Heating Load Calculator
Calculate heating load step by step: Q = ΣUA × ΔT + 0.33 × n × V × ΔT − internal gains (not below zero).
In short
Formula: Q = ΣUA × ΔT + 0.33 × n × V × ΔT − internal gains (not below zero).
Heating load
3.825 kW
- Fabric (U·A·ΔT)
- 3,000 W
- Ventilation (0.33·n·V·ΔT)
- 825 W
- Internal gains credited
- 0 W
- BTU/h
- 13,051
What this calculator does
Calculate heating load step by step: Q = ΣUA × ΔT + 0.33 × n × V × ΔT − internal gains (not below zero). Worked example, questions and limitations included.
Use it to turn Envelope U × A (sum over walls, roof, windows), Design indoor–outdoor temperature difference, Room volume, Air changes per hour, and the other shown inputs into a checked result you can compare, copy, or rerun with different assumptions.
The page shows the formula, a numeric worked example, and the assumptions that affect this thermal & hvac calculation.
Inputs and what they mean
- Envelope U × A (sum over walls, roof, windows)
- — Sum of U-value × area for each surface.
- Design indoor–outdoor temperature difference
- — number value.
- Room volume
- — number value.
- Air changes per hour
- — number value.
- Internal gains (people, lights, equipment)
- — number value.
How to use it
- Enter each value in the unit shown next to the box (use the unit converter first if your numbers are in other units).
- Check the breakdown to see every intermediate step.
- Read the limitations before relying on the result.
Formula
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).
Worked example
ΣUA 150 W/K at ΔT 20 K
- Fabric = 150 × 20 = 3,000 W.
- Ventilation = 0.33 × 0.5 × 250 × 20 = 825 W.
- Total = 3,825 W ≈ 3.83 kW.
Reading the result
The headline figure is the main answer. Any breakdown underneath shows the parts that make it up, so you can check the working and see what changes when you adjust an input.
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.
Common questions
What outdoor temperature should I use?
Use the published design temperature for your location from your national code or climate data. Check the formula, example, and limitations on this page before using the result for a real thermal & hvac decision.
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. Check the formula, example, and limitations on this page before using the result for a real thermal & hvac decision.
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.
Related tools
Calculate heat transfer step by step: Conduction Q = k A ΔT ÷ L; convection Q = h A ΔT; overall Q = U A ΔT.
Calculate cooling load step by step: Q = ΣUA × ΔT + 0.33 × n × V × ΔT + internal gains (sensible load, W).
Calculate energy consumption step by step: E = P_rated × load factor × hours/day × days/year.
Calculate equipment energy cost step by step: Cost = kWh × price per kWh + fixed or demand charges.