Free Manual J Calculator (Whole-House Estimate)
This is a whole-house estimate that follows the shape of a Manual J: heat loss through each surface, air leakage, solar and internal gains, and a sensible and latent split. It is not a real Manual J. It treats the house as one block with generalized insulation values, and it says plainly what it leaves out.
For what a real load calculation involves, read Manual J: the complete guide.
Whole-house Manual J style estimate
Northeast US (default)Zone 5A · 5°F heating / 88°F cooling design
Estimate only. Not a substitute for a room-by-room Manual J.
Heating load
36,700 BTU/h
31,200 BTU/h – 42,200 BTU/h · 18 BTU/h per sq ft
Cooling load (total)
24,500 BTU/h
2 tons · 12 BTU/h per sq ft
Sensible cooling
22,300 BTU/h
91% of the cooling load
Latent cooling
2,200 BTU/h
9% (moisture removal)
| Component | BTU/h | Share |
|---|---|---|
| Walls | 10,930 | 30% |
| Windows (conduction) | 12,481 | 34% |
| Ceiling / attic | 2,275 | 6% |
| Floor | 1,625 | 4% |
| Infiltration (air leakage) | 9,360 | 26% |
| Total (rounded) | 36,700 | 100% |
| Component | BTU/h | Share |
|---|---|---|
| Walls | 3,531 | 14% |
| Windows (conduction) | 2,496 | 10% |
| Solar gain through glass | 11,521 | 47% |
| Ceiling / attic | 1,330 | 5% |
| Infiltration (air leakage) | 1,123 | 5% |
| People (sensible) | 690 | 3% |
| Appliances and lighting | 1,600 | 7% |
| Latent (moisture) | 2,232 | 9% |
| Total (rounded) | 24,500 | 100% |
What this is NOT
- Not room by room. One block load for the whole house, so it cannot size registers, ducts or individual heads.
- No orientation. Solar gain is averaged across all directions; a west wall of glass is treated like any other.
- No duct losses or gains. Ducts in an attic or crawlspace can add a lot to both loads.
- No measured window specs. U-factor and SHGC come from the insulation level you picked, not the labels on your windows.
- Approximate design temperatures. A real Manual J uses the weather station for the home, not a regional value.
Assumptions behind this estimate
- Climate
- Northeast US (default) (zone 5A)
- Design temperatures
- 5°F / 88°F outdoor; 70°F / 75°F indoor
- Insulation level
- average
- Wall / ceiling / floor / window U
- 0.085 / 0.035 / 0.05 / 0.55
- Window SHGC
- 0.55
- Infiltration
- 0.5 ACH natural (heating)
- Net wall area
- 1,978 sq ft
- Window area
- 349 sq ft
- Ceiling / floor area
- 1,000 / 1,000 sq ft
- Volume
- 16,000 cu ft
- Occupants
- 3
Do this room by room in Tradewright
Real equipment matches, a 15-year cost comparison and a signed proposal, all from one visit.
Why the limits are the point
A free calculator that hides its assumptions looks more precise than it is. This one shows the load broken down by walls, windows, ceiling, floor, air leakage, sun, people and appliances, so you can see which inputs drive the answer and which ones you would want to measure.
A real Manual J goes room by room. It uses the measured area and orientation of every wall and window, the actual window U-factor and solar heat gain coefficient, the duct location, the local design temperatures and an air leakage figure from a blower-door test or a defined estimate. Those room loads are what you size registers, ducts and individual heads from, and they are what most permit offices and rebate programs ask for.
Use this estimate to sanity-check a quote, to see whether air sealing or windows matter more on a given house, or to start a conversation. Do not use it to order equipment or design ducts. The method is published by ACCA in Manual J, and equipment selection follows Manual S.
Sources
How the estimate works
Step 1: Geometry from a few inputs
Square footage, stories, ceiling height and shape give wall, window, ceiling and floor areas and the house volume.
Step 2: Envelope values
Year built or your insulation choice sets U-factors, window SHGC and air changes per hour.
Step 3: Heating: UA × ΔT
Each surface's U-factor × area × indoor-outdoor difference at the 99% design temperature, plus air leakage.
Step 4: Cooling: sensible plus latent
Conduction with a sun and attic premium, solar gain through glass, people, appliances and air leakage, plus moisture from people and outdoor air.
Questions
Is this a real Manual J?
No. It is a whole-house estimate that follows the same physics with generalized assumptions. It does not calculate rooms, orientation, duct losses or measured window specs, so it cannot replace a Manual J for equipment sizing, duct design, permits or rebates.
What does a real Manual J include?
Room-by-room loads from measured walls, windows, doors, ceilings and floors; window orientation, U-factor and SHGC; local design temperatures; infiltration; internal gains; and duct location and losses. The result is a load for each room and for the whole house.
How accurate is a whole-house estimate?
It can land close for a typical house and miss badly for an unusual one, such as a house with a wall of west-facing glass or ducts in a hot attic. The breakdown table shows which parts of the load drive the answer.
Can I use this for a permit or rebate?
No. Permit offices and rebate programs that ask for load calculations expect a proper room-by-room Manual J report.
Why does air leakage matter so much?
Every cubic foot of outdoor air that leaks in has to be heated or cooled and, in humid climates, dried. In older homes air leakage can be one of the largest parts of the load, which is why air sealing often shrinks the equipment you need.
What is sensible and latent load?
Sensible load changes air temperature. Latent load is moisture that has to be condensed out. Equipment must cover both, and the latent share is larger in humid climates.
Is Tradewright ACCA-approved?
Not yet. Tradewright is not on ACCA's list of approved Manual J software. Our load calculations follow the ACCA Manual J (8th edition) and Manual S methods, every input is visible, and reports show the assumptions behind each number. We plan to apply for ACCA approval in 2028 and will update this answer when that status changes.
How do I get a room-by-room Manual J?
Hire a contractor or designer who runs one, or use load calculation software. Tradewright builds a room-by-room Manual J from a LiDAR scan or a plan upload during the sales visit.
Related calculators and guides
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