BTU Calculator

1. Enter the room's square footage. Measure length times width. If the room is L-shaped, break it into rectangles and add them up.

2. Set the ceiling height. Standard is 8 ft. Vaulted or cathedral ceilings need more cooling capacity — the calculator adjusts automatically.

3. Rate the insulation. Older homes with single-pane windows are "Poor." Modern builds with double-pane Low-E glass are "Good."

4. Choose sun exposure. South and west-facing rooms with large windows get more solar heat gain. North-facing or shaded rooms need less cooling.

5. Add occupants and kitchen. Each person beyond 2 adds 600 BTU. Kitchens generate significant heat from cooking and appliances.

The calculator starts with a baseline of 20 BTU per square foot — the industry standard for residential cooling. This is more conservative than the 25 BTU/sq ft some competitors use, which tends to oversize systems.

From the baseline, five adjustment factors fine-tune the result:

Ceiling factor scales linearly: a 10-foot ceiling is 1.25× (10/8). Insulation ranges from 1.15× for poor to 0.9× for good. Sun exposure adds or removes 10%. Each person beyond two adds 600 BTU of body heat. Kitchens add a flat 4,000 BTU for cooking and appliance heat.

The calculator then matches the result to standard mini-split and window unit sizes (9K, 12K, 18K, 24K, 36K, 48K BTU) and shows the recommended tonnage.

Mini-split sizing on site. You're at a customer's home and need to quickly recommend a ductless unit size for a sunroom, garage conversion, or bonus room. Pull this up on your phone, enter the room dimensions, and give the customer a confident answer.

Window unit recommendations. When a customer calls asking what size window AC they need, walk them through the inputs over the phone and give them a specific BTU number instead of guessing.

Quick sanity check. Before running a full Manual J, use this as a gut-check on individual rooms. If your Manual J says a 200 sq ft bedroom needs 12,000 BTU, something's off.