How much concrete for a 20 ft footing?

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.

Almost every footing calculator divides the run by 20 and stops. 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 "40 bar diameters" figure repeated all over the web is the Grade 40 column of that same table. Grade 60 is what you actually buy, and its lap is 60 bar diameters. Using the Grade 40 number makes every splice 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, which no online calculator models. 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.