How do I calculate the heating load for a single room?

To calculate a room heating load, sum the heat loss through each exposed wall (area × U-value × ΔT), windows (area × U-factor × ΔT), ceiling if exposed to attic (area × U-value × ΔT), floor if over unconditioned space, and infiltration (volume × ACH/3600 × air density × specific heat × ΔT). The total is the room heating load in BTU/hr or watts.

Why do room loads matter for duct sizing?

Each room's heating and cooling load directly determines how much conditioned air (CFM) must be delivered to that room. Room loads feed into duct sizing — a room with a larger load needs a larger supply duct and register. Without room-by-room loads, duct systems are guessed and often poorly balanced. Use the CFM Calculator after calculating room loads.

What is an exposed wall in room load calculations?

An exposed wall is any exterior wall that separates the conditioned room from the unconditioned outdoors. Interior walls between two conditioned rooms are not exposed and have negligible heat loss. The number of exposed walls depends on the room's position — corner rooms have two, rooms in the middle of a floor may have one, and corner rooms on the top floor may have three.

How does window orientation affect room cooling load?

Window orientation dramatically affects peak cooling load. West-facing windows receive the most solar heat gain in the late afternoon (peak summer cooling time), making them the most significant cooling load driver. South-facing windows get midday sun but can be shaded with overhangs. North-facing windows have the least solar gain. East-facing windows get morning sun only. This calculator accounts for orientation-specific solar heat gain using ASHRAE peak irradiance values.

🏠 Room Load Calculator

Room Load Calculator

Calculate heating and cooling loads for any individual room with wall-by-wall detail. Accounts for all exposed walls, windows by orientation (N/S/E/W), ceiling, floor, infiltration, and occupancy. Essential for duct sizing and room-by-room HVAC balancing.

Unit System:

🏠 Room Details

Room Name

Room Type

Floor Position i

Room Length *

Room Width *

Ceiling Height *

Floor Area: — ft²

Volume: — ft³

🏠 Room Orientation Diagram

🌡 Design Temperatures

Heating Setpoint *

°F

Outdoor Heating Design Temp * i

°F

Lookup design temp →

Heating ΔT (auto)

°F

Cooling Setpoint *

°F

Outdoor Cooling Design Temp * i

°F

Cooling ΔT (auto)

°F

▢ Exposed Walls

💡

Add each exposed exterior wall of this room. Interior walls between conditioned rooms do not need to be entered. Each wall entry includes its area, R-value, orientation, and window area.

▲ Ceiling & Floor

Ceiling Exposure i

Ceiling R-Value i

hr·ft²·°F/BTU

Floor Exposure i

Floor R-Value i

hr·ft²·°F/BTU

Roof Color / Absorptance i

Exterior Wall Color i

🌫 Infiltration & Occupancy

Infiltration ACH i

ACH

Regular Occupants i

ppl

Lighting Load i

📊 Room Load Results — Room

BTU/hr

🌡 Heating Load — Component Breakdown

Heat Loss by Component

❄ Cooling Gain — Component Breakdown

📊 Heating vs Cooling by Component

Detailed Results Table

Component	Detail	Heating (BTU/hr)	Cooling Sensible	Cooling Latent

Export:

How to Calculate HVAC Load for a Single Room

Enter Room Dimensions

Enter the room length, width, and ceiling height. Floor area and volume calculate automatically and display in the info strip below. The room diagram updates to show compass orientation. Use the length converter if your measurements are in mixed units.

Set Design Temperatures

Enter indoor heating setpoint (70°F / 21°C typical) and outdoor winter design temperature. Then enter cooling setpoint (75°F / 24°C) and summer outdoor design temperature. ΔT calculates automatically. Use our design temperature lookup for your specific city.

Add Each Exposed Wall

Click "Add Exposed Wall" for each exterior wall. Enter the gross wall area, R-value, and compass orientation (N/S/E/W). Then enter window area within that wall with U-factor and SHGC. The diagram updates to show which walls have been entered. Only add walls that face the outdoors — interior partition walls have negligible heat loss.

Set Ceiling and Floor

Select ceiling exposure (attic, flat roof, or interior/conditioned above) and floor condition (crawlspace, slab, or conditioned basement). Enter R-values for each. The U-value / R-value calculator can help determine effective R-values for composite assemblies.

Calculate and Use Results

Click Calculate Room Load for instant results. The CFM recommendation at the bottom tells you exactly how much supply air this room needs from the HVAC system. Feed this into the CFM calculator and duct sizing calculator to design the air distribution. For whole-house loads, use the load estimator.

Room-by-Room HVAC Load Calculations

Calculating HVAC loads at the room level — rather than just the whole-house level — is essential for proper air distribution system design. Each room's individual heating and cooling load determines how much conditioned air (measured in CFM) must be delivered to that space through the supply duct and register. Without room-by-room loads, duct systems are guessed, leading to hot and cold rooms, humidity imbalances, and occupant discomfort.

How Room Heating Load is Calculated

Room heating load (BTU/hr) is the sum of all heat loss pathways from that specific room. This includes: heat loss through each exposed exterior wall (wall heat loss), heat loss through windows in each wall (window heat loss), heat loss through the ceiling to the attic if exposed (roof/ceiling heat loss), heat loss through the floor if over a crawlspace or slab (floor heat loss), and infiltration heat loss from air leakage through the room's exterior surfaces (infiltration load). The formula for each component is: Q = A × U × ΔT, where A is surface area (ft²), U is the assembly U-factor (BTU/hr·ft²·°F), and ΔT is the indoor-outdoor temperature difference.

How Room Cooling Load Differs

Room cooling load is more complex because it includes solar heat gain through windows — which varies dramatically by orientation. West-facing windows on a summer afternoon can generate 580 W/m² (54 BTU/hr·ft²) of solar radiation, making them the dominant cooling load component in many rooms. This calculator uses orientation-specific peak solar irradiance values from ASHRAE fundamentals to account for this. See the solar heat gain calculator for detailed analysis. Cooling load also includes occupant latent and sensible heat gain, lighting heat, and equipment loads.

From Room Load to CFM

Once you have the room's sensible cooling load (or heating load), you can calculate the required air volume in CFM using: CFM = Q_sensible / (1.1 × ΔT_supply), where ΔT_supply is the temperature difference between supply air and room air. For cooling, supply air is typically 55°F (13°C) delivering to a 75°F (24°C) room — a ΔT of 20°F. Use the CFM calculator to compute this, then feed the CFM into the duct sizing calculator to determine the correct duct and register sizes for each room.

Related Tools for Complete Room HVAC Design

Complete all rooms with the whole-house load estimator
Size supply ducts with the duct sizing calculator
Calculate required CFM per room with the CFM calculator
Analyze window solar gain in detail with the solar gain calculator
Check latent load and humidity with the latent heat calculator
Convert R-values and U-factors with the U-value / R-value calculator

Frequently Asked Questions

Whole-house loads determine what size furnace or AC unit to buy. But without room-by-room loads, you cannot properly design the duct system. Each room needs a specific CFM of supply air proportional to its load — if you guess, you end up with some rooms too hot, others too cold, and poor humidity control throughout the house. Room loads feed directly into duct sizing and register selection.

It depends on the room's position in the house. A corner bedroom on an exterior corner has two exposed walls. A bedroom in the middle of a floor with only one exterior wall has one. A top-floor corner bedroom may have two exposed walls plus an exposed ceiling. Rooms fully surrounded by other conditioned rooms have zero exposed walls and essentially zero envelope heat loss. Always check your floor plan and count only walls that face the outdoors directly.

Solar heat gain through windows is the biggest variable in room cooling loads. West-facing windows receive intense afternoon sun during the peak summer cooling period (2–5 PM), generating up to 580 W/m² of solar radiation. South-facing windows get midday sun but can be shaded with overhangs. East-facing windows only get morning sun. North-facing windows get virtually no direct solar gain. A bedroom with one large west-facing window can have twice the cooling load of an identical room with only a north-facing window. This is why the solar gain calculator is so valuable for room-level analysis.

Use the effective R-value of the assembly — not just the insulation. For a 2×6 wall with R-20 batt insulation, the effective R-value including framing, sheathing, and drywall is approximately R-16 to R-19 (framing reduces effective R-value by 15–25%). For an attic with R-40 blown insulation, the effective R-value is close to R-38–40 since there is little framing. Use the U-value / R-value calculator to calculate the effective R-value of any composite assembly layer by layer.