Heat Loss Calculator

How to use this heat loss calculator

What this calculator does

This calculator estimates the steady heating power required to offset heat escaping from one room through its external walls, windows, doors, floor, ceiling, and normal air leakage. It applies the standard fabric-loss relationship Q = U × A × ΔT to each exposed element and adds a room-volume-based ventilation allowance. The result is an engineering estimate in watts, kilowatts, and BTU per hour. It does not replace a room-by-room survey, an airtightness test, a heating-system design standard, or a professional check of emitter temperatures and system controls.

When to use it

Use it to compare radiator or heater sizes during early planning, to see how insulation changes affect a room, to estimate the effect of a colder outdoor design temperature, or to identify which envelope components contribute most to heat loss. For final heat-pump, boiler, or radiator selection, confirm assumptions with a qualified designer.

How to calculate

1. The calculator opens with a complete demonstration room and a validated example workbook, so results and Download Excel are available immediately.
2. Replace Length, Width, and Height with clear internal room dimensions in metres.
3. Choose Room level, Wall insulation, and Number of external walls. Select Custom U-value only when you have a known whole-wall thermal transmittance.
4. Enter the Number of windows, Number of external doors, Ambient temperature, and Internal temperature. Results update live.
5. Read Power required, then review the exact element-by-element table. Download Excel exports the current inputs and outputs as a real workbook. Reset clears the demonstration data and disables export until a complete valid state is entered again.

Input guide

Length, Width, and Height are required decimal measurements in metres, each greater than zero. Values such as 5, 4, and 2.4 define the room volume and exposed surface areas; larger dimensions generally increase heat loss. Do not enter feet or include unit text. Room level determines whether the model adds the floor loss, ceiling loss, both, or neither. A middle-floor room usually has neither exposed surface; a single-storey room has both.

Wall insulation selects the wall U-value: 2.2, 1.0, or 0.6 W/(m²·K), with lower values indicating better insulation. The Custom wall U-value is required only in custom mode and must be positive. U-value describes heat flow per square metre per degree of temperature difference; the U.S. Department of Energy's explanation of window and door energy-performance ratings provides useful context for interpreting lower and higher values.

Number of external walls is an integer from one to four. This model conservatively counts the longest wall faces first. Number of windows and Number of external doors are whole-number counts from zero upward. Each window is represented by 1.5 m² at U 2.5, and each door by 1.8 m² at U 2.4. For unusually large glazing or doors, a detailed element-area calculation is more accurate. Ambient temperature and Internal temperature accept Celsius values; internal temperature must be higher. A larger temperature difference increases every loss component in direct proportion.

Output guide

Power required is the total estimated steady heating capacity in kilowatts. Heat loss is the same result in watts, while Heating output converts it to BTU/h. Net opaque wall area subtracts modeled window and door area from the selected exposed wall area and cannot fall below zero. Design temperature difference is internal minus ambient temperature. Heat-loss coefficient is the combined watts-per-kelvin rate before multiplying by the temperature difference. The table shows each Building element, its modeled Area, U-value, Coefficient, and final Heat loss. High values identify the biggest opportunities for envelope improvement; zero is valid only when that element is not exposed or has zero count.

Worked example

The startup example uses a 5 m × 4 m × 2.4 m top-floor room, two external walls, mediocre wall insulation at U 1.0, two windows, one external door, an outdoor temperature of – 10 °C, and an indoor target of 20 °C. The room volume is 48 m³ and the temperature difference is 30 K. The model totals the wall, glazing, door, ceiling, and air-change coefficients to about 103.9 W/K. Multiplying 103.9 W/K by 30 K gives about 1,697 W, displayed as approximately 1.70 kW and 5,790 BTU/h. Minor one- or two-watt differences can occur because the interface rounds displayed component values while calculations retain full precision.