Foundation Depth Calculator

About the Foundation Depth Calculator

This calculator answers the two questions a residential footing has to satisfy: how deep its bottom must sit below grade, and how wide it must be to carry its load. It follows the International Residential Code (IRC) minimums and presumptive soil bearing values used across most of the US, with a metric mode for the same rules.

Depth and width are governed by different things. Depth is set by frost and the code minimum — not by load. Load and soil strength set the width. Treating one as the other is the most common mistake in footing estimates.

How Depth Is Determined

At least 12 in below undisturbed ground for exterior footings (IRC R403.1.4).
Below the frost line (R403.1.4.1). Soil that freezes under a footing expands and lifts it — frost heave. The frost depth is set by your local building department (IRC Table R301.2(1)); across the US it ranges from effectively zero along the Gulf Coast to 48–60 in or more in the northern states.
The required depth is the greater of the two. Frost-protected shallow foundations (R403.3), which use rigid insulation to raise the frost line, are the main exception.

How Width Is Determined

Wall footing: Width = Wall load per foot (plf) ÷ Allowable bearing (psf)

Column footing: Area = Column load (lb) ÷ Allowable bearing (psf); side = √Area

Footings must also be at least 12 in wide and 6 in thick (IRC R403.1.1), so a light load on good soil is governed by the minimum rather than the formula. Widths are rounded up to the next ¼ in — a footing is never sized smaller than the load needs.

Presumptive Soil Bearing Values

Without a geotechnical report, the IRC allows these allowable bearing pressures (Table R401.4.1). They already include a margin of safety, so they are used directly — not divided by a further safety factor.

Soil	USCS	psf	kPa
Crystalline bedrock	—	12,000	574.6
Sedimentary and foliated rock	—	4,000	191.5
Sandy gravel and/or gravel	GW, GP	3,000	143.6
Sand, silty sand, clayey sand, silty gravel, clayey gravel	SW, SP, SM, SC, GM, GC	2,000	95.8
Clay, sandy clay, silty clay, clayey silt, silt, sandy silt	CL, ML, MH, CH	1,500	71.8

Worked Example

An exterior wall carrying 2,800 plf on silty sand (2,000 psf) where the local frost depth is 36 in:

Depth = greater of 12 in and 36 in = 36 in (3 ft) to the bottom of the footing — frost governs
Width = 2,800 ÷ 2,000 = 1.4 ft = 16.8 in, rounded up to 17 in — load governs over the 12 in minimum

The same wall on clay (1,500 psf) needs 2,800 ÷ 1,500 = 1.87 ft, or 22.5 in.

When You Need an Engineer

Expansive soils — clays with a plasticity index of 15 or more need special design (IRC R403.1.8).
Fill, organic or soft soils, or bearing values below the presumptive table.
High groundwater, slopes, or footings near an existing foundation or retaining wall.
Commercial buildings or heavy loads, which fall under the IBC and usually require a soils report.

This tool gives code-minimum estimates for planning and bidding. It is not a foundation design — the engineer of record and your building department have the final say.

How to Use the Foundation Depth Calculator

1Choose the units: Select US (in, psf, lb) or metric (mm, kPa, kN).
2Pick the footing type: Choose a wall (strip) footing or a column (pad) footing.
3Enter the frost depth: Type your local frost depth from the building department, or 0 where there is no frost.
4Add load and soil for sizing: Optionally enter the wall load per length or the column load, and the allowable bearing pressure or a soil type.
5Read the results: See the minimum footing depth and, with a load, the required footing width or size.

Frequently Asked Questions

How deep should a foundation be?

Below the local frost line and at least 12 in (305 mm) below undisturbed ground, under IRC R403.1.4. Frost depths range from 0 in much of the southern US to 48 in or more in the northern states and Canada; your building department publishes the local figure.

Why must footings go below the frost line?

Water in soil above the frost line freezes and expands, lifting shallow footings (frost heave) and cracking walls. Footings below it rest on soil that does not freeze.

How is footing width calculated?

Width = load per unit length ÷ allowable bearing pressure. A wall carrying 2,000 lb per ft on soil with 1,500 psf capacity needs a footing 1.33 ft (16 in) wide; the IRC also sets minimum widths by the number of stories.

What bearing pressure should I use?

From a soils report if there is one. Otherwise IRC Table R401.4.1 presumes 1,500 psf for clay and silt, 2,000 psf for sand and 3,000 psf for sandy gravel.

What about frost-protected shallow foundations?

In cold climates a shallow footing can be protected with rigid insulation instead of digging below the frost line, following IRC R403.3 and ASCE 32. It needs careful insulation design.