HVAC Sizing Calculator: What Size AC and Furnace Does Your House Need?
Most homes need roughly one ton of air conditioning for every 400–600 square feet, but the right size depends on your climate, ceilings, insulation, sun exposure and how many people live there. Enter your details below for a BTU and tonnage estimate, plus a furnace output figure — then confirm with a contractor’s Manual J.
How this calculator works
The estimate starts from a base cooling load for your climate zone (in BTU per square foot), then adjusts for the things that actually move the number: ceiling height changes the volume of air you condition, insulation quality changes how fast heat leaks in or out, sun exposure changes solar gain, and each person and an open kitchen add measurable heat. Cooling capacity is reported in tons (1 ton = 12,000 BTU/hr) rounded to the nearest half ton, because that is how equipment is actually sold.
For heating, the calculator uses a separate set of BTU-per-square-foot factors that rise sharply in colder zones, then converts the required output into the input rating you will see on a furnace nameplate at two common efficiencies (80% and 96% AFUE). The breakdown table shows every adjustment so you can see why your number landed where it did.
Quick reference: tonnage by home size
| Home size | Approx. cooling load | Typical A/C size |
|---|---|---|
| 600–1,000 sq ft | 14,000–24,000 BTU | 1.5 tons |
| 1,000–1,500 sq ft | 24,000–36,000 BTU | 2–2.5 tons |
| 1,500–2,000 sq ft | 36,000–42,000 BTU | 3 tons |
| 2,000–2,600 sq ft | 42,000–54,000 BTU | 3.5–4 tons |
| 2,600–3,200 sq ft | 54,000–60,000 BTU | 5 tons |
These are starting points, not final answers. The same square footage in Phoenix and in Minneapolis needs very different cooling, and a leaky 1970s house needs more than a tight new build of identical size. That is exactly why the tool above asks for climate, insulation and sun rather than square footage alone.
Why bigger is not better
An oversized air conditioner cools the air fast but shuts off before it removes humidity, leaving rooms clammy and triggering frequent short cycles that wear out the compressor and waste energy. An undersized unit runs constantly on the hottest days and never quite catches up. The goal is a unit matched to the actual load — which is why HVAC professionals run a Manual J calculation rather than guessing from floor area.
Frequently asked questions
How many square feet does a ton of AC cover?
As a rough rule, one ton covers about 400–600 square feet, with the lower end in hot climates and the upper end in cool, well-insulated homes. The calculator refines this for your specific situation.
Is this the same as a Manual J?
No. A Manual J load calculation models every wall, window and duct in your house. This tool gives a fast, reasonable estimate for planning and for sanity-checking a contractor’s proposal, but you should always get a full Manual J before purchasing.
What size furnace do I need?
The calculator returns a furnace heat-output figure and the corresponding input rating at 80% and 96% AFUE. In cold climates heating usually drives the equipment size; in hot climates cooling does.
Does ceiling height really matter?
Yes. A room with 10-foot ceilings has 25% more air volume than the same floor area at 8 feet, so the calculator scales the load accordingly.
Want the reasoning behind every number? Read the complete guide to sizing an HVAC system, or see how the two main approaches stack up in Manual J vs. rule-of-thumb sizing.