Replacement AC sizing from your site-visit notes, no blueprint needed.

How the replacement load is calculated.

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.

A worked example for a 1,200 sq ft Phoenix replacement.

1,200 sq ft single-story on slab · ZIP 85003, Phoenix, AZ · built 2008, average air tightness, three occupants, ducts inside conditioned space · replacing a 3-ton condensing unit. Existing ductwork stays.

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 replacement sizing mistakes.

1. Matching the old nameplate. The unit coming out was often picked with a square-foot rule. Replacing like-for-like repeats whatever oversizing it had.
2. Ignoring upgrades since the last install. New windows, attic insulation or air sealing lower the load. Enter the house with those upgrades in place.
3. Upsizing to fix one hot room. A room that never cools is usually a duct or airflow problem. A bigger condenser short-cycles the rest of the house and still starves that room.
4. Skipping the heating side on a heat pump swap. For heat pumps, the heating load at design temperature often governs. Check the capacity at design temperature, not only the 47°F rating.
5. Treating the load as the equipment size. The load is the target. Manual S then checks a specific model's capacity at your design conditions before you quote it.

The problem

Where replacement quotes go wrong.

• Manufacturer sizing tools point you toward their own equipment. You end up picking a brand before the homeowner has agreed to a price.
• A square-foot rule ignores envelope, glass and climate, so it often lands a replacement a size too big, with short cycles and high indoor humidity to follow.
• A spreadsheet Manual J usually gets done back at the office. By then the first quote has already gone out on a rule of thumb.

Whole-house Manual J in chat

Describe the home; get a sized result with citations.

1. Describe the home. Square footage, location, wall construction, window count, insulation level. The chat asks follow-up questions when it needs them.
2. Run the load. BuildSolver calculates cooling and heating per ACCA Manual J 8th Edition with IECC climate-zone defaults for the ZIP, and the same inputs always return the same load.
3. Get the result. Block load, sensible/latent split and the nominal tonnage the load points to. Client PDF export with your company branding is on the Pro and Team plans.

The output is a Manual J load number. Quote from the load across brands and pick the model after the homeowner signs. Manual S equipment selection (Pro and Team plans) runs separately when you need to match a specific coil.

BuildSolver vs manufacturer software

BuildSolver next to manufacturer sizing software.

BuildSolver takes the job description in chat on your phone and returns a Manual J load you can match to any brand's equipment.

Common questions about replacement sizing.

Related calculators.

• Manual J calculator The load method behind this page, with state design data and a worked example.
• Manual S equipment selection Check a specific replacement unit against the calculated load.
• Retrofit sizing without a blueprint What to measure on the walkthrough when there are no plans.
• How to size a heat pump Balance point, design-temperature capacity and backup heat.
• Furnace size calculator Replacing the furnace too? The heating side of the same calculation.
• HVAC design temperatures by ZIP The outdoor design conditions this load uses, and where they come from.