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AC Size Calculator

Whole-house air conditioner tonnage from area, climate and insulation.

Your measurementsEvery assumption editable01
System sizeCalculated live02

Result

4tons

4 tons — 48000 BTU/hr

Base load1800 sq ft × 25 BTU/sq ft
45000 BTU/hr
Total load
45000 BTU/hr
Calculated tonnage
3.75 tons
Nearest equipment sizesystems are sold in half-ton steps
4 tons

Rounding to the nearest half ton rather than up is deliberate. Slightly undersized equipment runs longer, dehumidifies better and holds a steadier temperature than slightly oversized equipment.

Square-footage sizing is a starting point for a conversation with a contractor, not a specification. Any contractor who sizes a replacement system by floor area alone and nothing else is guessing too.

If your existing system keeps up on the hottest day but runs almost continuously, it is correctly sized. Continuous running at design temperature is what correct sizing looks like, not a fault.

Enter the conditioned floor area, your climate and how well the house is insulated and sealed. The result rounds to the nearest half ton, because that is how equipment is sold — and because rounding up is the wrong instinct in this one category.

Why use this tool?What it does differently03

Why use this tool?

Rounds to real sizes

Systems come in half-ton steps. The result lands on one rather than leaving you to guess which way to go.

Insulation adjustment

A new, well-sealed house and a draughty older one of identical area can differ by forty per cent in load.

Rounds to nearest, not up

Deliberate. Slightly undersized equipment runs longer, dehumidifies better and holds a steadier temperature.

Itemised working

Base load, the insulation adjustment and the calculated tonnage are all shown so the figure can be checked.

How this worksThe method04

How this ac size calculator works

Conditioned floor area multiplied by a climate load in BTU per square foot gives a base figure, which is then adjusted by a percentage for insulation and air-sealing quality — down fifteen per cent for a new tight house, up twenty-five for a poorly insulated one.

That total divides by 12,000 to give tons, and rounds to the nearest half ton because that is the granularity equipment is manufactured in.

Rounding to nearest rather than up is the important detail. An oversized system satisfies the thermostat quickly and shuts off before it has dehumidified, which is why an over-specified house often feels clammy despite hitting its set temperature.

How to use itStep by step05

How to use it

  1. Step 1: Measure conditioned area

    Total floor area that the system serves. Exclude garages, unconditioned basements and unheated lofts.

  2. Step 2: Choose the climate load

    Mild, moderate, hot, or very hot and humid. This drives the result more than anything else.

  3. Step 3: Assess the envelope

    New and well sealed, average, older and draughty, or poorly insulated.

  4. Step 4: Treat it as a starting point

    Use the figure to sanity-check a contractor's proposal rather than to place an order.

Example usageWorked figures06

Example usage

An average 1,800 sq ft house
At 25 BTU per square foot with no insulation adjustment, that is 45,000 BTU/hr — 3.75 tons, rounding to 4 tons.
The same house, well sealed
A fifteen per cent reduction takes it to 38,250 BTU/hr, or 3.19 tons — rounding to 3. A tighter envelope genuinely removes a whole equipment size.
An older draughty house
The same 1,800 sq ft at plus twenty-five per cent is 56,250 BTU/hr, or 4.69 tons — rounding to 4.5. Insulating first is almost always cheaper than sizing up.
Frequently asked questionsCommon questions07

Frequently asked questions

What size air conditioner do I need for 2,000 square feet?

In a moderate climate with average insulation, roughly 50,000 BTU/hr — about 4 tons. Climate and envelope quality move that substantially in both directions, which is why both are inputs here.

Why not round up?

Because oversized cooling short-cycles. It reaches the set temperature before removing moisture, so the house ends up cold and damp, and the compressor wears out faster from repeated starts.

My system runs constantly on hot days — is it too small?

Probably not. Continuous running at design temperature is what correct sizing looks like. A system that cycles frequently on the hottest day of the year is oversized, not healthy.

Is this a substitute for a Manual J?

No. A real load calculation accounts for window area and orientation, infiltration, duct losses and local design temperatures. Use this to scope a project or check a quote.

Does the number of storeys matter?

It does in practice — upper floors gain more heat through the roof, and a two-storey house often benefits from zoning rather than a single larger system. This calculates a single combined figure.

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