Heat Loss Calculator UK

Calculator Inputs

Use exposed areas and realistic U-values for the room or zone you want to assess.

Project or Room Name

Length (m)

Width (m)

Height (m)

Volume Override (m³)

Leave blank to use length × width × height.

Air Changes per Hour (ACH)

Internal Temperature (°C)

External Temperature (°C)

Safety Factor (%)

Annual Degree Days

Heating Hours per Day

Thermal Bridge Allowance (W/K)

Exposed Wall Area (m²)

Wall U-Value (W/m²K)

Roof/Ceiling Area (m²)

Roof U-Value (W/m²K)

Floor Area (m²)

Floor U-Value (W/m²K)

Window Area (m²)

Window U-Value (W/m²K)

Door Area (m²)

Door U-Value (W/m²K)

Formula Used

The calculator combines fabric heat loss, ventilation heat loss, and an optional thermal bridge allowance. It then multiplies the total coefficient by the design temperature difference.

Fabric Heat Loss Coefficient (W/K) = Σ(Area × U-value)

Ventilation Heat Loss Coefficient (W/K) = 0.33 × ACH × Room Volume

Total Heat Loss Coefficient (W/K) = Fabric + Ventilation + Thermal Bridge Allowance

Total Design Heat Loss (W) = Total Heat Loss Coefficient × (Internal Temp − External Temp)

Recommended Output (W) = Total Design Heat Loss × (1 + Safety Factor ÷ 100)

Estimated Annual Energy (kWh) = Total Heat Loss Coefficient × Degree Days × 24 ÷ 1000

Use exposed building element areas only. Insert realistic U-values from design data, surveys, product sheets, or project specifications.

How to Use This Calculator

Enter the room name and measured dimensions.
Add a manual volume only if your usable heated volume differs from geometric volume.
Set the internal design temperature and the outdoor winter design temperature.
Enter the ACH value for the room or zone.
Provide exposed wall, roof, floor, window, and door areas.
Enter a U-value for each building element.
Add a thermal bridge allowance if you want a fuller estimate.
Choose a safety factor for emitter or boiler sizing.
Click the calculate button to show the result above the form.
Use the CSV or PDF buttons to export your findings.

Example Data Table

Example Item	Sample Value	Unit
Room Size	5.00 × 4.80 × 2.40	m
Calculated Volume	57.60	m³
Internal / External Temperature	21 / -3	°C
Temperature Difference	24	°C
Fabric Coefficient	23.72	W/K
Ventilation Coefficient	13.31	W/K
Thermal Bridge Allowance	2.00	W/K
Total Heat Loss Coefficient	39.03	W/K
Total Design Heat Loss	936.72	W
Recommended Output with 10% Margin	1030.39	W

FAQs

1. What does this calculator estimate?

It estimates room or zone heat loss for UK-style heating design. It combines transmission through building elements with ventilation losses and optional thermal bridge allowance to show the heating output needed under design conditions.

2. What U-values should I use?

Use measured, specified, or surveyed U-values for the actual building elements. For quick early-stage checks, you can use provisional values, but detailed design should always use project-specific data.

3. What is ACH in the form?

ACH means air changes per hour. It estimates how often indoor air is replaced. Higher ACH values increase ventilation heat loss and can significantly raise the heating requirement.

4. Why is external temperature important?

The outdoor design temperature sets the winter temperature difference. A colder external value creates a larger ΔT, which increases both fabric and ventilation heat loss.

5. Should I include a safety factor?

A modest margin can help with emitter sizing and practical selection. However, excessive oversizing may reduce efficiency, so use a sensible allowance rather than a large guess.

6. Does this replace a full compliance calculation?

No. This tool is excellent for design checks, comparisons, and planning. Formal compliance work may require broader project data, regulated methods, and specialist assessment.

7. Why can annual energy differ from meter data?

Actual energy use depends on weather, controls, occupancy, setbacks, solar gains, internal gains, and system efficiency. The annual figure here is an estimate based on the degree days you enter.

8. Can I use this for one room only?

Yes. It works well for a single room, a zone, or repeated room-by-room checks. For full buildings, calculate each space or aggregate areas and volumes carefully.