Sonotube Concrete Calculator

Round forms are paper or plastic tubes that hold wet concrete in a hole or on a footing while it sets. Builders buy them by diameter, and the volume of one form is the area of its circle times the height you fill. The calculator multiplies that volume by the count and works out the bags for the whole order. Sonotube is a Sonoco brand name, but the math is the same for any round form.

How to use this calculator

1. The shape is fixed at Round, because a form is a circle when seen from above.
2. Enter the diameter in inches, the usual unit for forms, unless your plans give it in feet.
3. Enter the height in the Height box, from the bottom of the hole to the top of the concrete. Include the part of the form that stands above the ground.
4. Enter how many identical forms you are pouring in the How many box.
5. Read the cubic yards for a ready-mix order or the bag count for a mix-it-yourself job. Enter a price per bag to see an estimated cost.

Worked examples

Choosing the diameter

Builder's tubes come in diameters from 6 to 12 inches, which covers most posts, footings and small columns. Round column forms go wider, from 6 to 36 inches, so a large porch column or pier has a tube to match.

Start from the post, column or anchor you are setting. Its base or bracket decides the smallest tube that fits around it with room for the concrete. The area of a circle grows with the square of its diameter, so a few extra inches of width adds more concrete than it first appears.

Height and the part above grade

Enter the full height of the form: the buried part, from the bottom of the hole up to ground level, plus the part that stands above grade. Both parts are concrete you have to buy, and the above-grade part is the one people often leave out.

How deep the hole goes is set by your local frost depth and building rules, not by this calculator. Confirm the depth with your building department before you dig, rather than copying a depth from another job.

A form that stands above grade needs bracing to hold it plumb while the concrete sets. The wet mix is heavy, as the weight of a round column shows, so brace the form before you start pouring. Concrete dropped a long way can separate, so follow the placement advice on the mix label for a tall form.

Identical columns and bags per column

Set How many to the number of identical forms. The calculator multiplies the volume of one form by that count, so the total concrete grows in step with the number of forms.

To see the bags for one column, set How many to one and read the bag line. The calculator rounds up once for the whole order, so the bags for every column together can come out a little lower than the one-column count multiplied out.

Each 80 lb bag yields 0.6 cu ft, so a cubic yard takes 45 bags of that size. Many residential columns are only a fraction of a yard, so their bag counts are a fraction of that too.

Common questions