Sonotube Concrete Calculator: How Much You Need

Sonotube Concrete Calculator: How Much You Need

A sonotube concrete calculator figures the volume of a round column form so you order the right amount of ready-mix or bags. You measure the tube diameter and the pour depth, and the tool returns cubic yards, cubic feet, and how many bags it takes. That is the whole job: get the two numbers right and the rest is arithmetic.

I have poured hundreds of tube footings for decks, pergolas, mailboxes, and pole barns. The number one reason guys come up short is guessing the volume instead of running it. A concrete tube form looks small until you fill a dozen of them and realize you needed another half yard. Below is how the math actually works and where people get tripped up.

How much concrete for a sonotube

Concrete is sold by volume, and a sonotube is just a cylinder. The formula is the radius squared, times pi, times the height, all in feet, then divided by 27 to get cubic yards:

cubic yards = (3.1416 × radius_ft × radius_ft × depth_ft) ÷ 27

Radius is half the diameter. So a 12-inch tube has a 6-inch radius, which is 0.5 ft. If you pour that tube 4 feet deep:

• radius: 6 in ÷ 12 = 0.5 ft
• 3.1416 × 0.5 × 0.5 × 4 = 3.14 cubic feet
• 3.14 ÷ 27 = about 0.116 cubic yards per tube

That is roughly three and a half 80-lb bags for a single 12-inch tube poured 4 feet deep. Run our sonotube calculator and it does this instantly, including the bag count for 40, 60, and 80-lb mixes.

Sonotube volume by diameter

Diameter drives volume harder than depth because the radius gets squared. Bump a tube from 8 inches to 12 inches and you more than double the concrete per foot. This table shows cubic feet per foot of depth and cubic yards for a common 4-foot pour, so you can size an order fast.

Tube diameter	Cubic feet per foot	Cubic yards at 4 ft deep	80-lb bags at 4 ft deep
6 in	0.196	0.029	about 1.3
8 in	0.349	0.052	about 2.3
10 in	0.545	0.081	about 3.6
12 in	0.785	0.116	about 5.2
16 in	1.396	0.207	about 9.3
18 in	1.767	0.262	about 11.8

Numbers are rounded. Bag counts assume an 80-lb bag yields about 0.6 cubic feet. For a full breakdown of any cylinder shape, the concrete cylinder calculator handles odd diameters and depths the table does not list.

Do not forget the flared base

A lot of footings are not straight tubes. If you set a bell-shaped base or a bigfoot-style footing under the tube, add that volume separately. Calculate the tube on its own, calculate the base as its own shape, then add the two. Same rule applies if you overfill the top to form a cap or crown for water runoff. Guessing that extra always leaves you short.

Add a waste factor

Add 5 to 10 percent for spillage, tubes that bow out under load, and subgrade that swallows a little at the bottom. Cardboard forms flex when the wet concrete pushes on them, so the real diameter mid-pour is often a touch bigger than the label. On a batch of tubes that adds up. Ordering a little extra beats stopping a pour to run to the store for more bags.

Bags or ready-mix

Under about a half yard total, bags usually win on cost and cleanup. Once you cross roughly a yard, short-load ready-mix starts to make sense even with the small-load fee, because mixing 40-plus bags by hand is a long, ugly day. If you are pricing a run of deck or fence footings, the bags of concrete per fence post guide shows how the per-hole count scales, and it is the same logic for tube footings.

Whatever you pour, size the whole job first. Our full walkthrough on how much concrete do I need covers slabs, footings, and columns together so you order once and pour once.

FAQ

How much concrete does a 12-inch sonotube hold?

A 12-inch tube holds about 0.785 cubic feet per foot of depth. Poured 4 feet deep that is roughly 3.14 cubic feet, or about 0.116 cubic yards, which is close to three and a half 80-lb bags.

How many bags of concrete for a sonotube?

It depends on diameter and depth. A 10-inch tube at 4 feet deep runs about three and a half to four 80-lb bags. Run your exact size through the sonotube calculator to get a bag count for 40, 60, or 80-lb mix.

What size sonotube do I need for a deck footing?

Most residential deck posts sit on 8 to 12-inch tubes, but your footing size comes from the load and your local code, not the tube alone. Check your permit requirements, then size the concrete once you know the diameter and depth.

How deep should a sonotube be?

Below your local frost line, plus the footing thickness. In cold climates that can mean 4 feet or more. A tube that does not reach past frost will heave, so depth is set by code and soil, not convenience.

Can I pour multiple sonotubes at once?

Yes, and you should size them together. Calculate one tube, multiply by the count, then add your waste factor to the total. Ordering per tube instead of per job is how people end up a bag or two short.

Does the cardboard tube stay on the concrete?

The exposed portion is usually stripped after the concrete sets, and the buried portion can be left in place. Strip it within a day or two while the cardboard peels cleanly.

Size your footings in seconds with our sonotube calculator and order the right amount the first time.