Concrete footing calculator

Continuous strip footings for a load-bearing wall. We size the width from IRC Table R403.1 by your soil and story count, then give the concrete volume in cubic yards or cubic metres — with the waste factor and the rounding-up step done for you.

01 How the math works

A strip footing is a long rectangular prism of concrete, so its volume is three numbers multiplied together: the length it runs, its width, and its thickness, all in consistent units.

The width is the part most calculators leave to you. It isn't a number you pick because it looks sturdy — it comes out of IRC Table R403.1, keyed to the soil's bearing value and the number of stories the wall carries. Lighter load or stronger soil, narrower footing; heavier load or weaker soil, wider. For conventional wood-framed walls the floor is 12 in (305 mm) wide and 6 in (152 mm) thick, and nothing in the table goes below that.

The run length is the total length of continuous footing — the building perimeter plus any interior bearing walls. For a plain rectangle with no interior bearing walls, that's just the perimeter, 2 × (length + width).

02 Worked example

Take a 24 ft × 24 ft detached garage (7.32 m × 7.32 m), one story, wood-framed walls on sand that bears 2,000 psf. Table R403.1 puts the footing at its minimum: 12 in (305 mm) wide, poured 6 in (152 mm) thick.

With no interior bearing walls, the footing runs the perimeter: 2 × (24 + 24) = 96 ft (29.26 m). Volume is 96 ft × 1 ft × 0.5 ft = 48 ft³ = 1.78 yd³ (1.36 m³). Add a 10% waste factor and round up to the ready-mix half-yard step: 2.0 yd³ — or 1.5 m³ in metric.

As bagged premix at the standard 0.60 ft³ yield per 80 lb bag, the same footing is 88 bags with the waste included. That's the cue to call the ready-mix supplier, for the reason in the section below.

03 When this calculator is wrong

The width from Table R403.1 is a prescriptive number with assumptions behind it, and those assumptions are where a footing gets under-sized.

• The table width assumes a specific load. Those widths are derived for roughly 50 psf ground snow, 20 ft (6.10 m) of roof tributary width, 16 ft (4.88 m) of floor tributary width, and standard wall heights. A wider house or a deeper span carries more load onto the same footing. The IRC's own adjustment: for every 2 ft (610 mm) the house is wider than that basis, add 2 in (51 mm) of footing width and 1 in (25 mm) of thickness.
• It stops at 1,500 psf. The prescriptive table does not cover soil weaker than 1,500 psf. Organic soils, uncompacted fill, or soil of unknown bearing need a soils report and an engineered footing — the table isn't a substitute for either.
• It sizes a line load, not a point load. A girder, post, or column drops a concentrated load at one spot that a continuous footing width doesn't address. That location needs a spread (pad) footing or a thickened section sized to the point load on its own.
• The run is perimeter plus interior bearing walls. A center beam carried on a bearing wall or a thickened-edge line runs its own footing. Count it. Taking the outside rectangle alone under-orders every footing a house has on the inside of its walls.

04 What to do with the result

A cubic yard of concrete is about 60 bags of 80 lb premix — 4,800 lb to mix and place inside the roughly 90-minute working time before it sets. Above 1 yd³ (0.76 m³), ready-mix is faster, more uniform, and usually cheaper per yard once the $50–150 short-load fee is spread across the pour. The garage above is 2.0 yd³ and 88 bags — well past that line.

The exception is access: if the truck can't reach the forms, bagged or pumped concrete is the call regardless of volume. Either way, order the rounded-up number. A footing poured short leaves a cold joint where the next batch meets it, and that's a weak plane you designed in by running out — order the extra half-yard instead.

05 Common questions

How wide should a concrete footing be?

The IRC minimum is 12 in (305 mm), and that covers a 1-story wood-framed wall on most soils. A 2-story wall on weak clay (1,500 psf) steps up to 15 in, and a 3-story to 23 in. Width tracks the soil's bearing value and the number of stories — that's what Table R403.1 does, and what the calculator above reports.

How thick does a footing have to be?

Minimum 6 in (152 mm) for a plain concrete footing, per IRC R403.1.1. Don't confuse thickness with how deep the footing sits: the bottom has to reach the greater of a 12 in below-grade minimum and the local frost line, which in cold regions runs well past that.

How much concrete do I need for a footing?

Run length × width × thickness, in consistent units, then divide by 27 to read cubic feet as cubic yards. The 96 ft × 12 in × 6 in garage footing above is 48 ft³ = 1.78 yd³. Add waste and round up to the supplier's half-yard step.

Does a footing need rebar?

The IRC allows plain (unreinforced) concrete footings for many one- and two-story cases, but expansive or weak soils, taller walls, and some local amendments require reinforcement — subject to local code. Where rebar is called for, size and space it separately; this calculator sizes concrete only.

What's the difference between a footing and a foundation wall?

The footing is the wide pad at the bottom that spreads the load onto the soil; the foundation (stem) wall sits on top of it and carries the building above. This page sizes the footing. The wall on top is a separate pour with its own volume.