ČSN EN ISO 13793

3 Definitions, symbols and units
3.1.7 frost depth
depth of penetration of frost into the ground

6 Climatic data
6.2 Frost depth in undisturbed ground
If $H_0$ is not known, an approximate value may be calculated from the following equation:
$H_0=\sqrt{\frac{7200\cdot F_d\cdot {\lambda }_f}{L+C\cdot {\theta }_e}}$
where
$H_0$: maximum frost depth in undisturbed, snow-free ground, in m;
$F_d$: is the design freezing index, in K·h;
${\lambda }_f$: is the thermal conductivity of frozen soil, in W/(m·K);
L: is the latent heat of freezing of water in the soil per volume of soil, in J/m³;
C: is the heat capacity of unfrozen soil per volume, in J/(m³·K);
${\lambda }_e$: is the annual average external air temperature, in °C.

Annex E (informative)
Worked examples
E.1 No frost insulation
$H_0=\sqrt{\frac{7200\cdot 47000\cdot 2,5}{(150+3\cdot 1,5)\times {10}^6}}=2,34m$
design freezing index $F_{50}$ = 47 000 K·h,
annual mean external temperature ${\lambda }_e$ = 1,5 °C.