Concrete Footing Guide: How to Calculate Volume, Depth, and Width
Introduction
Footings are the buried concrete strips that spread a wall or foundation’s weight over enough soil to prevent settling. Because they’re poured below grade, running short mid-pour is one of the worst places on a job site to discover a shortage. This guide covers how footing volume is calculated and how local code determines the depth and width you should be building to in the first place.
For related project planning, see our concrete calculators page, slab guide , and column guide .
Calculating Footing Volume
Footing width and depth are usually specified in inches, while length is measured in feet — the calculation converts everything to a single unit before multiplying, then reports the result in cubic yards, the way ready-mix suppliers price it.
Formula:
Length × Width × Depth = Cubic feet Cubic feet ÷ 27 = Cubic yards
Example:
20 ft footing, 12 inches wide, 12 inches deep
Width = 1 ft, Depth = 1 ft 20 × 1 × 1 = 20 cu ft 20 ÷ 27 = 0.74 cu yd → round up to 1 cu yd
For help with this calculation, use the Concrete Footing Calculator .
How Deep Should a Footing Be?
Footing depth is usually set by your local frost line to prevent frost heave — the freeze-thaw cycle that can crack or shift a footing that’s too shallow.
| Climate | Typical Frost Line Depth |
|---|---|
| Warm climates | 12–18 inches |
| Moderate climates | 18–36 inches |
| Cold climates | 36–48 inches |
| Very cold climates | 48+ inches |
Always check your local building code rather than assuming a standard depth applies everywhere. In colder regions, this is one of the most important requirements to verify before any excavation begins.
How Wide Should a Footing Be?
A common rule of thumb is twice the wall width sitting on top of it, often working out to 12–24 inches for residential walls.
| Wall Type | Typical Wall Width | Common Footing Width Range |
|---|---|---|
| Residential wall | 6–8 inches | 12–16 inches |
| Foundation wall | 8–10 inches | 16–20 inches |
| Retaining wall | 10–12 inches | 20–24 inches |
That said, engineered footings can vary significantly based on soil bearing capacity and the load the footing needs to support. This rule of thumb is a starting point for estimating material — not a substitute for an engineer’s spec on load-bearing structures.
Confirm Dimensions Before Ordering
Footing dimensions aren’t arbitrary — they’re typically set by your local building code based on the load above and your region’s frost line. Always confirm width and depth against your permit drawings before ordering concrete, since a mismatch between your estimate and the approved plans can mean a failed inspection.
Pre-order checklist:
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Frost line depth verified with local building department
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Footing width confirmed on permit drawings
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Footing depth confirmed on permit drawings
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Soil bearing capacity verified (if required)
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Allowance added (5–10% is often used as a starting point)
Worked Example
The project:
20 ft continuous footing
12 inches wide (1 ft)
12 inches deep (1 ft)
10% allowance
Step 1: Volume
20 × 1 × 1 = 20 cu ft
Step 2: Cubic yards
20 ÷ 27 = 0.74 cu yd
Step 3: Add allowance
0.74 × 1.10 = 0.81 cu yd → round up to 1 cu yd
Common Mistakes to Avoid
Mixing units (feet and inches) without converting before multiplying
Using a generic footing depth instead of checking the local frost line
Assuming the twice-the-wall-width rule applies to engineered footings without confirmation
Ordering concrete based on an estimate that doesn’t match the approved permit drawings
Important Limitations
Estimates Are Not Construction Specifications
This guide provides estimating methodology. Footing dimensions must comply with local building codes and engineering requirements.
For frost line requirements:
Always verify your local frost line depth with the building department. Frost depths vary significantly by region.
Frequently Asked Questions
How deep should a footing be?
Footing depth is set by your local frost line, typically 12–42 inches depending on climate. Always check local code.
What is the standard width for a footing?
A common rule is twice the wall width, often 12–24 inches for residential walls. Engineered footings may vary.
Why are footings poured below the frost line?
Soil below the frost line doesn’t freeze and thaw, so the footing won’t be pushed or cracked by frost heave.
Can I use a slab calculator for footings?
Yes — the volume formula is the same. Use the Concrete Slab Calculator for rectangular footings.
Do footings need reinforcement?
Reinforcement requirements depend on the footing design, loads, and local requirements. Check your project specifications.
How much concrete do I need for a footing?
Use the Concrete Footing Calculator for an instant estimate.
Ready to calculate your footing? Use the free Concrete Footing Calculator — just enter your measurements and get an instant estimate.
Complete Your Concrete Estimating Toolkit
Footings are a critical part of any foundation project, but they’re just one element of a successful concrete pour. Visit our concrete calculators hub for all 8 free concrete estimating tools — including slab calculators, column calculators, rebar calculators, gravel base calculators, and more. Each tool is backed by a detailed guide to help you estimate with confidence.