What does EN 673 calculate?

The U-value of the centre of a glazing unit, Ug, in W/(m²·K), under standard conditions, from the panes, the cavities, the gas and the emissivity of the coatings. It leaves out the spacer and the frame, which the window U-value of EN ISO 10077-1 adds.

What is the Ug of a 4-16-4 unit with argon and low-E glass?

1.10 W/(m²·K) by the site's EN 673 calculator, with argon at 90 % and a coating of emissivity 0.03. With air and no coating the same unit gives 2.74.

Which edition of EN 673 does the calculator follow?

The calculation of the 2011 edition, with triple glazing as its annex A sets out, and the surface coefficients of the 1997 edition with its amendment A1 (he = 23, hi = 8 W/(m²·K)). EN 673:2024 has since replaced the 2011 edition; the calculator has not been checked against it.

Glass · Standards

EN 673 explained: the U-value of insulating glass (Ug)

EN 673 («Glass in building — Determination of thermal transmittance (U value) — Calculation method») gives the U-value of the centre of a glazing unit, Ug: the heat that crosses it per m² for each degree of difference between inside and outside. The U-value calculator for insulating glass follows its calculation, cavity by cavity. CEN has since published a 2024 edition, EN 673:2024, which replaces the 2011 one (DIN Media lists DIN EN 673:2011-04 as replaced by DIN EN 673:2025-01); the calculator follows the method described below and has not been checked against the 2024 edition. This page summarises what the calculator implements; it does not reproduce the standard, which is sold by the national standards bodies. The title of the standard is taken from the catalogue entries below, consulted on 8 November 2026.

Open the U-value calculator with the example →

What the calculator works out with it

The centre-pane U_g of a double or triple glazed unit, in W/(m²·K).
For every cavity, the radiation part h_r and the gas part h_g separately, so you see where the heat goes.
The overall thickness of the unit.

What it asks for

Double or triple glazing, and the thickness of each pane.
The width of each cavity.
The fill of each cavity: Air, Argon 90 %, Krypton 90 %, Xenon 90 %.
The low-E coating in each cavity, by its emissivity: None — uncoated glass, ε = 0.837; Hard coat (pyrolytic), ε = 0.16; Soft coat, ε = 0.05; Soft coat, high performance, ε = 0.03; Best available, ε = 0.013.

How it is calculated, and what the site assumes

1/U_g = 1/h_e + Σ 1/h_s + Σ d/λ + 1/h_i h_s = h_r + h_g h_r = 4 · σ · (1/ε₁ + 1/ε₂ − 1)⁻¹ · T_m³ h_g = Nu · λ_gas / s, Nu = A · (Gr · Pr)^n, not less than 1

Centre-of-pane U-value of double and triple glazing by the calculation of EN 673: radiation and gas conductance of each cavity, the gas part from the Nusselt number (Grashof and Prandtl, vertical glazing, A = 0.035, n = 0.38), standard conditions h_e = 23 and h_i = 8 W/(m²·K), mean temperature 283 K and ΔT = 15 K; in triple glazing the 15 K is shared between the two cavities in proportion to their resistance, repeated until it settles (EN 673:2011, annex A). Gas fills are 90 % gas and 10 % air, their properties mixed in proportion. The emissivity is chosen from five classes: 0.837 (uncoated), 0.16, 0.05, 0.03 and 0.013.

Known deviations, as how we check the calculations lists them:

The surface coefficients are those of EN 673:1997 with its amendment A1, h_e = 23 and h_i = 8 W/(m²·K), the ones the ift report uses; the 2011 edition of EN 673 has h_e = 25 and h_i = 7.7. With the 2011 values the sum is 0.0014 m²K/W larger: a U_g of 1.1 comes out 0.002 W/(m²·K) lower and one of 0.6 less than 0.001 lower, so the calculator is on the safe side by that much.

An example worked by the calculator

A 4-16-4 unit with argon at 90 % and a soft low-E coating of emissivity 0.03:

radiation across the cavity h_r = 0.153 W/(m²·K): the coating has taken it down from 3.70 with plain glass on both faces;
gas h_g = 1.218 W/(m²·K), with a Nusselt number of 1.104: the argon has started to move;
U_g = 1.10 W/(m²·K). The same unit with air and no coating gives 2.74; the triple 4-16-4-16-4 with argon and two coatings, 0.56.

Open this example in the calculator. Its «show the working» gives every step with the figures.

What it leaves out

Centre of pane only: no spacer, glass edge or frame. That is the window calculator.
Vertical glazing only: sloped and horizontal units convect differently.
Emissivity by class, not by product, and taken as the corrected emissivity of EN 673: a data sheet's normal emissivity ε_n is a little lower than that. The exact coating decides the last decimal: for a declared value, use the manufacturer's figure.
Heat loss only: no g-value or light transmission.

The calculator is run against 18 published reference cases, side by side with the published figure and the tolerance we accept, on how we check the calculations.

A comparison and pre-dimensioning aid, not a certificate: a declared value is the manufacturer's tested figure for the exact coating.

Sources

Questions people ask

What does EN 673 calculate?: The U-value of the centre of a glazing unit, Ug, in W/(m²·K), under standard conditions, from the panes, the cavities, the gas and the emissivity of the coatings. It leaves out the spacer and the frame, which the window U-value of EN ISO 10077-1 adds.
What is the Ug of a 4-16-4 unit with argon and low-E glass?: 1.10 W/(m²·K) by the site's EN 673 calculator, with argon at 90 % and a coating of emissivity 0.03. With air and no coating the same unit gives 2.74.
Which edition of EN 673 does the calculator follow?: The calculation of the 2011 edition, with triple glazing as its annex A sets out, and the surface coefficients of the 1997 edition with its amendment A1 (he = 23, hi = 8 W/(m²·K)). EN 673:2024 has since replaced the 2011 edition; the calculator has not been checked against it.