BTU calculator

This is the page on the site where it matters most to say plainly what the method can and cannot do.

A real load calculation is a Manual J. It works room by room. It takes the actual wall construction, the insulation values, the orientation of every window and the specification of its glass, the measured or assumed air leakage, the duct losses and the location of the ducts. It produces separate sensible and latent loads, and it sizes the ductwork as well as the equipment. Done properly it takes hours, and any contractor worth hiring will do one.

What this page does is a screening estimate: a per-square-foot figure for your climate, moved by four coarse adjustments. It is genuinely useful for three things — judging whether a quote is in the right universe, catching an obviously oversized proposal, and putting a number in a budget before anyone visits. It is not useful for deciding what to install.

A per-square-foot estimate routinely lands twenty to thirty per cent away from a Manual J on the same house. That is why the result is a range: presenting one number would imply a precision the method does not have, and false precision is worse than an honest spread because you cannot tell it is wrong.

Almost everyone’s instinct with heating and cooling is that extra capacity is cheap insurance. With cooling in particular, it is the opposite, and understanding why is the most useful thing on this page.

An air conditioner does two jobs at once. It lowers the temperature — the sensible load — and it removes moisture from the air, the latent load. The second only happens while the coil is cold and air is moving across it, which means it needs run time.

An oversized unit satisfies the thermostat quickly and shuts off. The temperature is right; the humidity has barely moved. The house feels cold and clammy, people turn the thermostat down to compensate, and the problem gets worse. Meanwhile the compressor starts and stops all day, which is where the wear happens and where the inrush current shows up on the bill.

This is why equipment selection guidance puts a ceiling on how far above the calculated load you may size — around 15 per cent for cooling. The calculator applies the same rule: it picks the smallest manufactured size that covers the load and warns you when even that is too much headroom, because sometimes the honest answer is that the size below is the right machine.

Heating is more forgiving — an oversized furnace is inefficient and cycles, but it does not create a humidity problem. Even there, the extra capacity buys nothing on the coldest day it was sized for.

Before anything about the house, where it is decides the base figure — and the two loads cross over as you move north.

In zone 1 a house is sized by its summer and the winter load is an afterthought. By zone 6 it is the reverse, and the cooling equipment is along for the ride. Around zone 4 the two are comparable, which is the entire technical argument for a heat pump: one machine doing both jobs makes most sense where both jobs are the same size.

The state default in the calculator is an approximation and the page says so. California spans four zones on its own, and Texas three. If the example cities for your default zone do not sound like your weather, change it — it is the input that moves the answer most.

Four adjustments, and between the extremes they change the answer by nearly a factor of two on the same floor plan in the same city.

The envelope matters most. Insulation and air sealing together separate a 1970s house with original walls from a modern sealed one by a quarter in each direction. It is also the only one of these you can change, and improving it is the only way to make the equipment genuinely smaller and cheaper.

Glass is the weak point. Even good windows insulate several times worse than the wall around them, and they admit solar gain directly. A wall of south and west facing glazing can move a cooling load more than the insulation does.

Shade is free capacity. A mature tree on the west side of a house is worth more on an August afternoon than most equipment upgrades, which is why it is in the adjustment list at all.

Ceiling height is arithmetic. Conditioning moves air volume. Ten foot ceilings are twenty-five per cent more air than eight, and vaulted ceilings are the case where every per-square-foot figure in this industry quietly breaks.

Internal gains are smaller but real: about 600 BTU/h per person beyond the first two, and 1,200 for a kitchen. They matter more in a tight, well-insulated house, where they can become a significant share of the total — another reason rule-of-thumb sizing fails at the efficient end.

Heating and cooling uses more confusing units than any other trade on this site, and two of the confusions cost real money.

A ton is 12,000 BTU per hour. It has nothing to do with the weight of the equipment: it is the cooling effect of a ton of ice melting over a day, a unit inherited from before refrigeration was mechanical. Divide BTU per hour by 12,000 and you have tons.

A furnace is sold by its input, not its output. A 100,000 BTU/h furnace at 80% AFUE delivers 80,000 BTU/h to the house; the rest goes up the flue. Size the output to the load and then divide by the efficiency to get the number on the label — doing it the other way round buys a furnace a fifth too small.

SEER2 and HSPF2 are efficiency, not capacity. They tell you what it costs to run, not what it can do. A high-efficiency unit of the wrong size is still the wrong size, and a well-sized standard unit will outperform an oversized premium one on comfort every time.

The load is the same whatever you install. What changes is which machine makes sense, and that is mostly decided by what is already in the house.

The decision that has genuinely changed in the last decade is the heat pump in a cold climate. Units that held only a fraction of their capacity at freezing have been replaced by ones that hold most of it well below — but the headline rating is measured at 47°F, and the number that matters is the capacity at your design temperature. Ask for it explicitly; a supplier who cannot produce it is selling you the wrong generation of equipment.

Where there are no ducts, the arithmetic changes completely. Installing ductwork can cost as much as the system going in it, which is the point at which a ductless installation stops being a compromise and becomes the sensible answer.

The per-square-foot figures by climate zone are a screening survey, published as ranges and labelled as such. The internal gain figures and the method being approximated come from the residential load calculation standard; the limit on how far equipment may exceed the load comes from the companion equipment selection standard; the zones themselves are the energy code’s. All of it is listed on the sources page, dated.

This page will not be improved by adding decimal places. If you want a number you can buy equipment against, the improvement available is a Manual J on your actual house, and no amount of web-form refinement substitutes for it. That is an unusual thing for a calculator to say about itself, and it is the reason this one is worth using.

Nothing you type is sent anywhere — the whole calculation runs in your browser.

Two of these — AFUE and the difference between sensible and latent load — are where most of the money hides.