Concrete

How Many Bags of Concrete for a Footing?

The slab guide answers the patio question. Footings are a trench: length in feet, width and depth in inches. Use footing mode on the concrete bag calculator and check the working against the tape—not against a “bags per foot” habit from a different job.

Footing volume

ft³ = length_ft × (width_in ÷ 12) × (depth_in ÷ 12)

Convert both cross-section dimensions to feet before multiplying. Skipping the inch-to-foot step is the same class of error as forgetting slab thickness conversion—off by twelve, not by rounding.

Worked example: 20 ft × 12 in × 12 in

Yields are Quikrete Concrete Mix No. 1101 (0.30 / 0.45 / 0.60 ft³). Sakrete high-strength lists the same three for those bag weights. The 10% default is the site’s labeled handling overage for spill and uneven trench bottoms—editable on the tool.

Footing versus slab versus posts

How many bags of concrete walks a 10×10 slab and the ready-mix threshold in slab language. Post holes subtract the post from a cylinder—see bags per fence post. Round form-tube piers use the Sonotube concrete calculator. Footing mode is the continuous rectangle. Mixing the geometries is how a garage footing order becomes a patio bag count.

Ready-mix planning threshold

One cubic yard is 27 cubic feet—about 45 bags of 80 lb mix before waste. The example above is only ~0.81 yd³ with waste, still in bag territory for many DIYers, but a longer or deeper footing crosses the line fast. When the calculator’s bag count climbs toward a yard, price a short load before you commit to a pallet. That is a practicality threshold, not a code cutoff, and truck minimums vary by plant.

What still changes the trench

• Keyways, steps, and width changes—split into segments and sum volumes.
• Over-excavation and loose soil—what waste is for.
• Rebar displacement is usually small versus trench error; do not invent a rebar credit.
• Local footing size is from the plan or AHJ, not from this guide.

Segmented and stepped footings

Real footings rarely stay one perfect rectangle for the whole job. A garage may step with grade, widen under a post, or turn a corner with a thicker pad. The honest takeoff splits those into segments: run footing mode (or the formula above) for each constant cross-section, then add the cubic feet before converting to bags once. Rounding each segment to whole bags and summing will usually over-order compared with totaling volume first — same lesson as multi-post holes.

Example mindset: 12 ft of 12×12 trench, then 8 ft of 16 in wide by 12 in deep, is not “20 ft of 12-inch footing.” Compute each volume, sum, apply waste, then divide by bag yield. The calculator is happiest when you enter one segment at a time and keep a running note of net cubic feet. A phone note with three lines of ft³ beats a single optimistic bag total every time.

Width and depth mistakes that blow the order

Footing width on the plan is not always the width of the trench you dug after the walls sloughed. If the excavator left a 14 in trench where the plan said 12 in, volume rose before anyone opened a bag. Depth to frost or to undisturbed soil similarly belongs to the plan and the inspection, not to a website default. This guide will not invent a frost map; enter the dimensions you will pour.

Another common miss: treating a “12×12 footing” as 12 ft by 12 ft. In trade shorthand that phrase often means a 12 in by 12 in cross-section along a length. The calculator’s footing mode uses length in feet and width/depth in inches for that reason. Read the unit labels twice before you trust a bag total.

Bag size on a trench pour

Long continuous pours favor fewer, larger bags if your mixer and crew can keep up — fewer bags to open, fewer seams in the mix schedule. Short repair footings often stay on 60 lb bags because an 80 lb bag that sets while you re-auger a corner is wasted material. Unit price per cubic foot matters less than finishing the pour with a consistent mix. Glance at the yield printed on the bag you actually hold; if it is not Quikrete 1101 or a Sakrete sheet with the same three yields, do not force this site’s defaults onto a different product. Write the yield on your ticket next to the bag count so a helper grabbing the wrong pallet does not silently change the math.

Open footing mode: concrete bag calculator. Sources: methodology.