Manual J Calculator for HVAC Contractors.

Choose your state.

Each state page collects reference data for that state: ASHRAE design temperatures for the top metros, the energy code in force, and HVAC contractor licensing. The calculator does not load this data; it looks up design conditions from the project location. Pilot batch covers 10 states; more added as the data is verified.

• ArizonaAZ · 4 metros
• CaliforniaCA · 5 metros
• FloridaFL · 5 metros
• GeorgiaGA · 4 metros
• NevadaNV · 4 metros
• New MexicoNM · 3 metros
• North CarolinaNC · 4 metros
• South CarolinaSC · 4 metros
• TennesseeTN · 4 metros
• TexasTX · 5 metros

How Manual J load calculations work.

What is Manual J?

A full Manual J run takes the envelope (R-values, areas, window U-factor and SHGC), infiltration, occupancy, internal gains and local climate data for every room. The output is BTU/h per room and for the whole house, which Manual S then maps to equipment. For permit submission, use software on ACCA's approved Manual J list (for example Wrightsoft Right-J8 or Cool Calc Manual J) and check what the AHJ wants to see. BuildSolver runs the Manual J 8th Edition formulas for sales-phase sizing and client quotes.

When do you need a Manual J calculation?

Most retrofit and new-construction jobs fall into one of these cases.

1. Permit submission. IRC M1401.3 and IECC 2021 R403.7 require equipment sizing on loads from Manual J or another approved methodology for new construction; for change-outs in existing homes it depends on the code edition and the AHJ - for the full permit-grade workflow and AHJ checklist, see our permit-grade Manual J workflow.
2. Sales-phase quote workflow. A preliminary Manual J calculation gives a contractor a defensible quote number in about 90 seconds, while the homeowner is still at the kitchen table. BuildSolver fits here.
3. Equipment replacement. ACCA Manual S caps oversize at ≤115% for single-speed cooling and ≤140% heating of the MJ8 load. Run the load before you quote the change-out. Matching the old nameplate carries any original oversizing into the new system.

What inputs does the Manual J calculator need?

Four input categories drive the calc:

1. Building envelope - square footage, wall construction type, insulation R-values, window count plus U-factor and SHGC. Per ACCA MJ8 §5 (Heating Loads, Inputs and Assumptions).
2. Climate - outdoor design temperatures (ASHRAE 99% heating dry-bulb / 1% cooling dry-bulb): ASHRAE 2021 station values for 10 metro cities, an approximate IECC climate zone estimate for a ZIP code, not location-specific. Station tables for every other site are in the ASHRAE Handbook Fundamentals Chapter 14. State reference pages list design temps for the top metros - see Manual J for Texas for an example.
3. Infiltration tightness - MJ8 Tables 5A (heating) and 5B (cooling) categorize Tight / Semi-Tight / Average / Semi-Loose / Loose by floor area and wind shielding. Without a blower-door test, the contractor uses vintage and visible cues to land within ±1 category.
4. Internal gains - occupants (default = bedrooms + 1 per MJ8) and a 1,200 BTU/h sensible allowance that covers lighting, the refrigerator and plug loads (a BuildSolver default). Houses with LED lighting and ENERGY STAR appliances usually run below that allowance, so the estimate leans conservative.

Worked example for a 2,000 sq ft Houston single-story.

Inputs for a 1985-vintage Houston ranch: 2,000 sq ft single-story on slab, 4 bedrooms (5 occupants per MJ8), wood-frame R-13 wall + R-30 ceiling, 250 sq ft of low-e double windows (U = 0.32, SHGC = 0.40), "Average" infiltration per MJ8 Table 5A (no blower-door test on file).

Design conditions, Houston (IECC zone 2A; station KIAH, ASHRAE 2021 Climatic Design Conditions, 99% heating / 1% cooling, whole °F within 1°F): winter 99% DB 34°F, summer 1% DB 95°F, MCWB 77°F.

Sales-phase preliminary sizing - for permit submission use ACCA-approved Manual J software.

Numbers above are illustrative - actual loads depend on orientation, shading, and verified envelope data. For the full ACCA-grounded calculation behind these numbers, see our ACCA-grounded methodology.

Cost and turnaround.

Manual J calculation cost is a per-house fee when contractors subcontract the workup to a third-party engineer or ACCA-approved software service (Wrightsoft, Cool Calc). BuildSolver's free tier is built for the sales-phase quote at $0 per calc. It runs whole-house Manual J 8th Edition and returns a load in about 90 seconds on a phone or laptop. Use it to price the job, then order a permit load when the contract is signed.

The formula behind the quick estimate.

The on-page calculator runs the whole-house Speed Sheet simplification of ACCA Manual J 8th Edition, the same deterministic code BuildSolver's assistant uses for a first sizing pass. Every component is computed separately and summed; the tonnage is the total rounded to the nearest half ton.

Cooling total

Q_cool = transmission + solar + infiltration + internal + ductIndoor design 75°F dry-bulb, 50% RH. Sensible and latent parts are reported separately; SHR = sensible ÷ total.

Transmission

Q = U × A × (CLTD multiplier × ΔT)Walls, ceiling, floor, glass and doors. U = 1/R from year-built × climate-zone defaults; wall area from a square footprint at 8 ft per story.

Solar through glass

Q = SHGC × A_glass × zone solar factor × 0.775Glass is 15% of floor area, split evenly across N/E/S/W. The 0.775 factor is the average of the four orientation factors in ACCA Manual J 8th Edition Table 3A. The page gives no shading credit.

Infiltration

Q_s = 1.08 × CFM × ΔT; Q_l = 4840 × CFM × ΔWCFM = ACH × house volume ÷ 60 with ACH by air-tightness class. Both terms are scaled by the ASHRAE standard-atmosphere air-density ratio at the elevation you enter, or, left blank, where Census and USGS data pin down the ZIP's elevation. Otherwise the result states that no altitude correction was applied.

Internal gains

Q = 230 × occupants + 1,200 (sensible) + 200 × occupants (latent)Per-person sensible and latent gains plus a baseline for lighting, refrigerator and plug loads.

Ducts in attic

cooling × 1.12 of sensible; heating × 1.20Applied only when ducts run through an unconditioned attic.

Heating total

Q_heat = Σ(U × A × ΔT) + slab edge + 1.08 × CFM × ΔT + ductΔT = 70°F indoor − 99% heating design dry-bulb. A slab's loss is figured at the edge as F2 × exposed perimeter × ΔT (Manual J Table 5C). Infiltration uses winter ACH by tightness class with the same elevation correction. The furnace input bin, the heat pump size at 47°F and its capacity at design temperature come from the same heating tool.

Standards and data sources.

• ACCA Manual J 8th Edition - residential load procedure. The page uses its whole-house Speed Sheet simplification, so expect the result to differ from a full room-by-room calculation.
• IECC climate zone estimate - outdoor design conditions (1% cooling dry-bulb with a wet-bulb, 99% heating dry-bulb) as an approximate estimate for the ZIP's climate zone, not location-specific. Official city values are in ASHRAE Handbook - Fundamentals, Ch. 14.
• IECC 2021 climate zones (PNNL county map) - ZIP → county → climate zone; the zone sets design temperatures, solar factor and envelope defaults.
• ASHRAE Fundamentals Eq. 9.5 / 9.6 - sensible (1.08) and latent (4840) infiltration constants, with a standard-atmosphere density correction where Census and USGS data pin down the ZIP's elevation.
• USGS 3D Elevation Program (3DEP) - ground elevation at the 2020 census centers of population of the tracts overlapping the ZIP (block groups for tracts larger than about 10 sq mi), averaged by population in the ZIP's land area, for the air-density correction.
• ACCA Manual S - equipment sizing limits (cooling ≤115% of the Manual J load). The result warns once half-ton rounding lands above 107% of the load, before that limit is reached; equipment selection itself is a separate step.

Limitations of the quick estimate.

• The load is a whole-house average, so it cannot size individual supply registers or find one hot bonus room.
• Design temperatures are an approximate estimate for the IECC climate zone, not location-specific. For permit work use the design conditions for your exact location.
• Insulation levels and, unless you pick one, window type come from year-built × climate-zone defaults. Real upgrades since construction change the answer a lot.
• Glass is fixed at 15% of floor area, evenly split across orientations, with no shading credit. A west-facing wall of glass or deep overhangs are not modeled.
• The residential scope is 100-20,000 sq ft and 1-3 stories. Light-commercial, multifamily and VAV systems need a different procedure.
• Preliminary sales-phase estimate. Not ACCA-approved software output and not accepted for permit submission.
• Elevation is the population-weighted average for the ZIP unless you enter the site's own. A house far up or down a slope in a hilly ZIP sits at a different elevation. BuildSolver treats the air-density correction as small below about 2,000 ft. Where the data cannot pin a ZIP's elevation down and you enter none, no altitude correction is applied and the result says so.

Common Manual J mistakes to avoid.

1. Sizing by square footage. A square-feet-per-ton rule of thumb ignores climate, glass and air leakage. Two 2,000 sq ft homes in the same city can land a full ton apart once window type and tightness are entered.
2. Sizing to record temperatures. Manual J sizes to the 1% cooling and 99% heating design values from ASHRAE Handbook - Fundamentals, Chapter 14. Designing to the extreme builds in oversizing before any other input.
3. Leaving the envelope on defaults. If the windows were replaced or the attic re-insulated, say so. On a 1985 Houston ranch, left blank, window type falls back to a year-built default, and glass type can change the cooling number substantially. Pick the glass you actually see.
4. Stacking safety factors. Manual J already uses conservative design conditions. Rounding the load up, adding 10% and then buying the next size up is how a 2-ton house ends up with a 3-ton unit.
5. Forgetting the ducts. Ducts in a vented attic add a measurable share to both loads. Leaving that box unchecked under-sizes; checking it when ducts are inside conditioned space over-sizes.

Related calculators.

• Replacement AC sizing calculator Same load engine, framed for change-outs: compare the existing tonnage against the load.
• Manual S equipment selection Next step after the load: check a specific unit against the ≤115% single-speed cooling limit.
• How Manual S selection works The sizing limits by equipment type, walked through by hand on four jobs.
• Retrofit sizing without a blueprint Inputs to collect on a site visit when no plans exist.
• Manual J for new construction Plan-based loads with specified envelope values.
• Furnace size calculator The same load engine, read from the heating side: furnace input bin and heat-pump capacity.
• Duct sizing calculator Trunk and branch sizes from design CFM, longest run and available static pressure.
• Gas pipe sizing calculator Pipe size from the furnace nameplate input plus every other gas appliance on the line.
• Evaporative cooler size calculator Airflow for the sensible load with a swamp cooler in a dry climate.