Calculatly.

Sonotube Concrete Calculator — Bags and Yards

Concrete, bags and yards for any round concrete form tube, plus how many tubes to buy and how to cut them with the least waste. Results as you type.

Works for any round concrete form tube. Sonotube is a trademark of Sonoco Products Company and Calculatly is not affiliated with or endorsed by them.

Measure from the bottom of the hole to the top of the finished pier.

Frost depths here are typical figures only. The concrete footing calculator has the full frost table.

Adjust prices
$
$

Rough national-average starting points, not quotes. Last updated October 1, 2026.

—
Pick a tube diameter and height to see concrete, bags and how many tubes to buy.
ad-slot

Concrete per Foot of Form Tube

Every figure comes from π × radius² for a round tube, using 80 lb bags at 0.6 cubic feet. Tap a row to load that diameter.

DiameterCubic feet per footCubic yards per foot80 lb bags per footFeet per 80 lb bagFeet per cubic yard
0.1960.00730.333.06137.5
0.3490.01290.581.7277.3
0.5450.02020.911.1049.5
0.7850.02911.310.7634.4
1.0690.03961.780.5625.3
1.3960.05172.330.4319.3
1.7670.06542.950.3415.3
2.1820.08083.640.2812.4
3.1420.11645.240.198.6
4.9090.18188.180.125.5
7.0690.261811.780.083.8

Concrete by Tube Size and Depth

Figures are per tube, cubic feet with the 80 lb bag count beneath, and no waste allowance. Tap a cell to load it into the form.

Depth8 in10 in12 in16 in
2 ft
3 ft
4 ft
5 ft
6 ft
8 ft

Tube Diameter by What It Carries

Typical uses only. Pier diameter is set by the load and the bearing capacity of the soil, so confirm it with local code and an engineer for anything structural.

DiameterTypical useNotes
8 inLight deck posts, small pergolas, fence gate postsEnough bearing area only for light loads on firm soil. Often the smallest size a building department will accept for a deck post.
10 inStandard deck postsThe common choice for a 4x4 or 6x6 post on a single-storey deck. Holds 0.55 cubic feet per foot, under one 80 lb bag for each foot of depth.
12 inDeck beams, porch columns, carportsMore bearing area for the same soil, so footings carry a heavier tributary load. At 0.785 cubic feet per foot it is the size most merchants stock deepest.
16 inHeavy posts, two-storey decksUsed where the load per post is high or the soil is weak. A 4 ft pier takes 5.6 cubic feet, so plan on ready-mix for more than a few.
24 in and upCommercial columns, bridge and pole workEngineered work with reinforcement cages. At this size the tube is formwork for a designed column, not a DIY footing.

How much concrete does a form tube hold

A form tube is a cylinder, so the concrete inside it is π × radius² × height. With the diameter in inches, the radius in feet is the diameter divided by 24, which makes a 12 in tube π × 0.5² = 0.785 cubic feet for every foot of height. The same sum works for any concrete cylinder or round column, whatever the tube is called.

Take six 12 in tubes at 4 ft each: 0.785 × 4 × 6 = 18.8 cubic feet. Divide by 27 and that is 0.70 cubic yards, and divide by the 0.6 cubic feet an 80 lb bag yields and you get 32 bags after rounding up. The per-foot figure is the one worth memorising, because it scales to any depth. A 8 in tube is 0.35, a 10 in is 0.55, a 12 in is 0.79 and a 16 in is 1.40 cubic feet per foot.

Tubes are sold in fixed lengths, usually 12 ft, so the arithmetic that matters at the merchant is how many piers come out of one tube. Six 4 ft piers use exactly two 12 ft tubes. Eight 5 ft piers are different: each 12 ft tube gives two piers and wastes 2 ft, a sixth of everything you buy, so you need four tubes. The same job from 20 ft tubes gives four piers per tube and wastes nothing, using two. The calculator tests every stock length and names the lowest-waste option for your pier height.

For a handful of piers, bagged mix usually beats a truck, and the concrete bag calculator prices the two routes side by side. A filled tube is heavy, since concrete weighs about 150 lb per cubic foot, so a 12 in tube 4 ft deep carries roughly 470 lb. The concrete weight calculator totals a whole pour when you are planning a wheelbarrow route or a trailer load.

Bracing matters more than people expect. Wet concrete pushes outward hard, and a tube that is not held will bulge, lean or split at the seam, which is a blowout. Fit a collar near the top and another near grade, plumb the tube, and stake it in two directions before you pour. Fill in lifts and rod the mix so it settles evenly. Taller piers need an extra collar for every further 4 ft.

A belled base is a wider flare at the bottom of the hole that spreads the load over weaker soil and resists uplift. It is dug by hand, because a post-hole digger cannot undercut, and the concrete added is a frustum minus the cylinder it replaces. Depth is a separate question, since the base of the pier has to sit below the frost line for your area, and the footing calculator with frost depths lists typical figures by region.

Once the concrete has cured, the cardboard peels off above grade for a clean finish. Below grade it stays, because it rots away harmlessly and nothing is gained by digging it out. Strip it with a utility knife, cutting only the tube.

How to use this calculator

  1. 01Pick the tube diameter from the quick buttons or the list, or choose Other and type your own.
  2. 02Enter the pour height, measured from the bottom of the hole to the top of the finished pier, and the number of tubes.
  3. 03Tick belled base if you are flaring the bottom of the hole, and choose your frost zone for the depth check.
  4. 04Leave stock tube length on Auto and the calculator finds the lowest-waste option, or pick 12, 16 or 20 ft to compare what you would buy.
  5. 05Read the bag count and yards, then follow the buying plan to the merchant. Adjust prices to match your local quotes.

Common questions

How much concrete does a 12 inch sonotube hold per foot?
A 12 in tube holds 0.785 cubic feet for every foot of height, from pi times 0.5 squared. That is 0.029 cubic yards per foot, so about 34 feet of tube make a cubic yard. A 4 ft pour takes 3.14 cubic feet, or 6 bags of 80 lb mix.
How many bags of concrete fill a 10 inch form tube?
A 10 in tube holds 0.545 cubic feet per foot, which is 0.91 of an 80 lb bag per foot. A 4 ft pier needs 2.18 cubic feet, so buy 4 bags. With 60 lb bags the same pier takes 5, and with 40 lb bags it takes 8.
What size form tube do I need for a deck post?
Most deck posts sit in a 10 in or 12 in tube, with 8 in used for light decks and small pergolas. Taller or heavier structures, such as a two-storey deck, usually step up to 16 in. The right size is set by the load and the bearing capacity of the soil, so confirm it with your local building department.
How long are concrete form tubes sold in?
The common stock lengths are 12 ft, 16 ft and 20 ft, though stocking varies by supplier and many yards will cut a tube to length for you. A 12 ft tube is the usual choice for deck and porch piers. Check what your merchant carries before you plan the cuts.
How many piers can I cut from one 12 foot tube?
Divide 12 by the pier height and round down. That gives three 4 ft piers, four 3 ft piers, two 5 ft piers or two 6 ft piers. A 5 ft pier leaves 2 ft over on every tube, which is why a different stock length is sometimes cheaper.
Do I need to remove the cardboard tube after the concrete sets?
Below grade you can leave it, because it rots away or sits harmlessly against the soil and removing it gains nothing. Above grade it is usually peeled off once the concrete has cured for a day or two, for a clean finish. Peel it with a utility knife, taking care not to nick the concrete.
How do I stop a form tube from blowing out?
Wet concrete pushes outward with real force, so fix the tube with a collar near the top and one near grade, plumb it, and stake it in two directions. Backfill and tamp around the base before you pour. Fill in lifts rather than all at once, and rod the mix so it settles evenly.
What is a belled base and do I need one?
A belled base is a wider flare at the bottom of the hole, hand-dug and poured as one with the shaft. A 12 in shaft with a 24 in bell 12 in high adds about 1.05 cubic feet. It spreads the load over weaker soil and resists uplift, and an engineer or local code will tell you if the job calls for it.
How deep should a form tube pier go?
At minimum the base must sit below the local frost line, which runs from about 12 in in the deep south to 48 in or more in northern states. Your building department sets the actual figure, so ask them before you dig. Deeper is fine, and a tube cut shorter than the frost depth will lift in winter.
Is a form tube pier strong enough for a deck?
It can be, but the answer comes from the load, the soil and the pier diameter together rather than from the tube. A footing must spread the post load across soil that can carry it, and weak or wet ground needs a bigger base. For anything structural, take the numbers to your local building department or an engineer.
ad-slot
Related calculators

Quantities are planning estimates. Tube diameters and stock lengths are commonly stocked sizes and vary by supplier. Prices are user-editable defaults rather than quotes. Pier diameter, depth and reinforcement are set by the load, the soil and local building code, and by an engineer for anything structural. Sonotube is a registered trademark of Sonoco Products Company. Calculatly is not affiliated with, endorsed by or connected to Sonoco or any form tube manufacturer. Last updated: October 1, 2026.