Manual J load calculator

What a load calculation adds up

Every load calculation is the same four sums repeated per room. Conduction through walls, ceilings and glass: area × U-value × design temperature difference. Solar gain through glass: area × a gain factor that depends on which way the window faces. Infiltration: the volume of the room times its air change rate times the temperature difference times 0.018, the volumetric heat capacity of air. And internal gain: people, lights and appliances, which matter for cooling and are ignored for heating because the design condition is a winter night.

The sheet above does exactly that. The heating column drops the solar and internal terms; the cooling column keeps them and adds a flat 1.2 BTU/h per square foot for lights and plug loads.

Design temperature difference

The two ΔT fields are where most home-made calculations go wrong. Heating ΔT is your indoor setpoint minus the 99% winter design temperature for your location — not the record low. Cooling ΔT is the 1% summer design dry-bulb minus 75 °F, and it is almost always a small number: 20 °F in most of the country, 25 °F in the desert Southwest.

Screening values for the 70 °F winter and 75 °F summer indoor setpoints a load calculation is required to assume. The outdoor half is a county lookup with published limits, not a judgement call, and those are the numbers a permit reviewer will check.

Sources: EPA ENERGY STAR, National HVAC Design Report v3.1/3.2/3.3 (Rev. 14, revised 2025-01-15), Item 3.1 (read 2026-09-04) · EPA ENERGY STAR, National HVAC Design Report v3.1/3.2/3.3 (Rev. 14, revised 2025-01-15), Item 3.3 and Footnote 18 — Design Temperature Limit Reference Guide (read 2026-09-04)

Why the cooling number is smaller than you expect

The cooling design ΔT is often only 19 or 20 degrees. That is the whole reason houses get oversized: a contractor reasons from "it hits 98 outside" rather than from the difference between 95 and 75. Put the real numbers in and a 2,000 ft² house in zone 4 usually lands under three tons, not the four that gets quoted.

Manual S has a stated window, which is what makes "round up a size" checkable: the equipment's listed capacity at design conditions should land at 95–115% of the design load, or the next nominal size. A four-ton box on a three-ton load is not a safety margin — it is outside the window.

Sources: DOE Building America Solution Center, Air Conditioning (read 2026-09-04) · EPA ENERGY STAR, National HVAC Design Report v3.1/3.2/3.3 (Rev. 14, revised 2025-01-15), Item 4.1 and Footnote 26 (read 2026-09-04)

What this sheet is not

It is not a certified Manual J. It uses one U-value per assembly type rather than a full construction library, it applies a single infiltration rate to the whole house rather than a blower-door-derived envelope leakage, and it does not compute the latent load separately. Permit offices that require a load calculation want output from ACCA-approved software with the contractor's name on it. Use this to build the room list, catch an obviously oversized quote and understand what the software is doing.