Sonotube Calculator

A Sonotube is a round fibre form, so the concrete inside is a cylinder. The volume of a cylinder is pi times the radius squared times the height, as set out in the [Wikipedia article on the cylinder](https://en.wikipedia.org/wiki/Cylinder). With the diameter in inches and the height in feet, one foot of tube holds pi x (diameter / 24)² cubic feet.

A 12 inch tube holds 0.785 cubic feet per foot. A 4 foot tube holds 3.142 cubic feet, and six of them hold 18.85 cubic feet before waste. Divide by 27 for cubic yards. The calculator also gives the cross-section area in square feet, which is the same number as the cubic feet per foot, and the volume in cubic metres.

The table below lists the concrete per foot for diameters from 6 to 36 inches. The numbers grow with the square of the diameter, so doubling the diameter quadruples the concrete. A 24 inch tube holds four times as much as a 12 inch one.

The Sonoco data sheet for the Sonotube Builder's Tube concrete forms lists tube diameters of 6, 8, 10 and 12 inches in standard lengths of 4, 8 and 12 feet. It prints a chart of cubic feet of concrete per foot of height: 0.20, 0.35, 0.55 and 0.80, and it notes that diameter sizes are nominal and should be measured if critical.

The exact cylinder volumes are 0.196, 0.349, 0.545 and 0.785 cubic feet per foot. The chart rounds those figures up, by up to 2 percent, which is useful as a small built-in margin. The calculator uses the exact volume and gives you your own waste allowance, so you can see both numbers and choose.

Larger tubes, from 14 inches up to 36 inches and more, are commonly sold for column and pier forms, and the same circle formula applies. Sizes and wall strength vary by supplier, so check the form maker's data for the size you plan to buy.

For a 12 inch tube, each foot of height needs 0.785 cubic feet. The Sonoco chart says that is 1 1/3 bags of 80 lb mix or 2 bags of 60 lb mix per foot, while the exact volume divided by the label yield gives 1.31 bags of 80 lb and 1.75 bags of 60 lb.

A 4 foot tube then holds 3.14 cubic feet, or 5.24 bags of 80 lb mix and 6.98 bags of 60 lb mix. The yields come from the QUIKRETE Concrete Mix No. 1101 data sheet: 0.30 cubic feet for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb. If your brand prints a different yield, type it into the yield box.

Bags are rounded up for the whole job, not per tube, so the total for six tubes is 47 bags of 60 lb mix, not six times a rounded single-tube number.

For a post set in concrete, a handy starting size is 3 times the post width. The QUIKRETE data sheet for setting posts states that the hole diameter should be 3 times the post diameter. A 4x4 post (3.5 inches actual) gives 10.5 inches, so a 12 inch tube. A 6x6 post (5.5 inches) gives 16.5 inches, so an 18 inch tube. Enter the post width in the calculator and it shows the starting size. This is a rule of thumb, not a design.

For a footing under a deck, shed or building column, the diameter depends on the load, the soil bearing capacity and local code. That is an engineering decision and varies from site to site. This calculator tells you how much concrete the size you choose will take. It does not tell you what size is safe. Have deck and structure footings designed or checked by an engineer or your building inspector.

The Sonoco sheet lists decks, fence and lamp posts, swing sets, mailbox posts and basketball goals among the uses of the Builder's Tube.

The Sonoco sheet gives the following steps for the Builder's Tube. Dig the hole to the desired depth and place the tube in the hole, filling around the outside to secure it. If the tube extends more than 2 feet above ground, add a collar or wooden frame with side braces. The tube should be dry before it is filled, and it is single-use. The sheet says to erect all the tubes for the job in advance so the pour can be done at once.

It cautions against mechanical vibration, and says the tube should remain stationary until the concrete sets. The form is designed for below ground-line placement, but it can be used up to 3 feet above ground. The tubes are meant to stay in place, and if you want to strip one, use a release agent before pouring and remove the form after about 24 hours of cure by cutting it vertically.

The tube height in the calculator is the height of concrete, so include the part above ground if there is any. For a 5 foot tube set 4 feet deep, the tube height is 5 feet.

The height of each tube comes from the footing depth and any length above ground, not from the calculator. The [frost line](https://en.wikipedia.org/wiki/Frost_line) is the depth to which groundwater in soil is expected to freeze. The Wikipedia article explains that building codes take frost depth into account because of frost heaving, and that foundations are generally built below the frost depth. Your local building department gives the figure that applies to you.

Many jurisdictions also set a minimum footing size and depth for decks and porches, and ask for an inspection before the concrete is poured. Call your building department before you dig, and call the utility locating service so buried lines are marked.

Once you have the diameter and height from your plans, enter them with the number of tubes. The calculator returns the cubic feet, cubic yards, cubic metres, bags of each size and the weight of the concrete. A 12 inch tube 4 feet tall holds 440 lb of concrete at 140 pounds per cubic foot, so each tube is heavy once filled.

For a few tubes, bagged mix is practical. For nine 16 inch tubes at 5 feet you would need 116 bags of 80 lb mix, about 9,280 lb to move, against 2.56 cubic yards of ready-mix. Ready-mix is priced per cubic yard and often has a minimum load and a delivery fee, so enter both prices and compare. The prices in the calculator are placeholders.

Rebar, ties and anchor bolts take a little space but they are small. The calculator does not size reinforcement. That belongs in the plans, or with an engineer or inspector. Leave the reinforcement inside your waste allowance.

If you want gravel in the bottom of the hole, the gravel calculator turns a depth and an area into cubic yards. For block piers instead of poured ones, the CMU block calculator counts the blocks, and the stone calculator covers decorative stone around the footings.

This is an estimate. Follow the form maker's directions, the bag label and your local building code, and have a professional confirm structural footings.