# Footing Size Calculator

Calculate footing size based on column load and soil bearing capacity. Shows required width, depth, and concrete volume for square and continuous footings.

## What this calculates

A footing is only as good as the soil beneath it. This calculator sizes spread footings by dividing your total load (including a safety factor) by the soil's bearing capacity to find the required area. It works for both square column footings and continuous wall footings, and includes concrete volume for ordering.

## Inputs

- **Total Column/Wall Load** (lbs) — min 0 — Dead load + live load + any other loads
- **Soil Bearing Capacity** — options: Soft clay / Silt (1,000 PSF), Firm clay (1,500 PSF), Stiff clay / Sandy clay (2,000 PSF), Sand / Gravel (3,000 PSF), Dense sand / Gravel (4,000 PSF), Hardpan / Bedrock (6,000 PSF) — Consult a geotechnical report or use conservative values
- **Footing Type** — options: Square (Column Footing), Continuous (Wall Footing)
- **Wall Length (Continuous Only)** (ft) — min 0 — Only used for continuous (wall) footings
- **Safety Factor** — min 1, max 3 — Typical: 1.5 for residential, 2.0-3.0 for critical structures

## Outputs

- **Required Footing Area** (ft²)
- **Footing Width** (ft) — Minimum width (rounded up to next 6 inches)
- **Footing Width** (in)
- **Recommended Depth** (in) — Minimum depth based on projection from column face
- **Concrete Volume** (yd³) — Including 10% waste factor
- **Actual Soil Pressure** (PSF) — Load per square foot on the soil

## Details

## How Footing Sizing Works

The basic formula is: **Footing Area = Total Load / Soil Bearing Capacity**

For a square column footing, the width is the square root of that area. For a continuous wall footing, the width is the area divided by the wall length.

## Typical Soil Bearing Capacities

| Soil Type | Bearing Capacity |
|-----------|-----------------|
| Soft clay / Silt | 1,000 PSF |
| Firm clay | 1,500 PSF |
| Stiff clay / Sandy clay | 2,000 PSF |
| Sand / Gravel | 3,000 PSF |
| Dense sand / Gravel | 4,000 PSF |
| Hardpan / Bedrock | 6,000+ PSF |

PSF = pounds per square foot. These are general guidelines. A geotechnical report gives you the actual numbers for your specific site.

## Depth Guidelines

The minimum footing depth depends on local frost lines and structural requirements:

- **Residential (non-frost):** 8-12 inches deep
- **Frost-protected:** Must extend below the local frost line (24-48 inches in northern states)
- **Structural rule:** Depth should be at least 1/3 of the footing width

## Safety Factors

Residential footings typically use a safety factor of 1.5. Commercial structures may require 2.0-3.0. The safety factor accounts for uncertainty in soil conditions and load estimates.

## Frequently Asked Questions

**Q: How do I find my soil's bearing capacity?**

A: The most reliable method is a geotechnical report from a soil engineer. For residential projects, many building departments provide presumptive values based on your area's soil type. Sandy or gravelly soil typically supports 3,000 PSF; clay around 1,500-2,000 PSF.

**Q: What loads should I include in the total?**

A: Include dead load (weight of the structure itself), live load (occupants, furniture, snow), and any other loads like wind uplift or seismic forces. For a simple residential project, dead load is typically 10-15 PSF per floor and live load is 40 PSF for floors, 20-30 PSF for roofs.

**Q: What is the minimum footing size per IRC code?**

A: The IRC (International Residential Code) Section R403 requires a minimum footing width of 12 inches for single-story, 15 inches for two-story, and 18 inches for three-story conventional wood-frame houses on soil with 2,000 PSF bearing capacity.

**Q: Do I need rebar in my footings?**

A: Most residential footings require at least two horizontal bars of #4 rebar running the length of the footing. Square footings typically get a grid of rebar in both directions. Check your local code, as requirements vary significantly by jurisdiction.

**Q: Can I build a footing on fill soil?**

A: Building on fill is risky unless the fill was properly compacted in controlled lifts. Uncompacted fill can settle unevenly, cracking your foundation. A geotechnical engineer should evaluate any fill soil before you design footings on it.

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Source: https://vastcalc.com/calculators/construction/footing-size
Category: Construction
Last updated: 2026-04-08