Free tool
Indicative Room Sizing Guide
Before a survey, it helps to know roughly what size of system a room is in the region of — enough to plan, budget and ask sensible questions. This guide gives a capacity band and, more usefully, tells you which features of your room are the ones that will actually decide the answer.
What this tool can and cannot tell you
This is a screening guide, not a heat-load calculation, and it must not be used to select equipment. Floor area alone does not size an air-conditioning system: glazing, orientation, insulation, occupancy and heat-producing equipment routinely move the requirement by half again in either direction. Rooms with a significant process load — server and comms rooms, commercial kitchens, plant rooms — are not covered by any general rule and are flagged for a proper assessment instead of being given a number.
Why floor area alone does not size a system
Two rooms of exactly the same size can need very different systems. A north-facing bedroom in a well-insulated house and a west-facing garden room with a glazed wall can differ by a factor of two, and the floor plan looks identical for both.
What actually decides the requirement is how much heat gets into the room and how much stays there: the glazing and which way it faces, the construction of the walls and roof, how airtight the space is, how many people are in it, and what equipment is running.
That is why this guide asks about those things, and why it still gives a band rather than a figure. A survey measures them instead of asking you to estimate them, which is the difference between a screening estimate and a specification.
Your indicative band will appear here
Answer the questions and an indicative capacity band appears here, along with the things about your room that could move it. Nothing is sent anywhere.
How this is worked out
Our own screening method, written out in full. It is not taken from any published standard, and it is not a heat-load calculation — a survey does that.
Floor area is length × width. Volume is that area × ceiling height — the amount of air in the room, which is what the system actually has to condition, and the reason a tall room needs more than a short one of the same footprint.
A starting figure in watts is the floor area × a base rate for the room's use, which for this guide ranges from 100 W per m² for a bedroom to 200 W per m² for a conservatory.
That figure is then multiplied by a factor each for ceiling height, glazing, sun exposure, insulation and heat-producing equipment. Each factor is between 0.95 and 1.35, so the answers you give can move the estimate meaningfully in either direction without any one of them dominating.
Two answers deliberately change nothing at all in the arithmetic: whether the room is a loft conversion, and whether it is open plan. A loft is not automatically harder to condition — what decides it is the insulation, the roof glazing and which way the slope faces, and those are asked about directly. Open plan does not add load either, because the floor area already carries it. Both change how much weight to put on the result and what a survey should look at.
People beyond the number the base rate already assumes add 100 W each.
The result is placed in a capacity band — a range, not a single figure, and not the choice of a particular system. If it lands within 10% of the top of a band, the band is widened upward rather than rounded down, because a system that is slightly too small never quite satisfies the room.
These base rates and factors are ACE Climate's own indicative pre-survey figures. They are screening assumptions rather than design calculations, and they are not drawn from any published standard.
Spaces this guide will not put a number on
Server and comms rooms
Sized from the actual electrical load of the equipment, plus an allowance for resilience. Floor area is almost irrelevant, and cooling usually has to run continuously with a plan for what happens if it stops. This guide gives no band at all for one.
Commercial kitchens and restaurants
Cooking equipment, extract ventilation and a number of people that changes hour by hour make this a ventilation and heat-load design problem. You will get a band, and a clear flag that it is not enough to specify from.
Conservatories
Glazing and roof gains dominate everything else and vary through the day and the year. In some conservatories no reasonable amount of cooling holds a comfortable temperature on the hottest afternoons without shading as well, and we would rather say so than sell equipment that will disappoint.
Related services
What this tool is a first step towards.
Want this checked properly?
A survey measures the things this page had to ask you to estimate. It is free, there is no obligation, and you get an answer based on your building rather than on a general rule.
Before you rely on this
- This is indicative, not a specification. It is a guide to help you plan and ask better questions. It is not a design, not a quotation and not a substitute for a survey.
- A survey is what decides. An engineer measures the things this page can only ask you to estimate, and confirms where equipment can physically go, how it will be run and what the electrical supply allows.
- Installation and refrigerant work are regulated. Air-conditioning systems contain refrigerant under pressure and are connected to a fixed electrical supply. Installing, moving, opening, recharging or decommissioning one is work for a qualified, F-Gas registered engineer — never a DIY job.
- We do not publish a saving. Nothing on this page claims what you would save against any other way of heating or cooling a room. What you would save depends on what you have now, how you use it and what you pay for energy, and we will not put a number on that without knowing all three.