Room heating and cooling load calculator: U·A·ΔT, solar gain, ventilation, people and equipment, in W, BTU/h and tons

Estimate the peak heating and cooling load of a room from its walls, windows and roof (U-values), sun through the glass, air leakage and ventilation (sensible and latent), and the heat from people, lighting and equipment. Results in watts, BTU per hour and tons of refrigeration, with a breakdown. A simplified estimate, not a full ASHRAE or CIBSE method.

Keep it one tap away for everyday use.

Room
Gains
Loads
Breakdown

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A simplified estimate for study and planning, not a full ASHRAE or CIBSE load calculation: it uses steady temperature differences and peak sun, with no thermal mass, time lag or hour-by-hour sun. Check important designs with a qualified engineer.

Room and design conditions

Floor area (m²) Ceiling height (m) Altitude (m above sea level) Heating: inside (°C) Heating: outside design (°C) Heating: outside humidity (% RH) Cooling: inside (°C) Cooling: inside humidity (% RH) Cooling: outside design (°C) Cooling: outside humidity (% RH)

Design outdoor temperatures are the cold and hot conditions you design for, usually a value exceeded only a few hours a year at your location; look them up for your town. The example values are made up.

Walls, windows and roof

One element per line: name, area (m²), U-value (W/m²·K), then for glass the SHGC and the sun on it (W/m²), and for sunlit walls and roofs an extra temperature difference (K) for cooling

U-value: heat flow per m² per degree. Very roughly, 0.15 to 0.4 for an insulated wall or roof, 2 for an uninsulated cavity wall, 1 to 3 for windows; use the values for your own construction. SHGC is the share of sunlight a window lets in as heat (about 0.25 to 0.7). Sun is the solar irradiance on the glass at the peak hour (up to about 800 W/m² on a sunny facade, less with shading). The extra ΔT allows for sun-heated outer surfaces. Only list surfaces between the room and outside (or an unheated space).

Outdoor air and internal gains

Infiltration (air changes per hour) Ventilation outdoor air (L/s) People Sensible heat per person (W) Latent heat per person (W) Lighting (W/m² of floor) Equipment (W) Safety margin (%)

Let internal gains reduce the heating load (usually left off: the room must heat up when empty)

Infiltration is air leaking in through gaps: roughly 0.2 to 0.5 per hour for a tight modern room, 1 or more for an old draughty one. Ventilation is fresh air brought in on purpose. A seated person gives off very roughly 70 to 75 W of sensible heat and 45 to 55 W of moisture (latent) heat, more when active. All of these are rounded example figures; use your own.

Peak loads

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What it does

Before choosing a radiator, heat pump or air conditioner you need the room’s peak load: how much heat it loses on the coldest design day, and how much heat (and moisture) has to be removed on the hottest. This tool adds up the main parts:

Fabric: heat through each wall, window and roof, U × A × ΔT. For cooling, an extra temperature difference you choose allows for sun-heated walls and roofs.
Sun through glass: area × SHGC × the sunlight falling on it at the peak hour.
Outdoor air: air leaking in (infiltration, in air changes per hour) plus fresh air brought in on purpose (ventilation, L/s). It has to be heated in winter; in summer it brings in both heat (sensible) and moisture (latent), worked out from the humidity ratios inside and outside.
Internal gains: people (sensible and latent), lighting and equipment.

You get the heating load, and the sensible, latent and total cooling loads, in watts, kW, BTU/h and tons of refrigeration, the load per m² of floor, the sensible heat ratio, a bar chart and a breakdown table.

This is a simplified steady-state estimate. A full ASHRAE (heat balance, radiant time series) or CIBSE calculation also handles the heat stored in walls and furniture, the time lag before sun on a wall reaches the room, and the sun hour by hour. For study and planning, check important designs with a qualified engineer.

How to use it

Give the floor area, ceiling height and altitude (thinner air carries less heat).
Give the inside temperatures and humidity you want, and the outside design conditions for your town.
List each wall, window and roof that faces outside, with its area and U-value. Add the SHGC and sun for glass. Paste from a spreadsheet or open a CSV or Excel file.
Give the air leakage, ventilation, people, lighting and equipment, and a safety margin.

Worked example

A made-up 20 m² office, 2.7 m high, with 10.5 m² of external wall (U 0.35), a 3 m² window (U 1.6, SHGC 0.4, 400 W/m² of sun) and a 20 m² roof (U 0.2). Heating to 21 °C with −10 °C outside, the fabric loses 387 W and the 27.5 L/s of outdoor air 1,151 W; with a 10 % margin the heating load is 1,691 W (5,771 BTU/h). Cooling to 24 °C and 50 % RH with 32 °C and 40 % outside, the sun through the window alone adds 480 W. Two people, 200 W of lights and 300 W of equipment bring the cooling load to 2,048 W: 1,699 W sensible and 349 W latent, a sensible heat ratio of 0.83, or 6,989 BTU/h (0.582 tons).

Tips

Outdoor air is often the biggest heating item in a leaky room. Sealing gaps or using heat recovery cuts it.
Shade the glass: the sun through a west window can outweigh everything else on a summer afternoon.
Run the cooling case for the sunniest facade and hour; the peak is not always at noon.

FAQ

Why is latent load separate? An air conditioner must also condense moisture out of the air. A unit sized only for the sensible load will leave the room cool but clammy.

Should I add the internal gains to heating? Usually not: a room must still warm up when it is empty and dark. Tick the box only if the gains are always there.

Where the numbers come from

Steady-state fabric loss Q = U × A × ΔT, solar gain through glazing Q = A × SHGC × I, and the sensible and latent loads of outdoor air from mass flow and humidity ratios: standard building services engineering, as set out in textbooks such as McQuiston, Parker and Spitler, Heating, Ventilating and Air Conditioning (methods only).
Moist air properties from the ASHRAE Handbook, Fundamentals (2017, SI) chapter 1 equations, shared with fibub's psychrometric calculator.
Ranges for U-values, SHGC, sun, air leakage and heat from people are rounded general figures chosen by fibub, labelled as rough; no table from ASHRAE, CIBSE or any code is copied. The example room is made up.
Checked against an independent Python calculation written for the tests.

Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.

Last updated 2026-11-09.