How deep do concrete footings need to be?

Two separate requirements stack: the footing itself must be at least 6 in thick (IRC R403.1), and its bottom must sit below the local frost line — anywhere from 12 in in the southern US to 60+ in in the upper Midwest and Canada. Your building department publishes the local frost depth; it is not negotiable.

How wide should a footing be for an 8-inch wall?

The rule of thumb is twice the wall thickness — 16 in for an 8 in wall — which matches IRC Table R403.1(1) for most one- and two-story houses on average soil (1,500–2,000 psf bearing). Weak soils or three-story loads push widths to 20–24 in; that's an engineering decision, not a calculator's.

Can I pour footings directly against the trench walls?

Yes, if the trench is clean, stable and cut to size — earth-formed footings are standard practice for residential work. Expect to use 10–15% more concrete than the formed calculation because trench walls are never perfectly straight. Set the waste allowance to 15% for trench pours.

Do footings need rebar?

Codes require it in seismic design categories D0–D2 and where footings span soft spots; two continuous #4 bars top and bottom is the common residential detail. Even where code doesn't demand it, two #4 bars cost little and bridge minor soil inconsistencies. Follow your approved plans.

How much concrete for deck footings?

Deck piers are columns, not strip footings — use the concrete column calculator for round sonotube piers. This page's tool is for continuous footings under walls.

Concrete · Foundations

Concrete Footing Calculator

Total run × width × depth, with the right waste factor for trench or formed pours. Sizing guidance follows IRC Chapter 4.

MaterialsCalc Editorial TeamConstruction Estimating EditorsJuly 15, 2026

Contents

1.Footing sizes that pass inspection
2.The footing volume formula
3.Worked example: house perimeter footing
4.What goes wrong in trench pours

Quick answer

How much concrete do I need for a footing?

A 120 ft footing run at the standard 16×8 in section needs about 4.35 yd³ of concrete with a 10% waste allowance — roughly 8 tonnes of material. Width and depth are structural decisions, so confirm both against IRC Table R403.1 before ordering.

Rule of thumb:: A 16×8 in footing uses about 1 cubic yard per 30 linear feet.
Formula:: Order (yd³) = (Run × Width × Depth ÷ 27) × (1 + waste)

Footings by common run length

16 in wide × 8 in deep, 10% waste included. Cost uses the $150–180/yd³ delivered ready-mix range from our concrete cost guide. Bag counts aren't shown — at this scale, ready-mix is standard.
Footing run (16×8 in)	Order (yd³)	Approx. weight	Est. material cost
40 ft	1.45 yd³	2.7 t	$218–261
80 ft	2.90 yd³	5.3 t	$435–522
120 ft	4.35 yd³	8.0 t	$653–783
160 ft	5.79 yd³	10.6 t	$869–1,042
200 ft	7.24 yd³	13.3 t	$1,086–1,303

Footing sizes that pass inspection

Footing dimensions come from the code table, not from the concrete budget.

Condensed from IRC Table R403.1(1), load-bearing values 1,500–2,000 psf. Always confirm against the adopted code in your jurisdiction.
Supporting	Minimum width	Typical section
1-story wood frame	12 in (305 mm)	12 × 6 inOn ≥1,500 psf soil
2-story wood frame	15 in (380 mm)	16 × 8 inMost common residential pour
3-story / brick veneer	18–23 in	20 × 10 inVerify bearing capacity
Garden / landscape walls	2 × wall thickness	16 × 8 in for 8 in wallBelow frost line
Deck beams (continuous)	Per span tables	Use column calc for piersPiers are more common

The footing volume formula

Formula

V = Run × Width × Depth × (1 + waste)

Run: total length of all footings (ft or m)
Width: footing width (in or mm)
Depth: footing thickness (vertical) (in or mm)
waste: 0.10 formed · 0.15 earth-formed

For a rectangular building, Run = 2 × (length + width). Interior bearing walls add their own runs — walk the foundation plan and sum every footing line.

Worked example: house perimeter footing

Worked example

A 40 × 28 ft house with one 28 ft interior bearing wall. Footings are 16 in wide × 8 in deep, earth-formed in clean trenches.

1
Perimeter run
2 × (40 + 28) = 136 ft
2
Add interior bearing wall
136 + 28 = 164 ft total run
3
Convert section to feet
16 in = 1.333 ft · 8 in = 0.667 ft
4
Gross volume
164 × 1.333 × 0.667 = 145.8 ft³
5
Add 15% (earth-formed) and convert
145.8 × 1.15 ÷ 27 = 6.21 yd³

Result: Order 6.5 yd³. One truck, one pour, no cold joints in the foundation.

What goes wrong in trench pours

Footings feed directly into wall volume — if you're pouring a stem wall or full foundation wall on top, run the wall calculator next and schedule both pours with the plant at once. Plants discount combined orders more often than you'd think; see the cost guide for how delivery fees stack.

Frequently asked questions

Estimating the whole foundation?

Footings, walls and a slab each take two minutes — then hand the plant one combined order.

Open the Concrete Calculator

Foundation reading list

Sources & references

[1]2021 International Residential Code, Ch. 4: Foundations — International Code Council, 2021
[2]ACI 318-19: Building Code Requirements for Structural Concrete — American Concrete Institute, 2019
[3]IS 456:2000 — Plain and Reinforced Concrete: Code of Practice — Bureau of Indian Standards, 2000

Concrete Footing Calculator

Continuous (strip) footings for foundations, garden walls and deck beams. Enter the total run length.