Window U-value calculator: inputs, result and limits
Which inputs are required?
Enter visible glazing area Ag and projected frame area Af, their U-values Ug and Uf, plus the glazing-edge length lg and linear thermal transmittance psi_g. Areas are measured in elevation; psi_g must match the actual spacer-frame-glazing system.
What is returned?
Results are window area Aw, window U-value Uw, the window heat transfer coefficient H, heat flow at the selected temperature difference, and glazing/frame area fractions.
Method and interpretation
Uw = (Ag·Ug + Af·Uf + lg·psi_g)/(Ag + Af). Area-weighted glazing and frame terms are supplemented by the linear edge term. A small window has proportionally more frame and edge, so identical components can still produce a higher Uw value.
Calculator limits
Pre-design for multiple-glazed windows without mullions or opaque panels. Installation joint, reveal, shutter box, air leakage, solar gains, shading and condensation are excluded. Use declared product values for specification and compliance work.
Related next calculator
Source: DIN EN ISO 10077-1:2020-10, section 6.4.2.1.1, equation (2), checked locally; Lohmeyer Practical Building Physics, 10th ed., section 2.3.4.2, worked 1.23 m × 1.48 m window. The reduced calculator sets the mullion term in the complete standard equation to zero.