Manual J Load Calculator

What this tool does

Estimate residential heating loss and sensible cooling gain from building surfaces and air infiltration. Simplified estimator, NOT a certified ACCA Manual J calculation. Climate is descriptive; enter local design temperatures explicitly.

How to use the Manual J Load Calculator

1. Enter or select floor area in sqft.
2. Enter or select ceiling height in ft.
3. Enter or select net exterior wall area in sqft.
4. Enter or select wall r-value.
5. Enter or select ceiling r-value.
6. Enter or select window area in sqft.
7. Enter or select window u-factor.
8. Enter or select door area in sqft.
9. Enter or select door r-value.
10. Enter or select climate zone (editable design δt below).
11. Enter or select heating indoor-outdoor difference (°f).
12. Enter or select cooling outdoor-indoor difference (°f).
13. Enter or select infiltration ach.
14. Enter or select occupants.
15. Enter or select appliance heat gain (btu/h).
16. Enter or select window solar gain (btu/h per sq ft).
17. Read the calculated result; change any measurement to compare alternatives.

Formula

UA=wallArea/wallR+area/roofR+windows×windowU+doors/doorR; infiltrationCFM=area×height×ACH/60; heating=(UA+1.08×CFM)×heatingΔT; cooling=(UA+1.08×CFM)×coolingΔT+windows×solar+people×400+appliances

area

Floor area (sqft, before metric conversion)

height

Ceiling height (ft, before metric conversion)

wallArea

Net exterior wall area (sqft, before metric conversion)

wallR

Wall R-value

roofR

Ceiling R-value

windows

Window area (sqft, before metric conversion)

windowU

Window U-factor

doors

Door area (sqft, before metric conversion)

doorR

Door R-value

climate

Climate zone (editable design ΔT below)

heatingDT

Heating indoor-outdoor difference (°F)

coolingDT

Cooling outdoor-indoor difference (°F)

ach

Infiltration ACH

people

Occupants

appliances

Appliance heat gain (BTU/h)

solar

Window solar gain (BTU/h per sq ft)

Simplified estimator, NOT a certified ACCA Manual J calculation. Climate is descriptive; enter local design temperatures explicitly.

Worked example

For manual j load calculator, the following measurements illustrate the exact method: Floor area: 1500; Ceiling height: 8; Net exterior wall area: 1200; Wall R-value: 13; Ceiling R-value: 30; Window area: 200; Window U-factor: 0.35; Door area: 40; Door R-value: 5; Climate zone (editable design ΔT below): mixed; Heating indoor-outdoor difference (°F): 60; Cooling outdoor-indoor difference (°F): 20; Infiltration ACH: 0.5; Occupants: 4; Appliance heat gain (BTU/h): 2000; Window solar gain (BTU/h per sq ft): 50.

Results explained

Estimated heating loss (BTU/h)

Estimated heating loss (BTU/h) from the formula above. Simplified estimator, NOT a certified ACCA Manual J calculation. Climate is descriptive; enter local design temperatures explicitly.

Estimated cooling gain (BTU/h)

Estimated cooling gain (BTU/h) from the formula above. Simplified estimator, NOT a certified ACCA Manual J calculation. Climate is descriptive; enter local design temperatures explicitly.

Frequently asked questions