of air you need to have circulating

2. Get your data

– You’ll need to have an idea of the number of occupants who will be in the building, the free area % of the vents, and the velocity of air.

3. Use our formula

– Volume (m³/s) ÷ (Average Wind Velocity (m/s) x Cd) = Free Area required (m²).

Input your data into the formula to find out the area in sq m of louvres you will need. 

These calculation can be complex due to the nature of the data you need and the variables within that data. For help specifying your louvres and ventilation requirements, contact our technical team

Your classification

The CIBSE (Chartered Institution of Building Services Engineers) Guide B2 provides the following guidance, laying out the recommended ventilation rates for air quality.

ClassificationIndoor Air Quality StandardVentilation Range (l/s per person)Default Value (l/s per person)
Table 1: Ventilation requirements (reproduced from BS EN 13779-2007)

These rates are per person and apply to non-residential buildings. Please note that classroom ventilation has a separate standard which is covered in our guide here. Warehouses for the storage of whisky also have their own requirements following the move to reclassify whisky as a dangerous liquid. 

Building regulations Part F (NBS 2013b) require a minimum ventilation rate of 10l/s per person for office applications. As a rule the fresh air supply for other workplaces should fall between 5 and 8 litres per second per person (please note that there are other factors which apply but for a rule of thumb this should be sufficient).

Maintaining a good level of natural ventilation is beneficial in any workplace but the added risk of Covid 19 transmission has made the provision of airflow even more important.

Get an expert opinion and support calculating your requirements by contacting our team today.


How to do a ventilation calculation

Once you understand the requirements and your classification, you will need to apply these to your building. Below is our guide.

This guide is intended to provide a basic toolkit to allow you to calculate general ventilator sizes based on the number of occupants in a given building. Please note that other factors such as varying wind speeds and directions, solar gain and pressure drops (Pd) are not included in this guide but the calculations below should give you a rough estimate of requirements.

You can find average wind speeds and directions using this data from the MET Office.

Ideally, all natural ventilation systems will have a low level inlet and a high level outlet to enable a good cross flow of air within a given space. For example if you required 2m² of inlet vents then the equivalent number of outlet vents should also be installed

Based on cross flow, ventilation rates are calculated using the average wind velocity of the incoming air on the face of the louvres, the free area % of the vents being used and the volume of air required in m³ per second. 

The data you need (With example)

– Average wind velocity: (2m/s)
– The free area % of the vents: (48%)*
– Co-efficient of discharge (Cd) (0.29)*
– Use the product specification

(McKenzie Martin products free area % and Cd values can be found in the table below (Table 2). In this example, we are using the Maximair ‘A’ Series.)

– Volume required: 0.2m³/s 

(Calculate this by using the table above (Table 1) to determine the airflow you require. This then needs to be converted to m³/s which you can do by dividing by 1000. If we have an office of 20 people, we need 200l/s according to the table above. We divide 200 by 1000 to give us 0.2m³/s)

The formula for ventilation calculation:

Volume (m³/s) ÷ (Average Wind Velocity (m/s) x Cd) = Free Area required (m²)

0.2m³/s ÷ (2m/s x 0.29 = 0.58) = 0.34m² free area required

Now you have the free area requirement in m² you can calculate the Ventilator area required using the Louvre free area %.

Free Area required (m²) ÷ Louvre Free Area = Answer

Answer x 100 = Vent Area (m²)

An “A Series” weather louvre provides 48% free area so

0.34m² ÷ 48 = 0.007

0.007 x 100 = 0.7m²

PLEASE NOTE – if your calculator has a % function then dividing by 48% in the first calculation will remove the need to times the result by 100.

Once you have your overall m² measurement you can size your louvres accordingly. The above calculation would require a louvre 0.837m wide x 0.837m high for a total area of 0.7m². These sizes can be modified to suit the situation on site, for example a louvre 1m wide by 0.7m high would provide the same area.

Our team can help you to calculate the size of the louvres required. This can be a complex calculation due to variable doctors like wind speed. If you require support, contact us today.

Types of ventilation

There are two key types of ventilation in building: natural, and mechanical. Mechanical ventilation is driven by fans or some other mechanism. A number of factors can influencer an architect’s decision to use one or the other.

Why choose natural ventilation?

Natural ventilation has the benefit of being most cost- and energy-efficient, as it’s driven by the differences in pressure between inside and outside the building or between two rooms. It’s cost less to build, operate and maintain than mechanical ventilation, and can be more flexible in terms of appearance, making it a preferable choice for architects. 

Reasons to choose natural ventilation:

– Cost-effective
– Quiet
– No running costs
– Minimal maintenance 
– Allows a flow of fresh air
– Custom-made for every order
– A wide range of products to suit every project

Specifying your order

We have a wide range of products at McKenzie Martin to suit projects of all shapes and sizes. Our louvres, grilles and ventilation systems are made to measure to cater to a variety of buildings and uses. Some of our products along with the relevant ventilation calculation data that you’ll need can be found below.

ProductFree AreaCo-efficient discharge (Cd)Inlet per m² @ 2m/s Average win velocity m/2
Maximair “A Series” Weather Louvre48%0.29280 l/s approx.
Maximair 100 Louvre52%0.35360 l/s approx.
Maximair 75 Louvre43%0.33280 l/s approx.
Maximair 50 Louvre49%0.24230 l/s approx.
Acoustic Louvre 300mm30%0.28170 l/s approx.
Acoustic Louvre 150mm34%0.23155 l/s approx.
Acoustic Louvre 100mm33%0.22145 l/s approx.
Thermac 200048% (fully opened)0.29280 l/s approx.
Operating Box Louvre80% (fully opened)0.45720 l/s approx.
Fixed Blade Louvre50%0.17300 l/s approx.
Triple Bank Louvre46%0.30170 l/s approx.
Table 2. A full list of our products is available here.

To find out more about our products or to discuss your requirements, get in touch with our sales team on 0161 723 2234 or email Our experienced technical and design team design every product to order to ensure you get the results you want.