About this calculator
This calculator gives the design frost penetration depth used to decide how deep a footing must sit so that frost-susceptible ground beneath it does not freeze and heave. Every figure in it applies to Lithuania only. The climate data is Lithuania's building-climatology regulation, the coefficients are the ones Lithuanian foundation practice uses, and the requirement it serves is Lithuania's geotechnical design regulation. Other countries set frost depth by their own codes, tables, and maps; do not use this page outside Lithuania.
Frost depth is jurisdiction-dependent by nature, so the calculator shows its sources rather than a single number. It returns three figures:
- the normative frost depth of open, unheated ground at the chosen station and soil, from the formula Lithuanian practice uses on the current climate normals;
- the design frost depth, the normative figure adjusted for whether — and how — the building's own heat warms the ground next to its footings;
- the observed maximum frost depth actually measured at that station, straight from the regulation's own table, where it has one.
This calculator does not design the foundation. Lithuania's geotechnical regulation takes the design seasonal frost depth from the site's geotechnical investigation report and leaves the choice to the engineer. This page informs that judgement; it does not replace it.
Formula
Normative frost depth (SNiP 2.02.01-83 Cl. 2.27, as presented for Lithuanian conditions in the VGTU foundations textbook, eq. 4.8):
M_t = sum of the absolute values of the negative mean monthly air temperatures at the station
d_fn = d_0 x sqrt(M_t)
d_0 = 230 mm clays and clay loams
= 280 mm sandy loams, silts, fine sands
= 300 mm medium, coarse and gravelly sands
M_t is computed from STR 2.01.12:2024 Annex 2, Table 1 — the 1991–2020 mean
monthly air temperature at 18 Lithuanian meteorological stations plus a
country-wide row. The regulation replaced RSN 156-94 (the source the textbook
names for M_t) on 2024-10-01, and its approving order states that references to
RSN 156-94 in other acts now mean this regulation.
Design frost depth (SNiP 2.02.01-83 Cl. 2.28; textbook eq. 4.9 and Table 4.3):
d_f = k_h x d_fn
k_h = 1.1 unheated building
k_h, heated building, by floor construction and the indoor temperature next to the external walls:
-5 °C 0 °C 5 °C 10 °C 15 °C 20 °C
no basement, floor on ground 1.0 0.9 0.8 0.7 0.6 0.5
basement / technical underfloor 0.9 0.8 0.7 0.6 0.5 0.4
The textbook adds a caveat the calculator enforces: the reduced values (below 1.0
without a basement, below 0.9 with one) must not be used when the floor and
foundation are insulated, because the building's heat then does not reach the
ground next to the footing. With the insulation switch on — the default — k_h
is pinned at 1.0 or 0.9.
Both depths are rounded up to a whole millimetre.
Observed maximum frost depth is not computed: it is STR 2.01.12:2024 Annex 9, Table 1 verbatim — the maximum soil frost depth measured with a frost gauge at 14 stations over 1956–2020, at once-per-10-years and once-per-50-years recurrence. Four stations (Dūkštas, Klaipėda, Laukuva, Nida) and the country average have no row and the calculator says so.
What the regulation in force actually requires. STR 2.05.21:2016 "Geotechninis projektavimas. Bendrieji reikalavimai" lists frost depth among the things a shallow foundation's depth must be chosen against (Cl. 320.3) and exempts three cases (Cl. 321): ground that is not frost-susceptible, a footing base below the frost depth, and thermal insulation against frost. It gives no formula or table of its own — the design seasonal frost depth comes from the geotechnical investigation report.
Worked example
Vilnius, clay, a heated house at 20 °C with an insulated floor and foundation, no basement (the defaults).
STR 2.01.12:2024 Annex 2, Vilnius: Jan -3.7, Feb -3.1, Dec -2.0 °C (the only negative months)
M_t = 3.7 + 3.1 + 2.0 = 8.8
d_fn = 230 x sqrt(8.8) = 230 x 2.9665 = 682.3 -> 683 mm
k_h = 1.0 (heated, but the floor and foundation are insulated)
d_f = 683 mm
STR 2.01.12:2024 Annex 9, Vilnius: 102 cm once in 10 years, 124 cm once in 50 years
The calculator returns exactly this: design depth 683 mm, normative depth
683 mm, observed maximum 1,240 mm — and a warning, because the observed record
is deeper than the formula's figure. Switch the insulation off and k_h drops to
0.5 (342 mm); mark the building unheated and it rises to 1.1 (752 mm).
FAQ
Why is the observed record so much deeper than the formula's result?
Two different things are being compared. The formula uses the 1991–2020 climate
normals — average winters, which in Lithuania have been mild — while Annex 9
records the deepest frost measured at each station across 1956–2020, including
the cold winters of the 1960s–1980s, at 10- and 50-year recurrence. The formula's
result on today's normals is lower than the same formula gave on RSN 156-94's
older data (for Vilnius, M_t was 15.7 then and is 8.8 now). The calculator
shows both and warns when the record exceeds the formula; which one governs a
particular footing is the geotechnical engineer's decision under STR 2.05.21:2016.
Which figure should I use? The one your geotechnical engineer specifies in the site investigation report — that is what STR 2.05.21:2016 requires. Read the smaller figure as the least a footing in warmed ground can get away with under the formula, and the observed record as what the ground at that station has actually done.
Is the formula an official Lithuanian standard? No. SNiP 2.02.01-83 is a Soviet-era norm that is not in force in Lithuania. It is the method the VGTU textbook "Pamatai ir pagrindai" (Sližytė, Medzvieckas, Mackevičius, Technika, 2012, Sec. 4.4) presents for Lithuanian conditions, explicitly as an indicative calculation, alongside the LST EN ISO 13793 method for frost-protected shallow foundations, which this calculator does not implement. The calculator cites it as the origin of the formula, not as a governing document.
My site is far from every station. Pick the climatically closest station, or the country-average row for a rough figure, and treat the result as indicative — the regulation's own data has no finer resolution than its 18 stations.
The soil is coarse sand. Does frost depth still matter? STR 2.05.21:2016 Cl. 321.1 lets frost effects be disregarded for ground that is not frost-susceptible, and the textbook lists gravelly, coarse and medium sands (not silty, not in the capillary zone) as non-heaving. The calculator still reports the depth — the ground still freezes, which matters for pipes and for what the sand sits on — and notes that the footing depth is then governed by other criteria.
Why round the depths up? A footing base is set no shallower than the figure. Rounding a 682.3 mm result down to 682 would be a millimetre in the wrong direction.
Assumptions and limits
Jurisdiction: Lithuania only. Every constant, table row and requirement cited here is Lithuanian or is the method Lithuanian practice uses; other countries differ and this page must not be used for them.
Standards cited, and what each actually covers:
- STR 2.01.12:2024 "Statybų klimatologija" (in force from 2024-10-01) — Annex
2 Table 1, the monthly mean air temperatures
M_tis summed from; Annex 9 Table 1, the observed maximum frost depths. Both transcribed from the regulation's own text and checked row by row against it. - STR 2.05.21:2016 "Geotechninis projektavimas. Bendrieji reikalavimai" — Cl. 320.3 (frost depth must be considered) and Cl. 321 (the three exemptions). It provides no figure; the design frost depth comes from the geotechnical report.
- SNiP 2.02.01-83 — the origin of
d_fn = d_0 x sqrt(M_t),d_0by soil, andd_f = k_h x d_fnwith its Table 1. Not in force in Lithuania; used via the VGTU textbook, which reproduces it, and cross-checked against two independent reproductions of the same table.
Not modelled: the textbook's Table 4.1 minimum foundation depth by soil and
groundwater level (needs the groundwater depth and the soil's liquidity index);
the +0.1 k_h adjustment for footings projecting 1.5 m or more past the wall;
SNiP's fourth soil class (coarse-fragment soils, d_0 = 0.34 m), which the
Lithuanian source omits; the LST EN ISO 13793 frost-index method; and any
frost-protected (insulated) shallow foundation design.
k_h below 1 presumes the building is heated through every winter. During
construction, or in a building left unheated, the ground next to the footing
sees the full normative depth.
This calculator does not design the foundation. It informs the frost-depth input to a geotechnical design that a qualified engineer must make.