AC & Furnace Size Calculator — What Size HVAC Do You Need?
A real multi-factor load estimate, not the naive 20-BTU rule: area, ceiling height, insulation, climate zone, sun exposure, windows and occupants → cooling BTU + tons and heating BTU. Free, instant, no sign-up.
Ballpark, not a Manual J. This estimate gets you to the right conversation; before buying central equipment, have a contractor run a room-by-room Manual J load calculation — oversized AC short-cycles and leaves the house humid.
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What size AC for a 1,500 sq ft house?
1,500 sq ft × 20 BTU base = 30,000 → average insulation (×1.0), mixed climate (×1.0), 8-ft ceilings (×1.0), average sun and windows, 3 occupants (+600), kitchen (+4,000) ≈ 34,600 BTU ≈ 3 tons. The same house with good insulation in a shaded lot drops near 2 tons; with poor insulation in Phoenix it pushes past 3.5 tons — which is exactly why a flat per-square-foot rule misleads.
How we calculate this
- Cooling: area × 20 BTU base, × ceiling-height/8, × insulation (poor 1.2 / avg 1.0 / good 0.85), × climate (hot 1.15 / mixed 1.0 / cold 0.9), × sun (±10%), × windows (±8%), + 600 BTU per occupant over two, + 4,000 BTU for a kitchen. Tons = BTU ÷ 12,000, rounded to the nearest half-ton.
- Heating: area × climate rate (hot 32 / mixed 43 / cold 55 BTU per sq ft) × insulation × ceiling factor. Furnace output must meet this; input size = output ÷ AFUE (e.g. 80,000 BTU output ≈ 84,000 input at 95% AFUE).
- Factors follow common ACCA-style rules of thumb; a Manual J adds duct losses, orientation and per-room detail.