Replacing your AC or furnace? Enter your home's details below and get a BTU load and tonnage recommendation — no HVAC license or engineering degree required.
Most homeowners replace their HVAC system the easy way: they ask the contractor to put in the same size as the old one. It sounds reasonable. It's usually wrong. Old systems were often oversized from the start — installed by contractors who over-specced to avoid callbacks. An oversized AC cools your home fast but short-cycles constantly, never running long enough to pull humidity from the air. You end up cold and clammy, with a system that wears out faster.
The fix is a proper load calculation. That's what this tool does. It takes your square footage, ceiling height, insulation quality, local climate zone, and number of occupants and runs it through the same simplified heat-load model that HVAC engineers use. You don't need a license to use it. You just need your home's basic facts.
Once you have your BTU number, you're in a much stronger position when talking to contractors. You'll know if someone is trying to upsell you a larger unit than you need — and that happens more often than contractors would like to admit.
This table gives you ballpark ranges for average American homes in a mixed climate (ASHRAE Zone 3–4, average insulation). Your actual number will shift based on the factors above.
| Home Size | Estimated BTU Load | Tonnage | Common Unit Size | Typical Home Type |
|---|---|---|---|---|
| 800–1,000 sq ft | 18,000–24,000 BTU | 1.5 ton | 1.5-ton central AC | Small condo, apartment |
| 1,000–1,400 sq ft | 24,000–30,000 BTU | 2 ton | 2-ton central AC | Small ranch, townhouse |
| 1,400–1,800 sq ft | 30,000–36,000 BTU | 2.5 ton | 2.5-ton system | Typical 3BR ranch |
| 1,800–2,400 sq ft | 36,000–48,000 BTU | 3 ton | 3-ton central AC | Two-story family home |
| 2,400–3,000 sq ft | 48,000–60,000 BTU | 4 ton | 4-ton system | Larger two-story home |
| 3,000–4,000 sq ft | 60,000–72,000 BTU | 5 ton | 5-ton system or zoned | Large home, high ceilings |
Based on ASHRAE Zone 3–4, average insulation, 8–9 ft ceilings, 2–3 occupants. Actual loads vary significantly by climate, window area, and sun exposure.
BTU stands for British Thermal Unit — it's the amount of energy needed to raise one pound of water by one degree Fahrenheit. In the HVAC world, it's used to measure how much heat your system needs to add (heating) or remove (cooling) per hour to keep your home comfortable. A system rated at 36,000 BTU/hr can move 36,000 BTU of heat per hour — that's 3 tons of cooling capacity (one "ton" = 12,000 BTU/hr, a unit that dates back to the days when ice was used for cooling).
Bigger isn't better. A 5-ton system in a 1,600 sq ft home will blast cold air for 5–8 minutes, shut off, and repeat. It never runs long enough to dehumidify the air properly, and all those starts and stops stress the compressor. A correctly sized 2-ton system will run for 15–20 minute cycles, pull moisture out of the air, and last years longer.
Once you have a BTU estimate, write it down before you start getting quotes. When a contractor comes out, ask them what size they recommend and why. If they say 4 tons for a 1,600 sq ft home without any load calculation, that's a red flag. A good contractor will either do their own Manual J calculation (the gold-standard method) or at least explain their reasoning.
Your calculator result is a simplified estimate — it doesn't account for every window's orientation, attic insulation depth, or thermal bridging in your walls. But it gets you within 10–15% of reality, which is close enough to have an informed conversation. If a contractor's recommendation is more than one unit size away from your estimate, ask them to justify it in writing before you sign anything.