Who this is for: For homeowners sizing a replacement system and installers giving a first-pass quote — before paying for the formal Manual J the permit may require.
Square footage, climate zone and house details in — get the estimated BTU, tonnage and a size breakdown you can defend.
Quick answer: Simplified Manual J: base load = square feet × 22–35 BTU/h (by climate zone), adjusted up for sun, old windows, poor insulation and extra ceilings, down for shade and good insulation; each extra occupant adds 600 BTU/h. 1,500 sq ft in a mixed climate, sunny → ≈36,900 BTU/h → 3.1 tons → buy 3.5 tons.
sqft
ft
Sun exposure
Insulation
Large / older window area
Kitchen in the space
Estimated load
33,600 BTU/h
cooling/heating design
Exact tons
2.8
BTU ÷ 12,000
Buy this size
3 tons
half-ton equipment steps
Where the number came from
Base load (Mixed (Mid-Atlantic, OH valley))33,000 BTU/h
Occupants (2)+600 BTU/h
Total33,600 BTU/h
Resist upsizing. An oversized AC short-cycles before dehumidifying — cold, clammy, higher bills. If you’re between sizes after a real Manual J, take the smaller and let it run longer, drier cycles.
Simplified whole-house estimate per Manual J concepts. Formal sizing (windows, orientation, infiltration) belongs to a certified ACCA Manual J — most permits require it.
Core facts
Base factors
22–35 BTU/h per sqft by zone
Adjustments
Sun +10% · windows +10% · poor insulation +15% · shade −10%
People / kitchen
600 BTU/h per extra occupant · +1,200 kitchen
Tons
BTU ÷ 12,000, half-ton snap
Example
1,500 sqft mixed + sunny → 36,900 BTU → 3.5 tons
Status
Estimating tool — formal design per ACCA Manual J. Compiled September 2026.
How does this load estimate work?
It starts from the climate-zone rule of thumb (roughly 22–35 BTU/h per square foot depending on where you live), then adjusts: taller ceilings, sunny exposure, big or old window areas and poor insulation each push the number up; shade and good insulation pull it down; each occupant beyond the first adds 600 BTU/h and a kitchen adds a fixed allowance. The output is an estimated cooling/heating load in BTU/h, converted to tons (12,000 BTU/h each), snapped to the half-ton sizes equipment actually ships in. Example: 1,500 sq ft in a mixed climate, sunny lot, two occupants → about 36,900 BTU/h → 3.1 tons → a 3.5-ton system. A real Manual J measures windows, orientation, infiltration and ducts — treat this as the fast, honest ballpark that keeps you from the classic mistake of oversizing.
Common uses
Checking a contractor's 'rule of thumb 500 sq ft per ton' against your actual house
Sizing a mini-split per floor before getting three quotes
Comparing tonnage needs before/after an insulation upgrade
Explaining to a customer why the old 5-ton just got replaced by a 3.5-ton
Frequently Asked Questions
What is Manual J?
The ACCA's room-by-room load calculation standard — the formal way HVAC loads are sized in the US. Many permits require it. This calculator is a simplified, whole-house version of the same physics: envelope area × climate factor, adjusted for sun, glass, insulation and internals.
Why is oversizing a problem?
An oversized AC cools fast then shuts off before it dehumidifies — cold, clammy rooms, short-cycling compressor, higher bills. A slightly undersized-but-right system outperforms an oversized one in comfort and lifespan. The old '500 sq ft per ton' rule routinely doubles real loads.
How many BTU per square foot?
As a starting point: ~30 in hot climates (South FL, AZ), 25 warm (TX, GA), 22 mixed (Mid-Atlantic), 20 cold (North), up to 35 in very cold zones where heating dominates — before the adjustment factors. The calculator applies your zone automatically.
Does the same load cover heating and cooling?
Not exactly — heating depends on design temperature difference, cooling on solar gain and latent load. In heating-dominated zones size the furnace/heat pump on the heating number; the calculator's zone factors lean that direction.
Can I size a mini-split with this?
Yes for the whole-floor or whole-home number; split it per-room by share of floor area (see the CFM calculator's share method). Room-levelManual J still wins for tricky rooms — big west glass, kitchens.