Loan BTU Estimator
Most BTU calculators ask for square feet and stop. This one asks the questions that actually move the load — climate zone, insulation, sun, ceiling height, people, even the kitchen — and gives heating AND cooling, because a house has to survive both.
A first-pass sizing estimate — the honest kind, with its limits stated below. Final whole-house equipment sizing belongs to a Manual J load calculation; this gets you the right shortlist and exposes oversized quotes.
How it works
Cooling starts at 20 BTU per sq ft, then corrects: ceiling height adds 5% per foot over 8; insulation multiplies by 1.0 (good), 1.15 (average) or 1.35 (poor); sun exposure bends cooling by 0.9–1.12; every occupant beyond the first two adds 600 BTU; a kitchen adds 1,200. Heating is climate-driven: the factor runs 25 BTU/sq ft in zone 1 to 55 in zone 7, with the same ceiling and insulation corrections. Tonnage is cooling ÷ 12,000, reported as a decimal on purpose — real systems come in half-ton steps, and 2.88 tons means a 3-ton class unit, not a squeezed 2.5.
Method & worked example
Method: Cooling = area × 20 BTU/sq ft × ceiling factor (1 + 5% per ft over 8) × insulation (good 1.0 / average 1.15 / poor 1.35) × sun (0.9 / 1.0 / 1.12) + 600 BTU per occupant past two + 1,200 for a kitchen. Heating = area × zone factor (25 zone 1 to 55 zone 7) × ceiling × insulation. Tons = cooling ÷ 12,000.
Worked example: A 1,500 sq ft average-insulated home, zone 4: 34,500 BTU cooling (2.88 tons) and 69,000 BTU heating.
When to use this
- You are replacing an AC or furnace and a quote's tonnage smells oversized.
- You are adding a mini-split to a room, addition or garage conversion.
- You want heating AND cooling loads from one honest first pass.
How to use it, step by step
- Enter the floor area being conditioned.
- Pick your climate zone for the heating load.
- Rate the insulation honestly — average is the honest default.
- Add sun exposure, ceiling height, occupants and kitchen, then read both loads.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Floor area | Conditioned square feet | 400 sq ft room; 1,500–2,500 sq ft homes |
| Climate zone | Sets the heating factor 25–55 BTU/sq ft | Zone 1 Miami to zone 7 Duluth |
| Insulation | Envelope quality multiplier | Good 1.0; average 1.15; poor 1.35 |
| Sun / people / kitchen | Cooling corrections | Sunny ×1.12; +600 BTU per extra person; +1,200 kitchen |
Another worked example
A tough small case: 800 sq ft, 9-ft ceilings, zone 6, poor insulation, sunny exposure, 4 occupants and a kitchen. Cooling = 800 × 20 × 1.05 × 1.35 × 1.12 + 1,200 + 1,200 = 27,802 BTU (2.32 tons); heating = 800 × 50 × 1.05 × 1.35 = 56,700 BTU. Small footprint, serious loads — envelope beats area.
Frequently asked questions
How many BTU do I need for 1,500 sq ft?
Cooling: about 34,500 BTU (2.88 tons → a 3-ton class system) for an average-insulated home with 8-ft ceilings. Heating in climate zone 4: about 69,000 BTU. Same house, two different loads — in most of the country winter asks for more raw BTU than summer.
How many BTU for a 400 sq ft room?
About 9,200 BTU cooling at the average insulation factor — a 9,000–10,000 BTU window unit or mini-split head. A sunny west-facing room or a kitchen pushes toward the top of that range and beyond.
What is a ton of air conditioning?
12,000 BTU per hour — the cooling delivered by melting one ton of ice over a day, a unit old enough to be charming. Homes run 1.5–5 tons. Half-ton steps are standard: your decimal tonnage rounds to the nearest available size, with the oversizing warning above in mind.
Does insulation really change the size?
A poor envelope multiplies the load by 1.35 against 1.0 for a good one — on 1,500 sq ft that is the difference between roughly 2.5 and 3.4 tons of cooling. Sealing and insulation are one-time costs; the bigger machine bills you monthly for decades. An 800 sq ft drafty, sunny zone-6 space with a kitchen lands near 27,800 BTU cooling and 56,700 heating — small house, serious loads.
Mini-split or central?
Mini-splits shine for additions, single rooms and homes without ducts — and they zone beautifully. Central wins where ducts exist and whole-house evenness matters. The HVAC Cost Estimator prices both routes for your tonnage.
❄️ What will the system cost installed?
Load sized, the HVAC Cost Estimator prices the equipment — central AC, heat pump, furnace or mini-split — at your tonnage with a low–high range.
What size AC for a 2,000 sq ft house?
At the average settings here, roughly 46,000 BTU cooling (about 3.8 tons → a 4-ton class unit) in a mid climate — but zone, glass and insulation legitimately move the answer a full ton either way, which is why the inputs exist.
Does this replace a Manual J?
No — it is the screening estimate that tells you whether a quote's Manual J is plausible. For equipment purchase on a whole house, get the J-load; for room units, additions and quote-checking, this pass is the right tool.