# Heating Load Calculator

> Calculate heating load step by step: Q = ΣUA × ΔT + 0.33 × n × V × ΔT − internal gains (not below zero).

Page: https://freecalculator.tech/tools/heating-load-calculator
Last reviewed: 2026-11-03

## Formula

Q = ΣUA × ΔT + 0.33 × n × V × ΔT − internal gains (not below zero).

- Design ΔT = indoor set point − outdoor design temperature for your location.
- Internal gains are subtracted only if you enter them; many designers leave them at 0 for a conservative result.
- Simplified steady-state method.
- Q = ΣUA × ΔT + 0.33 × n × V × ΔT − internal gains (not below zero).
- Inputs used: Envelope U × A (sum over walls, roof, windows), Design indoor–outdoor temperature difference, Room volume, Air changes per hour, Internal gains (people, lights, equipment).

## Inputs

- Envelope U × A (sum over walls, roof, windows)
- Design indoor–outdoor temperature difference
- Room volume
- Air changes per hour
- Internal gains (people, lights, equipment)

## Worked example: ΣUA 150 W/K at ΔT 20 K

250 m³ room, 0.5 ACH.
1. Fabric = 150 × 20 = 3,000 W.
2. Ventilation = 0.33 × 0.5 × 250 × 20 = 825 W.
3. Total = 3,825 W ≈ 3.83 kW.

## Common questions

### What outdoor temperature should I use?

Use the published design temperature for your location from your national code or climate data.

### Where do U-values come from?

From building specifications or energy certificates. The calculator does not assume any.

### Why are internal gains optional?

Occupants and equipment are not always present on the coldest night, so they are often ignored for sizing.

### How do I use the Heating Load Calculator?

Enter the required values for Envelope U × A (sum over walls, roof, windows), Design indoor–outdoor temperature difference, Room volume, Air changes per hour, Internal gains (people, lights, equipment). The calculator applies the formula on this page and shows the main result with any supporting breakdown so you can check the arithmetic.

### What formula does the Heating Load Calculator use?

Q = ΣUA × ΔT + 0.33 × n × V × ΔT − internal gains (not below zero). The visible formula section above lists the calculation path and the edge cases the page handles, so the result can be checked without relying on the form alone.

### Can the Heating Load Calculator be used for exact decisions?

Use it as a calculation aid, not as a substitute for checking the underlying rule, contract, policy, or professional advice that applies to your situation. When a result depends on local rules, personal details, prices, or dates, enter those values yourself and confirm them before acting.

## Limitations and assumptions

- Steady state; no warm-up allowance.
- Ground-floor losses must be included in ΣUA yourself.
- Not a full code-compliant heat-loss calculation.
- Results are estimates for planning and learning. Real designs, installations and bids must be checked by a qualified, licensed professional against the codes that apply where you work.
- The result depends on the values you enter for this heating load calculator; it does not supply missing rates, rules, prices, dates, or assumptions for you.

## Sources

- ASHRAE — https://www.ashrae.org/technical-resources/ashrae-handbook

## Cite this page

- APA: Free Calculator. (2026, November 3). Heating Load Calculator. https://freecalculator.tech/tools/heating-load-calculator
- MLA: "Heating Load Calculator." Free Calculator, 3 Nov. 2026, https://freecalculator.tech/tools/heating-load-calculator.
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