Sonotube Calculator

What a Sonotube form is and what this page returns

Sonotube is Sonoco's brand name for the round fiberboard tubes that shape a concrete pier. The do-it-yourself version, Builder's Tube, comes in 6, 8, 10 and 12 inch diameters in 4 foot lengths, and the commercial line runs from 6 to 60 inches in 12 and 20 foot lengths. The tube holds wet concrete in a clean cylinder while it sets, then either stays buried in the hole or is cut away. Sonoco's product brochure describes the same product families and sizes.

These forms are designed for below grade work, though Builder's Tube can be poured up to 3 feet above the backfill line, with bracing once the tube stands 2 feet or more above ground. One caution from the same brochure: the Builder's Tube version is not meant to be used with a mechanical vibrator, so the concrete is consolidated by rodding instead.

The calculator below takes the tube diameter, the height, the number of tubes, a waste allowance and your bag prices, and returns the volume in cubic feet and cubic yards, the bags to buy, and the material cost. The post or rebar cage inside the tube is not subtracted, so treat the result as a small overestimate rather than a shortage.

The volume formula for a round tube

A tube is a cylinder, so the volume is the circular end area times the length. The end area of a circle is pi/4 times the diameter squared, which makes the whole formula:

Volume = pi/4 x diameter^2 x height

Work the units before the numbers. With the diameter and height in inches, the result is cubic inches, and it takes 1,728 of them to make a cubic foot. A 12 inch tube that is 4 feet long has an end area of 113.10 square inches from pi/4 times 12 squared, a volume of 113.10 x 48 = 5,428.7 cubic inches, and 5,428.7 / 1,728 = 3.14 cubic feet. Divide by 27 for cubic yards and the same tube holds 0.116 cubic yards. The conversion factors come from NIST Special Publication 811, Appendix B, the federal reference for unit conversions.

The table reads quickly in both directions. Doubling the diameter quadruples the volume, because the diameter enters the formula squared, and every extra foot of tube adds one more row unit. A 24 inch form 4 feet long holds as much concrete as sixteen 6 inch tubes of the same length.

Worked example: four 10 inch piers, 3 feet tall

A small deck needs four piers from 10 inch tubes set 3 feet into the ground. The end area comes to 78.54 square inches from pi/4 times 10 squared, so one tube holds 78.54 x 36 / 1,728 = 1.64 cubic feet. Four tubes take 4 x 1.64 = 6.54 cubic feet, which is 0.24 cubic yards before any allowance.

Nobody buys concrete to the third decimal. Spillage, uneven hole bottoms and a slightly proud pour at the top all consume mix, so add a waste allowance, commonly 10 percent. The order becomes 6.54 x 1.1 = 7.20 cubic feet, or 0.267 cubic yards. At the published yield of 0.60 cubic feet per 80 lb bag, that is 7.20 / 0.60 = 12 bags.

Twelve bags sounds like a lot until you stack it against a ready-mix delivery: most trucks have a minimum load, and a short pour of 0.267 cubic yards usually pays for the whole truck anyway. For a few piers, bags mixed in a wheelbarrow or a small mixer win on cost and on timing, because each tube is poured as soon as its mix is ready.

Bag yields and how many bags to buy

Bagged concrete is sold by weight and yields a fixed volume once water is added. Quikrete's data sheet for its standard Concrete Mix (No. 1101) lists 0.60 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag and 0.30 for a 40 lb bag. Those are the yields the calculator uses. A cubic yard of 27 cubic feet therefore takes 27 / 0.60 = 45 eighty pound bags, or 60 sixty pound bags.

The bag count per tube is the tube volume plus waste, divided by the yield, rounded up, because partial bags do not exist at the store. The table shows a 4 foot tube with a 10 percent allowance in both common bag sizes.

Two habits keep the count honest. First, check the yield printed on the bags you buy, because specialty mixes such as high early strength or fast setting blends list different numbers. Second, if a hole collapses wider than the tube, the extra space is concrete too, and a collapsed sandy hole can swallow another bag or two that no tube formula predicted.

Picking the diameter and setting the depth

The diameter usually follows the post it carries, not the concrete math. Common practice pairs a 4x4 fence or railing post with an 8 to 10 inch tube, and a 6x6 deck post with a 10 to 12 inch tube, leaving an inch or two of concrete around the wood on each side. Mailboxes and light posts often use 6 or 8 inch forms. These are shop conventions rather than code, so let the permit drawings win whenever they say something different.

Depth is a code question, not a preference. Piers generally bear below the local frost line on firm, undisturbed soil so that freezing ground cannot lift them, and the building department for your town publishes the required depth for decks and freestanding structures. Sonoco's own FAQ makes the same point: the base of the pier belongs below the frost line, and the local inspector or an engineer settles the details.

Below the tube, a wider footing pad spreads the load. Sonoco sells matching plastic Tube Base forms for this: the 22 inch base holds 2.0 cubic feet, the 28 inch base 4.5 cubic feet, and the 32 inch base 7.0 cubic feet. Those volumes are on top of whatever the tube itself holds, so a pier with a base needs both numbers added together.

Setting, bracing and pouring the tube

Dig the hole first, to the required depth and a few inches wider than the tube. Set the tube plumb, backfill around the outside in shovelfuls and tamp it in layers so the form cannot drift. Once the tube stands more than 2 feet above the ground, Sonoco's instructions call for a collar or a wooden frame with side braces, and the company's FAQ describes bracing at the top and bottom for anything up to 12 feet long. On a commercial site the same duty falls under OSHA 29 CFR 1926.703, which requires formwork to be designed, erected and braced for the loads it will carry.

Pour in lifts of a couple of feet rather than one drop from the wheelbarrow, and consolidate each lift by rodding with a scrap of rebar or a shovel. The Builder's Tube form is not rated for a mechanical vibrator, and a stuck vibrator can tear a wet fiber wall. Keep the tube stationary while the concrete sets, and screed the top slightly proud so water cannot pool on the pier.

Keep the tube dry until you pour. A soaked Builder's Tube loses strength and can sag or blow out under the weight of wet concrete, which is why the tubes ship wrapped and why pouring on a rain-soaked day needs a second look at the form before the first shovel of mix goes in.

Cost per pier and what the estimate leaves out

With the bag price entered, the calculator splits the material bill into concrete, tubes and total. Bag prices move with region and season, so the per-bag figure you type matters more than any number printed here. The table uses $6.50 per 80 lb bag and a 10 percent waste allowance on 4 foot tubes to show the shape of the cost curve.

The estimate covers concrete and the tubes themselves and stops there. Rebar or post anchors, a wider footing pad, gravel in the hole, delivery on large orders, sales tax and a mixer rental all sit outside it. On jobs past a yard or so of concrete, price a ready-mix truck as well, because the per-yard price beats bags once the count gets high, and a truck pour finishes a row of piers before the first batch sets.

Stripping the form and inspection day

Builder's Tube is designed to stay in place, and below grade there is no need to strip it unless local code says otherwise. If a smooth column shows above ground, apply a release agent before pouring, then cut the tube vertically with a knife and pull it off as soon as the concrete has cured for 24 hours. The standard Sonotube product line gives a wider window: stripping is easiest 24 to 48 hours after the pour, and the form should not stay on the column beyond 5 days.

Inspection usually happens before the pour. The inspector checks the depth against the frost line, the diameter, the bearing soil, the rebar or anchor placement, and the bracing on any tube standing tall. An inch of soil that falls into the tube before the pour is an inch of concrete displaced, so brush the inside walls clean and check the bottom of the hole one last time before the mixer starts.

After the pour, protect the pier from foot traffic and loading while it cures. Bagged mixes commonly reach design strength in about a month under normal conditions, and the bag you buy prints the strength and curing expectations for that blend, so read the label rather than assuming every mix sets on the same clock.