Square feet times a number is how equipment gets sized, and it is why so much of it is two to three times too big. This works the envelope instead, and shows you both figures.
Footprint and height, not total floor area — the envelope is what loses heat, and two storeys lose less per square foot than one.
R-value is resistance — higher is better. These are assembly values, close enough for a comparison but not a Manual J.
The familiar figures — 25 to 50 BTU per square foot depending on climate — were reasonable when they were coined. They describe housing with R-11 walls, single-pane windows and an air change every hour or two whether you wanted it or not.
Current construction is a different building. R-19 or better in the walls, R-38 in the ceiling, low-E glass, and air sealing that a 1970s house never had. The load per square foot has fallen by more than half, and the rule of thumb has not moved.
So a well-built 2,000 sq ft house that actually needs around 27,000 BTU/hr gets a system sized for 80,000. Both numbers are shown above; the gap is the point of this page.
An oversized furnace satisfies the thermostat fast and shuts off. That wastes fuel through repeated startup, wears the equipment, and gives uneven temperatures because the air never circulates long enough to mix.
Oversized air conditioning is worse, and the reason is moisture. A coil only removes humidity while it runs — moisture condenses on it over time. A short cycle pulls the temperature down quickly and stops before meaningful dehumidification has happened, so the house ends up cold and clammy, which people then respond to by lowering the thermostat further.
That is the signature of an oversized cooling system, and no amount of thermostat adjustment fixes it.
The breakdown above is worth reading. In a poorly insulated house, walls and ceiling dominate. Insulate them properly and the picture inverts — windows and air leakage become the largest terms, because glass is a poor insulator even when it is good glass, and air you have heated leaving the building takes its heat with it.
That is why air sealing usually returns more than adding another layer of attic insulation to a house that already has some.
Conduction through each surface is Q = A ÷ R × ΔT. Infiltration is Q = 1.08 × CFM × ΔT, where 1.08 comes from the density and specific heat of air. Cooling adds solar gain through glazing and internal gains from people and appliances, and carries a latent component for moisture that varies enormously by climate.
Tons are simply BTU/hr ÷ 12,000 — a figure that comes from the heat absorbed by a ton of ice melting over a day.
No. Use it to understand what drives the number, then have a Manual J done. If a contractor sizes from floor area alone, that is a reason to get another quote.
Because the temperature difference usually is. A 60°F winter difference against a 20°F summer one is three times the driving force, offset somewhat by solar and internal gains in summer.
Per square foot, yes — the same floor area has much less roof and slab exposed. The calculator accounts for that.