An office in England needs at least 10 litres per second of fresh air for each person, under Approved Document F of the Building Regulations. Many designs aim higher, commonly 12 l/s per person, to give more comfortable, lower-CO₂ air. Fresh air per person is the single most useful number in an office ventilation design. It's easy to explain, easy to check against a target, and it tells you more about how a room will feel than almost anything else on the drawing.
This guide covers the regulatory minimum, the targets designers use, how to work out what a floor or room actually gets, and how CO₂ readings show whether it's working.
1Fresh air per person: the unit
- 75 l/s of fresh air in
- 240 l/s circulated
Ventilation for people is measured as outdoor air delivered per occupant, in litres per second per person (l/s per person, sometimes written l/s/p). It is the outdoor air that counts, not the total air moving around the room. A fan-coil unit might circulate 240 l/s, but if only 75 l/s of that is fresh air from outside, the 75 l/s is the figure that matters for the people in the room.
The other common measure, air changes per hour (ACH), describes the volume of the room replaced each hour. ACH is useful for rooms where the contaminant isn't people, such as a plant room or a kitchen. For offices it hides the most important variable, how many people share the air. A room can have a respectable ACH and still be stuffy if it is full.
2The regulatory minimum: Approved Document F
In England, Approved Document F (Ventilation) gives the guidance that satisfies the Building Regulations for ventilation. Its 2021 edition, in force since June 2022, gives a whole-building ventilation rate for offices of 10 l/s per person, alongside extract rates for toilets, kitchens and other wet or polluting rooms.
Scotland and Wales have their own building standards and guidance, so check the version that applies to the project's location. The principle, a minimum outdoor air rate per person, is the same.
Treat 10 l/s per person as the floor of what is acceptable, not a design target. A design that only just meets it at the expected headcount has no margin for busy days, denser layouts or rooms that fill up.
3What designers aim for in practice
- About 850 ppm at 12 l/s per person
Many UK office specifications set a higher design rate, commonly 12 l/s per person, and some go further. The figure usually comes from three places:
- Industry guidance. The British Council for Offices (BCO) publishes a widely used guide to office specification, and its fresh-air recommendation has long sat above the regulatory minimum. Check the current edition for the figure, because it is reviewed with each update.
- The client's brief. Occupiers with a wellbeing agenda often ask for more fresh air, sometimes as a percentage uplift on the minimum.
- Certification. Schemes such as BREEAM, WELL and NABERS UK may reward, or require, ventilation beyond the minimum, and some need it to be measured or monitored.
More fresh air is not free. Twenty per cent more outdoor air means larger air-handling plant, larger ductwork, more fan energy, and more heating and cooling of that air, unless heat recovery claws much of it back. That trade-off is why the design rate deserves an explicit decision rather than a default.
4How occupancy density changes the sum
Fresh air per person is a division, and the denominator is people. A floor's installation delivers a fixed volume of outdoor air. How much each person gets depends on how many people share it.
A worked example:
| Floor as designed | Same floor, densified | |
|---|---|---|
| Net internal area | 1,000 m² | 1,000 m² |
| Occupancy density | 1 person per 10 m² | 1 person per 7 m² |
| People | 100 | 143 |
| Outdoor air delivered | 1,200 l/s | 1,200 l/s |
| Fresh air per person | 12.0 l/s | 8.4 l/s |
Same ductwork, same plant, and the floor has dropped below the regulatory minimum because the layout changed. This is why "how many people can this floor take?" is really a ventilation question, and why it should be answered before the desks are ordered.
To find the supportable headcount, run the division the other way:
With 1,200 l/s and a design rate of 12 l/s per person, the floor supports 100 people. At the 10 l/s minimum it supports 120. Anything beyond that needs more fresh air, not just more desks.
5Rooms, not just floors
- Boardroom: 4.7 l/s per person, ~1,530 ppm
- Open plan: ~12 l/s per person
Even when the floor-wide figure is fine, individual rooms can be starved. The meeting room is the classic case. Fresh air spread evenly across an open-plan floor might give a 20 m² room enough for two people. Put twelve people in it for an afternoon and the fresh air per person falls to a fraction of the target.
A good design checks the figure zone by zone, especially for:
- meeting and training rooms,
- boardrooms and client suites,
- breakout and tea-point areas at lunchtime,
- any room with a door that is usually closed.
The usual fixes are a dedicated fresh-air branch to the room, sized for its full occupancy, often with a motorised damper that opens when a CO₂ sensor says the room is in use. These rooms also need a way for air to leave, such as transfer grilles or a ducted return. Our guide to Cat A vs Cat B fit-out explains why these rooms are so often missed when a floor is fitted out.
6What CO₂ tells you
- Dedicated branch: 12.2 l/s per person, ~850 ppm
People breathe out carbon dioxide, so the CO₂ concentration in a room is a convenient stand-in for how well it is ventilated for the people in it.
- Outdoor air is roughly 420 ppm (parts per million).
- Around 1,000 ppm or below is a common design aim for a well-ventilated office.
- Readings that stay above about 1,500 ppm are widely treated as a sign of inadequate ventilation. The HSE's guidance on CO₂ monitoring in workplaces uses this level as the point to take action.
Approved Document F also recommends CO₂ monitoring in offices, so that occupants and building managers can see when a room isn't getting enough fresh air.
From l/s per person to ppm
In steady conditions, a room's CO₂ settles where the fresh air coming in balances what people breathe out. The relationship is simple:
A seated office worker breathes out roughly 0.0052 l/s of CO₂ (published values vary with activity and body size). That gives:
| Fresh air per person | Steady CO₂ (approx.) |
|---|---|
| 5 l/s | 1,460 ppm |
| 8 l/s | 1,070 ppm |
| 10 l/s | 940 ppm |
| 12 l/s | 850 ppm |
| 15 l/s | 770 ppm |
These are well-mixed estimates for full occupancy. Real rooms vary with how well the air mixes (see age of air explained). CO₂ also takes time to build up after people arrive, which is why a short meeting can feel fine and a long one doesn't.
7How to check a design
- Agree the design occupancy, floor-wide and for each enclosed room, including peaks such as all-hands meetings and lunchtime breakouts.
- Agree the design rate, the regulatory minimum and the brief's target.
- Sum the outdoor air per zone from the schedules: terminal unit fresh-air rates, fresh-air diffusers, heat-recovery units.
- Divide, and flag any zone below target at its design occupancy.
- Estimate CO₂ for the flagged rooms at full occupancy, and for the time they'll realistically be full.
- Decide the fix: a dedicated branch, a CO₂-led damper, a heat-recovery unit for the room, or a lower occupancy limit for it.
8Putting it in front of a client
The numbers on their own rarely land. What lands is seeing them change: dragging the occupancy up and watching the fresh air per person fall, or watching a meeting room's CO₂ climb through a full afternoon while the open plan stays green.
In an HVAC Viewer model, the figures panel shows fresh air per person, space per person, cooling and estimated CO₂ against the project's design targets, and every figure updates when the occupancy changes. The airflow is indicative, driven by the design figures rather than a full CFD study, but it makes the question visible, which is usually the point.
9Frequently asked questions
How much fresh air per person does an office need in the UK?
In England, Approved Document F gives a minimum whole-building ventilation rate for offices of 10 l/s per person. Many office designs target more, commonly 12 l/s per person, following industry guidance and the client's brief. Scotland and Wales have their own building standards.
What is a good CO₂ level for an office?
Outdoor air is about 420 ppm. Around 1,000 ppm or below is a common design aim for a well-ventilated office. Readings that stay above about 1,500 ppm suggest the room is not getting enough fresh air for the people in it.
How do I work out how many people an office can hold?
For ventilation, divide the outdoor air the system delivers by the design rate per person. A floor with 1,200 l/s of fresh air supports 100 people at 12 l/s per person, or 120 at the 10 l/s minimum. Space per person, means of escape and toilet provision also set limits.
Is air changes per hour the same as fresh air per person?
No. Air changes per hour describes how often a room's volume is replaced. Fresh air per person describes how much outdoor air each occupant gets. For offices, fresh air per person is the better measure, because the main source of stale air is the people in the room.
Why is my meeting room stuffy when the rest of the office is fine?
Usually because the room holds far more people than its share of the floor's fresh air was designed for, or because air can't easily leave the room. A dedicated fresh-air supply sized for the room's full occupancy, with a route for air out, normally fixes it.