Anything that conditions a space is sized against its volume rather than its floor area, because air is what gets warmed, cooled or treated. A 5 × 4 m room with 2.4 m ceilings is 48 cubic metres and needs roughly 1,200 W of cooling — about 4,100 BTU per hour. The same footprint under a 3.2 m ceiling is 64 cubic metres and needs a third more.
Floor-area rules of thumb are common because they are easy, and they are the reason so much equipment is the wrong size in older houses.
What are the working figures?
Rules of thumb for ordinary construction, ordinary glazing and no unusual heat sources.
| Job | Rough figure |
|---|---|
| Cooling load | ~25 W per cubic metre |
| Heating output | ~40 W per cubic metre |
| Pool volume | Surface area × average depth |
Heating needs more per cubic metre than cooling because the temperature difference it has to bridge is larger for longer. A British winter asks a radiator to hold a 20-degree gap for months; a British summer asks an air conditioner for perhaps a 6-degree gap for a fortnight.
When do the rules of thumb break?
On any room with an unusual heat balance, which is more of them than the figure suggests.
- A glass wall facing the sun. Solar gain can dominate the entire load, and a conservatory can need triple the cooling the formula gives.
- A kitchen. Cooking is a substantial heat source and is usually not in the model.
- Servers or equipment. A rack of hardware is a space heater with a job.
- Poor insulation. The figures assume ordinary construction; a solid-wall Victorian house is not that.
- Occupancy. Each person is roughly 100 W, which matters in a meeting room and not in a bedroom.
Whole-house sizing should not be done this way at all. A proper heat-loss calculation works room by room from the construction, the glazing and the air changes, and it is a different exercise from a rule of thumb.
Why is a bigger air conditioner worse?
Because it short-cycles. An oversized unit cools the air to the setpoint quickly and shuts off before it has removed much moisture, so the room ends up cold and clammy rather than comfortable — and the compressor wears faster for the repeated starts.
The same logic applies to boilers and heat pumps. Equipment sized close to the actual load runs longer at lower output, which is where the efficiency is, and modulating equipment is designed on that assumption.
What does Delta T do to a radiator?
It changes the output by roughly half, which makes it the single most important number in radiator sizing and the one least often stated.
Manufacturers quote output at Delta T 50 — the radiator running 50 degrees hotter than the room, which means a boiler flow around 75 °C. A heat pump running at 45 °C gives Delta T 25, and the same radiator then delivers barely half its rated figure.
That is why converting a house from a gas boiler to a heat pump usually means replacing radiators even though the heat loss has not changed. The room still needs 1,350 W; the radiator that used to supply it at 75 °C now supplies about 650 W at 45 °C.
Delta T also explains why oversizing a radiator is the cheap insurance it looks like. A radiator specified with 30 per cent headroom at Delta T 50 is a radiator that will still work if the house is later heated at a lower flow temperature, and the running cost of the larger unit is nil — output is set by the thermostat, not by the size of the panel.
How does a pool volume differ?
It is the one case where the volume is the water rather than the air, and it is the number every chemical dose derives from. An 8 × 4 m pool running from 1.2 m to 2 m holds 51,200 litres — surface area times average depth, where the average of a sloping floor is the mean of the two ends.
Getting it roughly right matters more than getting it exactly right. Chlorine, acid and algaecide are all dosed per cubic metre or per ten thousand litres, so a pool assumed to be a fifth smaller than it is will be persistently under-treated — and under-treated is the failure mode that produces algae.
The pump is sized from the same figure: a full turnover of the volume in a target number of hours sets the flow rate, and undersizing there produces poor circulation whatever the chemistry says.
Questions people ask
How many BTU per square foot? Around 20 for a typical room at standard ceiling height, which is a floor-area shortcut for the volume figure above. It breaks the moment the ceiling is not standard.
What is a ton of cooling? 12,000 BTU per hour, from the rate at which a ton of ice melts over a day. It is a unit of power despite sounding like a weight.
Do bathrooms need more heat? Usually yes — they are asked to run warmer than the rest of the house, and a towel rail contributes far less output than its size suggests.
Does a pool cover change the volume? No, but it changes everything else: evaporation, heat loss and chemical demand all fall substantially with a cover on.
Measure the volume once and size against it. The air conditioner BTU calculator and the radiator size calculator both start from room dimensions rather than floor area, and the pool volume calculator handles sloping floors and irregular shapes.