What size do I need?
Every calculator on this site, with what each one is actually sized against.
This is an estimate from the standard industry rule of thumb, not an engineering calculation. For anything you are about to spend money on, have the figure confirmed by a load calculation.
What each piece of equipment is sized against
| Equipment | Sized by | Answer in |
|---|---|---|
| ac unit | Floor area, climate and ceiling height | BTU |
| mini split | Floor area, climate and ceiling height | BTU |
| dehumidifier | Floor area, climate and ceiling height | pints per day |
| furnace | Floor area, climate and ceiling height | BTU |
| ceiling fan | Floor area, climate and ceiling height | inch blade span |
| generator | The electrical load you want to run | watts |
| solar system | Household size and peak hour demand | kW |
| water heater | Household size and peak hour demand | gallons |
Notice that only some of these are sized by floor area. Generators are sized by the load you plug into them and water heaters by when your household showers — using square footage for either produces a number with no relationship to reality, and it is a common enough mistake that retail staff make it.
Everything here runs in your browser with the arithmetic shown. There is no form, no email field and no contractor call afterwards, which is unusual in this category and is the main reason this site exists. Pick the equipment and what size do i need in about a minute.
Why climate changes the answer so much
| Climate zone | Cooling multiplier | Heating BTU per sq ft |
|---|---|---|
| Hot and humid (Gulf Coast, Florida, south Texas) | ×1.15 | 35 |
| Hot and dry (Arizona, inland California, Nevada) | ×1.1 | 32 |
| Mixed (mid-Atlantic, Carolinas, Tennessee) | ×1 | 45 |
| Cold (Midwest, New England, Pacific Northwest) | ×0.92 | 55 |
| Very cold (Minnesota, Maine, Montana) | ×0.88 | 62 |
The heating column spreads nearly two to one from the warmest zone to the coldest, while cooling spreads about a quarter. That asymmetry surprises people: a national rule of thumb for air conditioning is roughly defensible, and the same rule for heating is not. If you take one thing from this table, it is that a furnace sized off a generic online figure is far more likely to be wrong than an air conditioner sized the same way.
Where a rule of thumb stops being enough
Every number on this page comes from the standard industry rule of thumb, and a rule of thumb is not an engineering calculation. The real method is a Manual J load calculation, which models your wall and attic insulation, your window area and orientation, how leaky the building envelope is, and how much your ducts lose. It routinely lands a size away from any square-footage estimate — in either direction, which is the part people forget.
So treat this as a sanity check rather than a specification. Ten minutes to what size do i need tells you whether a quote is in the right neighbourhood, which is genuinely useful — it is enough to catch a contractor who has quoted you two sizes up because that is what was on the truck. It is not enough to order equipment from.
A Manual J load calculation is the real answer. It models your insulation, window area and orientation, air leakage and duct losses, and it routinely lands one size away from any rule of thumb — in either direction. Any contractor who sizes your system by square footage alone and nothing else is guessing.