How Much Concrete for Footings? Bag Counts by Size and Depth

How much concrete for footings? A typical 12 inch diameter by 48 inch deep deck footing takes 3.14 cubic feet of concrete, which is about 6 bags of 80lb mix. Multiply that by your number of footings and you have your order. Nine of those footings is 54 bags. Run your exact sizes through the Concrete Calculator to get the total.

How much concrete is in a 12 inch sonotube?

Round footings are the easy case. You pour into a cardboard form tube, so the shape is a clean cylinder. Here are common sizes for decks, porches, fences and small posts. Bag counts are 80lb bags rounded up, before waste.

• 8 in diameter x 36 in deep: 1.05 cu ft, about 2 bags
• 8 in diameter x 48 in deep: 1.40 cu ft, about 3 bags
• 10 in diameter x 48 in deep: 2.18 cu ft, about 4 bags
• 12 in diameter x 36 in deep: 2.36 cu ft, about 4 bags
• 12 in diameter x 48 in deep: 3.14 cu ft, about 6 bags
• 16 in diameter x 48 in deep: 5.59 cu ft, about 10 bags

Notice how fast the bag count climbs with diameter. Going from 12 inch to 16 inch at the same 48 inch depth nearly doubles the concrete, from 6 bags to 10. Depth costs you less than width does. That is just geometry, since the radius gets squared.

What is the formula for a round footing?

You only need one line of math for any tube size:

Cubic feet = 3.1416 x radius in feet squared x depth in feet

To get radius in feet, divide the diameter in inches by 24. So a 12 inch tube has a radius of 0.5 ft. Square that to get 0.25. Multiply by 3.1416 to get 0.785 sq ft of area. Multiply by 4 ft of depth and you land on 3.14 cubic feet. That matches the chart above.

Once you have cubic feet, bags are simple. An 80lb bag yields 0.60 cu ft. A 60lb bag yields 0.45 cu ft. So 3.14 divided by 0.60 is 5.2 bags, and you round up to 6. Always round up. Half a bag short in the middle of a pour is a bad afternoon.

How much concrete for a strip or trench footing?

Strip footings run under walls, garages, sheds and retaining walls. They are a long rectangle, so it is easiest to think in cubic feet per linear foot, then multiply by your run length.

• 12 in wide x 6 in deep: 0.50 cu ft per linear foot. A 20 ft run is 10.0 cu ft, or 0.37 cu yd, about 17 bags.
• 16 in wide x 8 in deep: 0.89 cu ft per linear foot. A 20 ft run is 17.8 cu ft, or 0.66 cu yd, about 30 bags.
• 16 in wide x 8 in deep, 40 ft run: 35.6 cu ft, or 1.32 cu yd, about 60 bags.
• 24 in wide x 12 in deep: 2.00 cu ft per linear foot. A 30 ft run is 60.0 cu ft, or 2.22 cu yd, about 100 bags.

The formula is even simpler than the round one:

Cubic feet per linear foot = (width in inches / 12) x (depth in inches / 12)

Then multiply by total length. A 16 by 8 footing works out to 1.333 x 0.667, which is 0.89 cu ft per foot. Over 40 feet that is 35.6 cu ft. Divide by 27 to convert to cubic yards and you get 1.32 cu yd.

How many bags for a deck footing?

One footing is not the real question. Your deck has several, and that is where the number gets away from you.

Say you have 9 footings at 12 inch diameter by 48 inch deep. That is 6 bags each, so 54 bags total. At 0.60 cu ft per bag, 54 bags is about 32 cubic feet, or roughly 1.2 cubic yards. That is a lot of mixing. Fifty four bags is also over a ton of material to haul, stack and tear open.

This is the point where a lot of people switch. Somewhere around 1 to 2 cubic yards, ready-mix delivery starts making more sense than bags, both for your back and for consistency between footings. Worth reading bags vs ready-mix before you commit either way. If your project has slabs or piers too, the how much concrete do I need guide covers the whole picture.

Add 5 to 10 percent waste on top of whatever you calculate. Tubes sag. Trenches slump inward. You spill some. Nobody has ever regretted one extra bag.

How deep do footings need to be?

Deeper than you want them to be, usually. Footing depth is governed by your local frost line and by code, not by what looks sturdy.

The frost line is how deep the ground freezes in winter where you live. Water in the soil expands when it freezes, and that expansion lifts anything sitting above it. If your footing bottom is above the frost line, your deck will heave up in winter and settle back in spring. Do that a few seasons and posts twist, ledgers pull, and stairs go crooked.

Footings must bear below the frost line. That depth varies enormously by region, from almost nothing in warm climates to several feet in cold ones. Your building department has the number. Call them. Do not guess, and do not copy a depth off a video shot in another state.

This is also why the 48 inch depth shows up so often in footing charts. It clears the frost line in a lot of cold regions. Your number may be more or less.

What are sonotubes and bell-bottom footings?

Sonotube is a brand name that stuck, the way people say Kleenex. They are waxed cardboard form tubes. You dig the hole, drop the tube in, backfill around it, cut it level, and pour. The tube holds the shape while the concrete cures and keeps loose soil from collapsing into your pour.

Buy the tube diameter that matches your calculation, and cut it to length before you set it. Brace the top so it stays plumb.

Bell-bottom footings flare out at the base, like a bell. The wider foot spreads load over more soil and helps resist frost lifting the pier. Some regions require them. They also take more concrete than a plain cylinder, since the flare is extra volume the cylinder formula does not capture. If you are pouring bells, add the flare volume separately or ask your supplier what the form holds.

Do footings need a permit?

Often yes. Footings are structural. They carry the whole load of whatever sits on them down into the soil, and if they are wrong, everything above them is wrong too.

Footing size, depth, spacing, reinforcement and soil bearing capacity are all set by local code, and larger or unusual structures may need an engineer to sign off. This article gives you volume math, not structural approval. Nothing here overrides your inspector, your code book or an engineer.

Check with your building department before you dig. Get the permit if one is required. An inspector who looks at an open hole is cheap. An inspector who looks at a finished deck sitting on bad footings is not.

Bottom line

A 12 inch diameter by 48 inch deep footing takes 3.14 cubic feet of concrete, about 6 bags of 80lb mix. An 8 inch by 48 inch takes 1.40 cu ft, roughly 3 bags. Strip footings at 16 by 8 inches run 0.89 cu ft per linear foot. Multiply by your footing count or run length, add 5 to 10 percent for waste, and confirm depth against your local frost line and code. Punch your sizes into the Concrete Calculator and it will total the cubic yards and bags for the whole job at once.