AC Tonnage Calculator
Estimate the cooling load of a whole house in tons from its size, climate, insulation and windows — not from square footage alone.
Method reviewed October 22, 2026. Version 1.0.0. How it works
Result
- Total cooling load
- 27,395BTU/h
- Required capacity
- 2.28tons
Estimated cooling load ≈ 2.3 tons. A 2.5-ton system is the nearest standard size; confirm it with a Manual J calculation before buying.
- Suggested nominal size
- 2.5tons
- Sensible load
- 24,691BTU/h
- Latent (moisture) load
- 2,704BTU/h
- Sensible heat ratio
- 0.9
- Floor area per ton
- 876ft²
Cooling load breakdown
- Walls
- 2,579BTU/h
- Ceiling / roof
- 3,067BTU/h
- Windows (conduction)
- 2,352BTU/h
- Windows (solar)
- 8,820BTU/h
- Doors
- 256BTU/h
- Air leakage (infiltration)
- 3,286BTU/h
- Occupants
- 1,720BTU/h
- Appliances & lighting
- 1,200BTU/h
- Duct losses / gains
- 4,115BTU/h
Show the working
- Footprint and perimeter (square plan)2,000 ft² / 1 = 2,000 ft²; perimeter = 4 × √2,000 = 178.9 ft
- Net wall area1,431 ft² gross − 300 ft² windows − 40 ft² doors = 1,091 ft²
- Conditioned volume2,000 ft² × 8 ft = 16,000 ft³
- Cooling design temperature difference91 °F − 75 °F = 16 °F
- Total cooling load24,691 sensible + 2,704 latent = 27,395 BTU/h
- Convert to tons (12,000 BTU/h per ton)27,395 / 12,000 = 2.28 tons
Why square footage alone gets it wrong
The familiar “one ton per 400–600 square feet” rule ignores almost everything that drives a cooling load. In an analysis of 40 ACCA Manual J calculations published on ACCA’s blog, the average house needed about one ton per 1,400 square feet — so the rule of thumb tends to oversize, sometimes by a factor of two.
This calculator replaces the rule with a simplified block load. It estimates heat flowing in through walls, ceiling, windows and doors, sunlight through glass, outdoor air leaking in, and heat from people and appliances, then adds an allowance for ducts outside the conditioned space.
What the inputs change
- Climate sets the outdoor design temperature and summer humidity. Choose the zone closest to you, or enter your own design temperatures for a sharper estimate.
- Insulation and air sealing set the wall and ceiling R-values and the air-leakage rate.
- Windows matter most in most homes: their area, U-factor and solar heat gain coefficient (SHGC) together are usually the biggest part of the cooling load.
- Ducts in a hot attic can add 20 % or more to the load; ducts inside the conditioned space add nothing.
From load to equipment size
The load is divided by 12,000 BTU/h per ton. We then suggest the smallest standard size at or above 95 % of the total load, because cooling equipment is sold in half-ton steps and a modest shortfall on the hottest hours of the year is better than chronic oversizing.
The sensible heat ratio (SHR) tells you how much of the load is temperature versus moisture. Humid climates have a lower SHR, which points to equipment and airflow settings that remove more moisture.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Conditioned floor area
- 2,000 ft²
- Stories above grade
- 1 story
- Average ceiling height
- 8 ft
- Climate
- Zone 4 — mixed (e.g. Baltimore)
- Insulation & air sealing
- Average (code-built 1990s–2000s)
- Window type
- Double pane, clear
- Window area (% of floor area)
- 15 %
- Foundation
- Slab on grade
- Duct location
- Attic/crawl — typical
- Occupants
- 4
Result
- Total cooling load
- 27,395 BTU/h
- Required capacity
- 2.28 tons
- Suggested nominal size
- 2.5 tons
- Sensible load
- 24,691 BTU/h
- Latent (moisture) load
- 2,704 BTU/h
- Sensible heat ratio
- 0.9
Estimated cooling load ≈ 2.3 tons. A 2.5-ton system is the nearest standard size; confirm it with a Manual J calculation before buying.
Assumptions
- Indoor design conditions are 70 °F for heating and 75 °F for cooling (ACCA Manual J defaults).
- The house is modelled as a square footprint with two 20 ft² doors; homes with complex shapes have more wall area and a somewhat higher load.
- Opaque surfaces use an equivalent temperature difference for sun-heated surfaces: walls +10 °F, ceilings under a vented attic +30 °F.
- Window solar gain uses an all-orientation average of 60 BTU/h·ft² (calibrated so typical results fall in the range published Manual J studies report) multiplied by the window SHGC and an interior-shading factor of 0.7.
- Summer air leakage is taken as half of the winter natural air-change rate.
- Each occupant adds 230 BTU/h sensible and 200 BTU/h latent heat; appliances and lighting add 1,200 BTU/h.
- Duct losses are a fixed percentage of the sensible load depending on duct location (0 %, 10 %, 20 % or 30 %).
Limitations
- This is a screening estimate, not an ACCA Manual J load calculation. Use it to sanity-check a quote or plan a project, not to finalise equipment selection.
- Orientation, shading by trees or neighbouring buildings, actual window placement, and room-by-room loads are not modelled; a whole-house figure cannot size individual duct runs.
- Climate presets are rounded values for one representative city per zone. Design temperatures vary considerably within a zone — enter your local values for a better estimate.
- Conditioned basements are not modelled; below-grade wall losses are excluded.
Frequently asked questions
Is this a Manual J calculation?
No. ACCA Manual J is a room-by-room procedure with detailed construction data, orientation and local design conditions, and it is what building codes require for equipment sizing. This calculator is a whole-house screening estimate that follows the same physics in simplified form. Use it to sanity-check a quote; ask your contractor for the Manual J report before you buy.
How many square feet does a 3-ton AC cool?
It depends on the house. In this calculator a typical code-built house in a mixed climate comes out at roughly 800–900 ft² per ton; a tight, well-insulated house with low-E windows can exceed 1,300 ft² per ton. Published Manual J studies report averages well above 1,000 ft² per ton.
Should I round up to the next size?
Only when the load is close to the next size. Manual S, ACCA’s equipment-selection standard, allows air conditioners to be only modestly larger than the calculated load. A two-stage or variable-speed system tolerates some oversizing better than a single-stage unit.
Why does my result differ from my contractor’s?
Contractors often size from the old system or a rule of thumb. If their number is much larger than this estimate, ask how it was calculated. If it is a Manual J result, it should take precedence — it uses details this tool cannot see.
Sources
- Manual J — Residential Load Calculation — Air Conditioning Contractors of America (ACCA). Manual J is the ANSI-recognised standard for residential equipment-sizing loads required by building codes.
- Manual J Load Calculations vs. Rules of Thumb — ACCA HVAC Blog (A. Bailes). Square feet per ton is not a sizing method; Manual J results commonly exceed 1,000 ft² per ton.
- Proper Sizing of HVAC Systems — U.S. DOE Building Science Education Solution Center. Rules of thumb lead to oversized systems; loads should be calculated with Manual J.
- Heat Loss from Buildings — The Engineering ToolBox. Conduction heat loss Q = U·A·ΔT with U = 1/ΣR.
- Calculating heat loss by infiltration (factsheet, 1988) — Washington Energy Extension Service (via AIVC). Infiltration heat loss Q = 0.018 × ACH × volume × ΔT.
- Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.
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