FreeHVACCalculators

HVAC Load Calculator

Enter the building’s own numbers — R-values, window ratings, air leakage, design temperatures — and see the heating and cooling loads with a component-by-component breakdown.

Method reviewed October 22, 2026. Version 1.0.0. How it works

Your inputs

Unit system
Stories above grade

About 40–60 gr/lb in humid climates, 20–35 in mixed climates, 0 or less in dry climates.

Whole-wall R is lower than the cavity insulation label because framing bridges the insulation.

Tight new homes ≈ 0.2–0.35; average ≈ 0.5–0.7; leaky older homes ≈ 1.0 or more.

Heat loss per foot of exposed slab edge per °F. Energy-code tables list roughly 0.73 for an uninsulated slab edge and lower values with edge insulation.

0 % for ducts inside conditioned space; 10–30 % for ducts in an attic or crawlspace.

Result

Total cooling load
27,395BTU/h
Required capacity
2.28tons
Design heat loss
42,487BTU/h

Design heat loss ≈ 42,487 BTU/h; cooling load ≈ 27,395 BTU/h (2.3 tons).

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

Heat loss breakdown

Walls
5,555BTU/h
Ceiling / roof
3,733BTU/h
Windows (conduction)
8,232BTU/h
Doors
896BTU/h
Slab edge
7,313BTU/h
Air leakage (infiltration)
9,677BTU/h
Duct losses / gains
7,081BTU/h
Show the working
  1. Footprint and perimeter (square plan)
    2,000 ft² / 1 = 2,000 ft²; perimeter = 4 × √2,000 = 178.9 ft
  2. Net wall area
    1,431 ft² gross − 300 ft² windows − 40 ft² doors = 1,091 ft²
  3. Conditioned volume
    2,000 ft² × 8 ft = 16,000 ft³
  4. Cooling design temperature difference
    91 °F − 75 °F = 16 °F
  5. Total cooling load
    24,691 sensible + 2,704 latent = 27,395 BTU/h
  6. Convert to tons (12,000 BTU/h per ton)
    27,395 / 12,000 = 2.28 tons
  7. Heating design temperature difference
    70 °F − 14 °F = 56 °F
  8. Design heat loss including ducts
    42,487 BTU/h

Who this is for

It is the detailed version of the AC Tonnage and Furnace Sizing calculators: instead of picking “average insulation”, you enter the actual values. It suits energy auditors, designers, students and homeowners who have an energy-audit report or construction drawings to work from.

Method

Each part of the envelope is treated as a heat path:

  • Conduction: Q = U × A × ΔT, where U = 1 ÷ R. Walls use an effective whole-wall R-value (lower than the insulation label, because studs bridge the insulation).
  • Sun on opaque surfaces is approximated with a higher effective temperature difference: +10 °F for walls and +30 °F for a ceiling under a vented attic.
  • Solar gain through glass: window area × SHGC × an average solar factor × an interior-shading factor.
  • Air leakage: 1.08 × CFM × ΔT for temperature and 0.68 × CFM × Δgrains for moisture, with CFM from the air-change rate and house volume.
  • Slab floors lose heat along their exposed edge: F-factor × perimeter × ΔT.
  • People and appliances add internal heat in summer.

A duct allowance is added last as a percentage of the sensible load.

Using the breakdown

The breakdown shows where the load comes from. If windows dominate the cooling load, shading or a lower-SHGC window does more than added attic insulation. If air leakage dominates heating, air sealing is usually the cheapest improvement. Change one input at a time and watch which line moves.

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
Outdoor cooling design temperature (1 %)
91 °F
Outdoor heating design temperature (99 %)
14 °F
Indoor cooling set point
75 °F
Indoor heating set point
70 °F
Design moisture difference (outdoor − indoor)
35 gr/lb
Effective whole-wall R-value
11 h·ft²·°F/BTU
Ceiling / attic R-value
30 h·ft²·°F/BTU
Window area (% of floor area)
15 %
Window U-factor
0.49 BTU/h·ft²·°F
Window SHGC
0.7
Exterior doors
2
Natural air changes per hour (winter)
0.6
Foundation
Slab on grade
Slab edge F-factor
0.73 BTU/h·ft·°F
Duct loss / gain allowance
20 %
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

Design heat loss ≈ 42,487 BTU/h; cooling load ≈ 27,395 BTU/h (2.3 tons).

Assumptions

  • 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.

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.
  • Conditioned basements are not modelled; below-grade wall losses are excluded.

Frequently asked questions

What design temperatures should I use?

The 99 % heating and 1 % cooling dry-bulb temperatures for the nearest weather station. Your local building department, an energy-code compliance document or your HVAC contractor’s Manual J report will list them. They are lower and higher than average weather — that is the point: equipment is sized for conditions exceeded only about 1 % of the year.

What is a whole-wall R-value?

The R-value of the wall assembly as built, including the framing that interrupts the insulation, plus sheathing, drywall and air films. A 2×4 wall with R-13 batts typically performs closer to R-10 or R-11 as a whole.

How do I find my air-change rate?

A blower-door test reports ACH50 (air changes at 50 pascals). Natural air changes are much lower; a common rough conversion divides ACH50 by a factor of about 10–20, depending on climate, building height and exposure. Without a test, use the guidance under the field.

Sources

  1. 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.
  2. 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.
  3. Heat Loss from Buildings — The Engineering ToolBox. Conduction heat loss Q = U·A·ΔT with U = 1/ΣR.
  4. Calculating heat loss by infiltration (factsheet, 1988) — Washington Energy Extension Service (via AIVC). Infiltration heat loss Q = 0.018 × ACH × volume × ΔT.
  5. 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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