Turning a Room's Cooling Load in Watts into the Tonnage of Unit That Fits
Sizing an air conditioner for your own home usually starts with a watt figure. A heat-loss report, an online estimator or an installer's quick sum gives you something like "this room needs about 3.5 kW of cooling". Then you go looking at units and the sizes come back in tons — half a ton, one ton, one and a half. This page turns the load you already have into the size class the equipment is sold in, so you can tell whether the model in front of you is the right one.
What Drives the Watt Figure in the First Place
Glass and which way it faces
Insulation and the roof above you
People, cooking and screens
Air leakage and open doorways
Working a Quoted Watt Load Through to a Model on the Shelf
Whether the number came from a proper survey or from a rule of thumb, the path from a load in watts to a unit you can actually buy is short.
Put the load in watts on the left
Type 2 100, 3 500, 5 300 — whatever the estimate gave you. If it is written in kilowatts, either multiply by a thousand first or set the left dropdown to kW and enter it as it stands.
Round to the nearest size the market sells
Nothing is built at 0.91 TR. Residential units step in halves and quarters, so a calculated 0.91 lands on a one-ton model and a 1.51 lands on a one-and-a-half.
Reverse it to test a unit you have found
Spotted a 1.5 ton unit and want to know what load it covers? Press the swap button (↔) for TR → W and read 5 275 W. If your room needs 6 000, that model is short.
Take the figure to the installer
The copy button gives you the bare number with no unit attached — easy to paste into a message or a quote request when you are comparing what two companies have proposed.
Room and House Sizes with Their Cooling Load and Tonnage
Worked examples for a temperate climate and reasonably modern construction. The watt column is a planning estimate, not a substitute for a survey — take it as the shape of the answer rather than the answer itself.
| Space | Floor area | Cooling load (W) | Tonnage / unit size |
|---|---|---|---|
| Single bedroom | 12 m² | 1 400 W | 0.40 TR → half-ton unit |
| Large bedroom or study | 18 m² | 2 100 W | 0.60 TR → three-quarter ton |
| Top-floor room, warm roof space | 20 m² | 3 200 W | 0.91 TR → one ton |
| Living room, one exposed wall | 28 m² | 3 500 W | 1.00 TR → one ton |
| Open-plan living and kitchen | 40 m² | 5 300 W | 1.51 TR → one and a half tons |
| Small apartment, ducted | 65 m² | 7 000 W | 1.99 TR → two tons |
| Three-bedroom house, multi-split total | 110 m² | 12 300 W | 3.50 TR → three and a half tons |
| Detached house with heavy glazing | 160 m² | 17 600 W | 5.00 TR → five tons |
The two 20-something rooms are the interesting pair: a 20 m² room under a hot roof and a 28 m² living room land on the same one-ton unit, because the roof adds as much as the extra eight square metres does. Floor area alone was never going to get you there — it is a starting point that construction, glazing and orientation then push around by a third in either direction.
What Helps When You Are Comparing Quotes
Watts and tons move together as you edit
Nudge the load up and down while you think about that extra window, and the tonnage keeps pace — useful for seeing how far you are from the next size up.
Check a shortlisted model the other way round
Swapping to TR → W tells you what a unit you have already found actually covers, which is the honest way to compare two installers' proposals.
Kilowatts on the dropdown for European spec sheets
Mini-split brochures outside North America print capacity in kW, so switch either side and the same page reads those figures without any hand arithmetic.
A clean number to send to the installer
Copy gives the plain value with nothing attached, so it drops straight into an email, a message or a comparison note without tidying up.
Home Cooling Sizing Questions
What actually goes wrong if I buy a unit two sizes too big?
It hits the set temperature fast, shuts down, and starts again a few minutes later. That is short cycling, and it has three costs. The room never feels comfortable, because the coil has not run long enough to pull moisture out of the air — you end up cold and damp. Efficiency drops, since a compressor is at its worst during start-up. And the extra starts wear the machine. A right-sized unit runs long and steady, which is exactly what you want.
Why do watts-per-square-metre rules of thumb vary so much between sources?
Because each one quietly assumes a climate and a building. A figure written for a well-insulated apartment in a mild summer might be 80 W/m²; the same rule written for a hot, humid region with single glazing runs past 200 W/m². Neither is wrong at home and both are useless abroad. Use them to sanity-check a quote, and get a room-by-room calculation before committing real money to a whole-home installation.
Does the tonnage on a heat pump describe its heating output too?
No. Tonnage is a cooling rating, and a heat pump's heating capacity is a separate number that changes with the outdoor temperature — often higher than the cooling figure in mild weather and falling away as it gets colder. That is why cold-climate models publish capacity at two or three outdoor temperatures. Size on the cooling load if summer is your problem, but check the heating column at your design winter temperature before assuming one unit covers both jobs.
My calculation says 1.2 tons but everything sold jumps from 1 to 1.5. What now?
The size steps exist because manufacturers build a handful of compressor and coil combinations, not a continuous range. With an inverter unit the gap matters far less: it modulates down to a fraction of its rating and simply runs at partial output most of the time, so the 1.5 is a reasonable choice. With an older fixed-speed unit the smaller size, running longer, will usually feel better and dehumidify more than the larger one.
Does a higher efficiency rating mean the unit cools more?
No — efficiency and capacity are different axes. A seasonal efficiency rating tells you how much electricity the unit uses to deliver its cooling over a season; the tonnage tells you how much cooling it delivers. Two one-ton units both move 3 517 W of heat, but the efficient one might do it on 900 W of electricity where the cheap one needs 1 300. Pick the capacity first from the load, then pick the efficiency you are willing to pay for.
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