HVAC Size Calculator
Enter your room dimensions and conditions to find the right AC size — too small never cools, too big wastes money and never dehumidifies.
Runs entirely in your browser — your numbers never leave this page.
How it works
AC sizing starts with a simple rule — ~20 BTU per square foot — then adjusts for reality:
BTU = sq ft × 20 × climate × sun × (ceiling ÷ 8) + kitchen bonus
- Climate: mild zones need less; hot zones (regular 95°F+) need ~30% more.
- Sun: a sunny south/west room gains ~10%; a shaded north room needs ~10% less.
- Ceilings: 10-ft ceilings hold 25% more air than 8-ft — scale up proportionally.
- Kitchens: ovens and fridges add ~4,000 BTU of heat load.
- Tonnage: 12,000 BTU = 1 ton. A 180 sq ft bedroom ≈ 6,000–8,000 BTU (0.5–0.75 ton).
- Don't oversize: bigger isn't better — short-cycling leaves humidity behind and wears the compressor.
Frequently asked questions
How many BTU per square foot?
Roughly 20 BTU/sq ft as a baseline, adjusted up for hot climates, high ceilings, sun exposure, and kitchens.
What size AC for a 12x15 bedroom?
180 sq ft × 20 = 3,600 BTU base; with normal adjustments, a 6,000 BTU unit covers it comfortably.
What does "ton" mean for an AC?
One ton of cooling = 12,000 BTU/hr — the heat needed to melt a ton of ice in 24 hours. A 24,000 BTU unit is a 2-ton unit.
Is a bigger AC better?
No — oversized units short-cycle: they cool fast but shut off before dehumidifying, leaving the room cold and clammy while wearing out faster.
How do I size AC for a whole house?
Use this per-room then sum, or better, get a Manual J load calculation — insulation, windows, and ductwork change the answer a lot.
Does ceiling height matter?
Yes — a 10-ft ceiling has 25% more air volume than 8-ft. Scale the BTU up proportionally.
What about a kitchen or sunroom?
Kitchens add ~4,000 BTU for appliances. Sunrooms with lots of glass can need 30-50% more than the base rule.
Window vs mini-split sizing?
Same BTU math. Mini-splits are more efficient at part-load, so they tolerate slight oversizing better than window units.
What SEER rating should I get?
Higher SEER = lower running cost. In hot climates, the payback on a high-SEER unit is fast; in mild ones, mid-range is fine.
Can I use this for heating too?
Roughly — heat pumps are sized similarly, but cold-climate heating loads are bigger than cooling loads. Size for the larger of the two.