What size air conditioner do I need?

The rule everyone gives you

This Old House states it plainly:

1. Measure the square footage of the room.
2. Multiply the square footage by 20 BTU per square foot.
3. Adjust for ceiling height, sunlight exposure and occupancy.

Its worked example: a 300 square foot room typically requires at least 6,000 BTU.

Note the word room. The rule is written for a room, and the page then applies it to whole houses, which is where the trouble starts.

The table that disagrees with it

Fixr does not give a per-square-foot figure. It gives this:

Our arithmetic, marked as ours. Divide each row by the top of its own area band:

The rule of thumb is roughly right for a small space and badly wrong for a large one. At 1,000 square feet it asks for 11% more than the table. At 3,500 square feet it asks for 70,000 BTU against 46,000, which is 52% more, our arithmetic.

Our reading: this is not a contradiction between two publishers so much as a flat rule being used outside its range. Heat gain does not scale with floor area alone, because a bigger house has proportionally less outside wall and roof per square foot of floor. A single multiplier cannot hold across that, and Fixr's falling figures are what that looks like in a table.

What the wrong size costs

Fixr's tonnage table, where 12,000 BTU is one ton on its own labelling:

Now put This Old House's own case next to it. Its HVAC expert visited a homeowner whose 4-ton unit kept freezing up, in a 1,300 square foot house that likely needed only a 2.5-ton system.

Our arithmetic, marked as ours. For that house:

Four methods, four answers, and the installed one is the outlier. On Fixr's table, 4 tons installed is $6,950 to $11,600 against $4,500 to $8,000 for 2.5 tons. That homeowner paid somewhere between $2,450 and $3,600 more, our subtraction, for a unit that then froze up.

Why oversizing breaks things

This Old House lists the consequences, and they are mechanical rather than financial:

• Short cycling. The unit cools quickly, then shuts off before it has dehumidified, which it describes as a clammy feeling.
• Increased energy consumption, because frequent on-off cycles waste energy.
• Reduced lifespan, because short cycling puts extra wear on the compressor.

That last one is the expensive part. Our AC compressor page prices what a compressor replacement costs, and our lifespan page collects what the sources say about how long these machines should last. An oversized unit buys you a higher price now and a shorter life after it.

What to ask for instead

This Old House's expert is specific: insist that the contractor performs a proper heat-loss and cooling-load calculation based on the house, rather than sizing from floor area.

Our note, marked as ours. That calculation accounts for insulation, window area and orientation, ceiling height, air leakage and occupancy. The rule of thumb accounts for none of them, which is why This Old House lists ceiling height, window placement, insulation quality and room purpose as adjustments on the very same page. Once you are adjusting for four factors, you are doing the calculation badly rather than doing it quickly.

A contractor who sizes your system from square footage alone, in a phone call, has told you something useful about the quote.

One more table that is only arithmetic

Fixr also prices central air by house size:

Our arithmetic. Every row is $2.90 to $7.20 per square foot, multiplied out. $2,320 divided by 800 is $2.90; $25,200 divided by 3,500 is $7.20. The table contains one price band, repeated eight times.

We keep finding these. Our radiant floor page found a seven-row version shared by two publishers, and our furnace page found another. The useful part of this one is what it does not do: notice that the cost table scales straight with area while Fixr's own BTU table does not. The two tables on the same page use different logic, and the sizing one is the one built from data.

Before you accept a size

1. Ask for a heat-loss and cooling-load calculation in writing. It is the only method either publisher actually endorses.
2. Check the proposed tonnage against the BTU table above. If it is more than half a ton above it, ask why.
3. Bigger is not safer. Short cycling, humidity and compressor wear are the price of the margin.
4. Ask what the quote assumes about your insulation and windows. If it assumes nothing, it used floor area. What the windows themselves cost to put right is in our windows section, and the glazing is the part that touches the load, on our double versus triple pane page.
5. For a single room, the 20 BTU rule is fine. This Old House prices window units at 5,000 to 25,000 BTU, which is the range that rule was written for.
6. Tonnage is only half the specification. The other half is the efficiency rating, on our SEER rating page.