Formula & worked example
Q=Σ(U·A·ΔT)+ρ·cp·Vdot·ΔT; Qdesign=Q·(1+margin).
A 120 m² envelope at U=0.45 W/m²K and 25 K temperature difference loses 1.35 kW by transmission before infiltration.
Heating & HVAC engineering calculator · Updated Sep 17, 2026
Heating Load Calculator — U-Value, Area, Infiltration & Heater Size
Estimate design heating load from envelope UA loss plus infiltration, with watts, BTU/h, kW and a configurable sizing margin.
Inputs
Live result
Temperature difference 25 K
Envelope loss 1.35 kW
Infiltration/air load 1.4243374 kW
Block heating load 2.7743374 kW (9,466.4322 BTU/h)
Design output with margin 3.0517711 kW
Assumptions & limits
- Uses a lumped envelope U-value and area for a transparent block-load estimate.
- Solar/internal gains and thermal bridges are not silently credited.
Frequently asked questions
What does this Heating Load Calculator calculate?
Estimate design heating load from envelope UA loss plus infiltration, with watts, BTU/h, kW and a configurable sizing margin.
What units are supported?
Inputs state their unit basis and results include practical SI and US customary equivalents where useful, so the calculation can be checked against field data and equipment schedules.
Can I use this for final equipment selection?
Use it for transparent first-pass sizing and engineering cross-checks. Final selections should use measured conditions, manufacturer performance data and applicable codes or design standards.