Sizing a window unit, mini-split, portable AC, or space heater comes down to BTU. This tool starts from the square footage and adjusts for sun, people, kitchens, and climate, so you get a cooling and a heating number instead of guessing at the store. It is right for a single room or zone; whole-home central systems still want a full load calculation. Runs in your browser.
The workhorse rule for cooling is about 20 BTU per square foot of floor space. It is a solid starting point for a single room because it captures the biggest driver, the volume of air you are conditioning. From there, real rooms differ: a west-facing room baking in afternoon sun runs hotter, a shaded north room runs cooler, a kitchen dumps appliance heat, and every extra person adds warmth and humidity. This calculator applies each of those adjustments so the number reflects the actual room, not an average one.
A room that needs 6,000 BTU of cooling in Georgia might need 12,000 BTU of heating in Minnesota, because heating load is driven by how cold it gets outside on a design night. That is why this tool gives you both figures and asks for your climate on the heating side. If you are sizing a heat pump or a combo unit, size to the larger of the two loads and confirm low-temperature output for cold climates.
Homeowners searching for BTU are often mid-decision on a purchase or a service call. If you are the tech in the room, quoting the right unit and billing it cleanly is what turns a sizing question into a paid job. FieldForge lets you say the scope and the estimate or invoice builds itself from your price book, records the equipment model and serial for the warranty, and texts a card payment link before you leave. For a quick document today, use the free HVAC invoice generator.
About 6,000 BTU of cooling on the 20 BTU baseline, adjusted for sun, people, and whether it is a kitchen.
12,000 BTU per hour equals one ton of cooling. A 12,000 BTU mini-split head is a one-ton unit.
Only slightly. A badly oversized cooler short-cycles and never dehumidifies. Match the load, do not double it.