Room-by-Room Load Calculator
Estimate how much heating and cooling each room needs — and the whole-house total — from each room’s floor area, exterior wall, windows and what is above and below it.
Method reviewed October 25, 2026. Version 1.0.0. How it works
This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.
Result
- Total heating load
- 12,617BTU/h
- Total cooling load
- 8,555BTU/h
- Total cooling load
- 0.71tons
3 rooms: about 12,617 BTU/h of heating and 8,555 BTU/h (0.71 tons) of cooling in total.
- Cooling — sensible
- 7,183BTU/h
- Cooling — latent
- 1,371BTU/h
Heating load by room
- Room 1
- 7,048BTU/h
- Room 2
- 2,784BTU/h
- Room 3
- 2,784BTU/h
Cooling load by room
- Room 1
- 4,716BTU/h
- Room 2
- 1,919BTU/h
- Room 3
- 1,919BTU/h
Show the working
- Design conditionsHeating ΔT 56 °F (70 °F inside), cooling ΔT 16 °F (75 °F inside); walls R-11, ceiling R-30, windows U-0.49, SHGC 0.7
- Room 1300 ft², 35 ft exterior wall, 45 ft² windows → heating 7,048, cooling 4,716 BTU/h
- Room 2150 ft², 12 ft exterior wall, 15 ft² windows → heating 2,784, cooling 1,919 BTU/h
- Room 3150 ft², 12 ft exterior wall, 15 ft² windows → heating 2,784, cooling 1,919 BTU/h
- Duct losses and gains20 % added to each room’s heating and sensible cooling
Why room loads matter
A whole-house load tells you how big the equipment should be. Room loads tell you where the heating and cooling has to go: how much air each supply register should deliver, which rooms need a bigger duct, and how to divide a home into zones. A corner bedroom with two big windows and an attic above can need several times the airflow of an interior room the same size.
ACCA Manual J, which codes require for residential equipment sizing, works room by room for exactly this reason.
How each room is calculated
Each room uses the same component method as our HVAC Load Calculator:
- Walls and windows from the room’s exterior wall length, ceiling height and window area.
- A ceiling only if the room has a roof or attic above it.
- A slab edge or crawlspace floor only if it is on the lowest level.
- Air leakage shared out by room volume.
- People and, for a kitchen, appliance heat.
- The same duct allowance for every room.
The climate, insulation, window type and duct location apply to the whole house. The room results add up to the whole-house total.
What it does not include
Windows are not split by compass direction, so a west-facing room that gets afternoon sun will usually need more cooling than shown. Heat moving between rooms through interior walls is ignored, and doors and basement walls are not counted per room. For a design you will build ducts from, use a full Manual J.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Climate
- Zone 4 — mixed (e.g. Baltimore)
- Insulation & air sealing
- Average (code-built 1990s–2000s)
- Window type
- Double pane, clear
- Duct location
- Attic/crawl — typical
- Ceiling height
- 8 ft
- Number of rooms
- 3 rooms
- Room 1: floor area
- 300 ft²
- Room 1: exterior wall length
- 35 ft
- Room 1: window area
- 45 ft²
- Room 1: roof or attic above?
- Yes (top floor)
- Room 1: floor
- Slab on grade
- Room 1: people
- 2
- Room 1: kitchen appliances?
- No
- Room 2: floor area
- 150 ft²
- Room 2: exterior wall length
- 12 ft
- Room 2: window area
- 15 ft²
- Room 2: roof or attic above?
- Yes (top floor)
- Room 2: floor
- Slab on grade
- Room 2: people
- 1
- Room 2: kitchen appliances?
- No
- Room 3: floor area
- 150 ft²
- Room 3: exterior wall length
- 12 ft
- Room 3: window area
- 15 ft²
- Room 3: roof or attic above?
- Yes (top floor)
- Room 3: floor
- Slab on grade
- Room 3: people
- 1
- Room 3: kitchen appliances?
- No
Result
- Total heating load
- 12,617 BTU/h
- Total cooling load
- 8,555 BTU/h
- Total cooling load
- 0.71 tons
- Cooling — sensible
- 7,183 BTU/h
- Cooling — latent
- 1,371 BTU/h
3 rooms: about 12,617 BTU/h of heating and 8,555 BTU/h (0.71 tons) of cooling in total.
Assumptions
- Uses the same component method and constants as our HVAC Load Calculator, applied to each room: walls and windows from exterior wall length and window area, ceiling for top-floor rooms, slab edge or crawlspace floor, air leakage shared by room volume, people, kitchen appliances and duct losses.
- Indoor design temperatures: 70 °F heating, 75 °F cooling. Climate presets are representative values for each IECC zone; choose Custom for your own.
- Rooms are treated independently; heat moving between rooms through interior walls is ignored.
Limitations
- An estimate, not a Manual J. Orientation of each window, shading, room-specific air leakage and internal walls are simplified.
- Doors and below-grade walls are not included per room; add them with the Building Heat Loss Calculator if they matter.
Frequently asked questions
Is this a Manual J calculation?
No. It follows the same physics in a simplified form, and it is useful for planning, checking a quote or comparing rooms. Manual J also accounts for window orientation and shading, construction details, internal walls and room-by-room air leakage, and it is what codes and many utility programs require.
How do I measure the exterior wall length?
Add the lengths of the room’s walls that face outdoors, in feet. A corner room has two exterior walls; an interior room has none. Do not count walls next to a garage as interior — treat them as exterior.
Why do the room totals differ from the whole-house calculator?
The whole-house calculator assumes a square floor plan and spreads windows evenly. Entering each room’s real walls and windows usually gives a more realistic total, especially for long, narrow or many-cornered houses.
Can I use the room results to size registers?
They are a starting point. A room’s share of the total cooling load is roughly its share of the system airflow. Check the register size with the Register and Grille Sizing Calculator.
Sources
- Heat Loss from Buildings — The Engineering ToolBox. Conduction heat loss Q = U·A·ΔT with U = 1/ΣR.
- Calculating heat loss by infiltration (factsheet, 1988) — Washington Energy Extension Service (via AIVC). Infiltration heat loss Q = 0.018 × ACH × volume × ΔT.
- 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.
- 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.
- Proper Sizing of HVAC Systems — U.S. DOE Building Science Education Solution Center. Rules of thumb lead to oversized systems; loads should be calculated with Manual J.
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