BTU Calculator – How to Size an Air Conditioner or Heater for a Room
A BTU calculator estimates how much heat a room gains on a hot afternoon or loses on a cold night. That figure, in BTU per hour (BTU/h), is the capacity your air conditioner or heater needs. This calculator gives you two ways to get it. The Quick estimate needs only the floor area and a few details. The Detailed method builds the load up from walls, windows, insulation, air leakage and the people and appliances in the room. Both methods round up to the next standard AC size, heater size or furnace rating.
The Quick Estimate: Square Footage Rules
For cooling, the quick method uses the ENERGY STAR room air conditioner sizing chart. A 150–250 ft² bedroom needs about 6,000 BTU/h, and a 450–550 ft² living room about 12,000 BTU/h. The chart is then adjusted: −10% for a heavily shaded room, +10% for a very sunny one, +600 BTU/h for each regular occupant above two, and +4,000 BTU/h for a kitchen. Rooms with ceilings above 8 ft are scaled up by height ÷ 8, since the chart assumes a standard ceiling.
For heating, the quick method multiplies floor area by a BTU per square foot figure for your climate. That's about 30 in hot climates and 60 in cold ones. It then applies an insulation factor. It's a rough guide: published tables disagree by about ±10 BTU/ft².
The Detailed Method: Heat Gain and Heat Loss
The detailed method follows the logic of an ACCA Manual J, simplified to one room at peak conditions. Heat moving through each surface is Q = A × ΔT ÷ R (area times temperature difference, divided by the R-value). For windows it's Q = U × A × ΔT. Sunlit walls get an extra 10 °F in cooling, and an attic is treated as 30 °F hotter than the outdoor air. Air leakage is calculated from the room volume and air changes per hour: CFM = volume × ACH ÷ 60. Its sensible load is 1.08 × CFM × ΔT, and its moisture (latent) load is 0.68 × CFM × Δgrains.
Solar gain through windows is often the biggest single item in a cooling load. Each window adds area × SHGC × solar factor × shading. East glass peaks in the morning and west glass in the afternoon, so both cases are calculated and the larger one is used. Each person adds 230 BTU/h of sensible heat and 200 BTU/h of latent heat. Lights and electronics add 3.412 BTU/h per watt.
Choosing the Right Equipment Size
Air conditioners are sold in fixed sizes. Room units come in 5,000, 6,000, 8,000, 10,000 and 12,000 BTU/h and up. Mini-splits start at 9,000 BTU/h, and central systems come in half-ton steps from 1.5 to 5 tons. Pick the smallest size at or above your load. An oversized AC short-cycles: it cools the air before it has removed the moisture, so the room feels cold and damp. The size gauge marks anything more than 25% above your load as oversized. For heating, the calculator rounds electric heaters up to the next 250 W step. It also shows the furnace input rating needed at your AFUE efficiency, because a furnace's output is only a percentage of its input.
Whole-House Totals
Add a card for each room to build up a house. The totals table shows each room's share and recommends a central system size for the total. It also gives a per-room mini-split size for multi-zone planning. Adding up the rooms overstates the whole-house peak a little, because east and west rooms reach their solar peaks at different times of day.