Concrete footing calculator

Three shapes, one formula

A strip footing under a wall and a pad under a post are the same sum: length × width × depth. A round deck pier is π × radius² × depth. The answer comes back in cubic yards because that is how ready-mix is sold — 27 cubic feet to a cubic yard, not 3.

A 20 ft strip footing 16 in wide and 8 in deep is 17.8 cubic feet. Add 10% for a trench dug in earth and you are ordering 0.72 cubic yards, which is 33 × 80 lb bags. Bag yields are QUIKRETE's published figures for general-purpose concrete mix: 80 lb → 0.60 cu ft, 60 lb → 0.45, 50 lb → 0.375, 40 lb → 0.30.

The rebar mistake: laps are not free

Rebar comes in 20 ft sticks. Where two sticks meet they have to overlap, and the overlap length is set by code. IRC Table R608.5.4(1) gives the tension lap splice for Grade 60 bar as 30 in for #4, 38 in for #5 and 45 in for #6.

Divide the run by 20 and stop, and the count comes out short. On the 96 ft perimeter of a 24 × 24 ft garage that gives five sticks per run. The real answer is six: the first stick covers 20 ft, but every stick after it only adds 20 − 2.5 = 17.5 ft of new footing. Two runs of bar, and you are two sticks short — which you discover with the trench dug and the truck booked.

Watch the grade, too. The figures above are the Grade 60 column of IRC Table R608.5.4(1), which is what big-box rebar is unless the tag says otherwise. The "40 bar diameters" rule of thumb is the Grade 40 column of that same table — 20 / 25 / 30 in for #4 / #5 / #6, against the 30 / 38 / 45 in used here. Lay a splice out from it on Grade 60 bar and it is a third too short.

Corner bars are not counted here. Where two runs meet, the bars have to be tied round the corner with a bent or L-shaped bar lapped into both runs — add one lap length per leg, per corner.

What the code actually asks for

• Thickness. IRC R403.1.1 sets 6 in as the minimum for a concrete footing.
• Width. The same section wants the projection either side of the wall to be at least 2 in and no more than the footing thickness. That is why a 16 in footing under an 8 in wall is a common combination — 4 in each side.
• Reinforcement. In Seismic Design Categories D0–D2, IRC R403.1.3 calls for a No. 4 horizontal bar 3–4 in up from the bottom of the footing.
• Sloping ground. R403.1.5 requires the top of a footing to be level and the bottom to slope no more than 1 in 10. Steeper than that and the footing has to be stepped. This page assumes one footing of constant cross-section, so enter each step as a separate footing and add them up.

Bags or a truck

The crossover sits right inside the range people search for. A 20 ft footing at 0.72 cubic yards is under the 1 cubic yard minimum load most plants will deliver, so you either pay a short-load fee or mix 33 bags. The 96 ft perimeter of a 24 × 24 ft garage is 3.5 cubic yards — 157 bags, about 6.3 short tons through your hands. That one is a truck.

One thing this calculator will not tell you: how wide and how deep the footing has to be. That depends on the load above, your soil's bearing capacity and your local code. Get those two numbers from your building department, then come back for the volume.

Where the rebar model stops applying

The splice arithmetic is only right while the job looks like the job it assumes. Four ways it can quietly stop being your job:

• The stick is fixed at 20 ft. That is the length rebar is stocked and priced in, but plenty of stores sell 10 ft and 12 ft bar. Shorter sticks mean more splices, and each splice eats another lap length — the count here will be low, and the shorter the stick the worse it gets.
• Offcuts are never carried from one footing to the next. With the count set above 1, each pad or pier is given its own whole sticks. That is deliberate and it errs high: a 3 ft leftover is often unusable anyway once you allow for cover. But on a job with many small pads it is not the cheapest possible order.
• Round piers get no steel at all in these outputs. A pier normally takes a vertical dowel or a small cage rather than a continuous run, so the rebar rows read zero. That is not a statement that the pier needs none.
• The lap is the Grade 60 tension figure from one table. Bar tagged Grade 40 or Grade 80, epoxy-coated bar, or a splice designed by an engineer rather than taken from the IRC table all use different lengths. Read the tag before you trust the splice.

None of the dimensions is checked for being buildable either. The footing count carries no upper limit, and neither do the dimensions above it: we cannot cite a source for where a number stops being realistic, and a cap chosen without one would reject a correct entry from someone with an unusual job. Enter 500 pads and you get the honest concrete for 500 pads. Whether the size is structurally right is a different question, and not one this page asks.

Two more things this page has no view on: the depth you type is not checked against your frost line, and a footing that changes width — widening under a column, say — has to be entered as separate footings. Enter one shape, get one shape's worth of concrete.

Two assumptions on the concrete side, for completeness. The round pier is a straight-sided cylinder, which is what a tube form gives you — a belled base, or a hole augered in earth without a form, holds more than π × radius² × depth. And the finished weight is normal-weight concrete at 150 lb per cubic foot; a lightweight mix weighs less and that output is then high.