Sonotube Calculator

A Sonotube is a single-use cylindrical form for pouring round concrete columns, piers and footings. It is a spirally wound fibre tube with a waxed inner liner so the cured concrete releases cleanly. Sonotube is Sonoco’s brand name, and like Dumpster or Sheetrock it has become the generic term — you will see the same product sold as a concrete form tube, a column form, or a Builder’s Tube. Standard round forms run 6 in to 36 in in diameter, commercial forms up to 60 in, and the hardware-store Builder’s Tube comes in 6, 8, 10 and 12 in diameters at 4, 8 and 12 ft lengths.

A 12 in tube holds 0.785 ft³ per foot of depth. An 80 lb bag yields 0.60 ft³, so you need about 1.3 bags of 80 lb per foot — 6 bags for a 4 ft pier, 7 bags for a 5 ft pier. In 60 lb bags it is about 1.75 per foot: 8 bags for 4 ft. In cubic yards a 12 in pier takes 0.029 yd³ per foot, so nine 4 ft piers is a little over a third of a yard.

An 8 in tube holds 0.349 ft³ per foot. That is one 80 lb bag for every 1.7 ft of depth — so 3 bags for a 4 ft pier and 4 bags for a 5 ft pier, rounding up. In 60 lb bags, one bag covers about 1.3 ft, so a 4 ft pier takes 4 bags. Metric users will see a 25 kg bag cover roughly a foot of an 8 in tube, which is where the common "one bag per foot" rule of thumb comes from.

An 18 in tube holds 1.767 ft³ per foot, which is 2.9 bags of 80 lb mix or 3.9 bags of 60 lb per foot of depth. A 4 ft 18" sonotube takes about 12 bags of 80 lb; a 5 ft one takes 15. At that size you are near the point where mixing bags stops making sense: four 18 in piers at 5 ft deep is about 1.3 cubic yards, which is a ready-mix delivery.

A 12 in diameter tube 48 in (4 ft) deep holds 3.14 ft³, or 0.116 cubic yards. That is 6 bags of 80 lb mix, 7 bags of 60 lb, 9 bags of 50 lb, or 11 bags of 40 lb, with each figure rounded up to a whole bag. Add your waste allowance on top — at 10% the 80 lb figure becomes 7 bags, and one spare bag beats a second trip mid-pour.

Use V = π × d² ÷ 4 × h, with the diameter and height both in feet, then divide by 27 for cubic yards. A quick check: the volume in cubic feet per foot of depth is 0.785 for a 12 in tube, and it scales with the square of the diameter — so a 24 in tube holds four times as much per foot as a 12 in one, not twice. Multiply by the number of piers and add 5 to 15% for waste.

Divide the tube volume in cubic feet by the finished yield of your bag: 0.60 ft³ for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb, and 0.30 for 40 lb. Always round up — a partial bag is a whole bag, and a bag left over is cheaper than a pier poured in two batches an hour apart. The calculator above shows all four bag sizes at once so you can pick on weight rather than on arithmetic.

A 12 inch sonotube takes 0.785 ft³ (0.029 yd³, 0.022 m³) for every foot of depth. A 3 ft pier is 2.36 ft³, a 4 ft pier is 3.14 ft³, and a 5 ft pier is 3.93 ft³. Ten 4 ft piers come to 31.4 ft³, or 1.16 cubic yards — just past the point where a ready-mix delivery beats a pallet of bags.

Measure the length you need, then wrap a strip of stiff paper, a length of hose, or a sheet of newspaper around the tube and align its edges. That gives you a square line all the way round — cutting freehand almost always spirals. Score and follow the line with a sharp utility knife for two or three passes, or cut it with a fine-tooth handsaw or a recip saw with a wood blade. Support the offcut so it does not tear the last inch as it falls.

A 6×6 post is 5½ in actual, so a 6 in tube leaves no room at all. For a post standing on a pier with a metal post base, a 10 in or 12 in tube is the usual choice — that gives a bearing area and a pier stout enough to hold the anchor. If the post is being embedded in the concrete rather than surface-mounted, go to 12 in minimum so there is at least 3 in of concrete cover all around the timber. The pier diameter is governed by the load and the soil bearing capacity, though, not by the post: check the footing table in your local code or the deck plans.

It depends on the load and the soil. A plain cylinder bearing on firm, undisturbed soil is adequate for many light deck and pergola piers, and it is what most prescriptive deck details show. Softer soil, heavier loads, or a code that sets a minimum bearing area means you need to spread the load — either a belled base flared out at the bottom of the hole, or a square or round pad cast beneath the tube. The footing options in the calculator add that concrete to the total. Your building department has the final say on both the bearing area and the depth.

For short, lightly loaded deck piers in compression, many prescriptive details do not require any. Add it — and follow a real schedule — when the pier is tall and slender, when it carries lateral load or uplift, when it is part of a permitted structure, or whenever the plan calls for it. A typical arrangement is 2 to 4 vertical #4 bars in tubes up to 12 in, more in larger forms, with roughly 3 in of cover to the form, circular ties where the pier acts as a column, and a dowel tying the pier to any pad beneath it. Bar size, lap length and tie spacing come from the engineer or the code, not from a rule of thumb.

It is a shorthand mix proportion: 4 parts gravel or coarse aggregate, 2 parts sand, and 1 part Portland cement by volume, with just enough water to make it workable. It sits close to the classic 1:2:3 and 1:2:4 nominal mixes and lands around 3,000 psi when it is batched carefully and not over-watered. For piers and footings, bagged mix or ready-mix is more consistent than site batching; the 4-2-1 rule is most useful when you are mixing from bulk material and have no scale.

Common alternatives are plastic or composite footing forms with an integral flared base, pre-cast concrete deck blocks where code allows them, helical or screw piles driven by machine, a plain hole augered in firm clay and poured without any form, a length of PVC or corrugated drainpipe for small posts, or a site-built plywood box for a square pier. Each has a trade-off: deck blocks sit above the frost line, an unformed hole gives no control over diameter or finish, and screw piles need equipment. Fibre tubes remain the cheapest way to get a clean, true round column.

Dig to full depth and clean the bottom of the hole. Cut the tube to hole depth plus the height you want above grade. Drop it in, plumb it in two directions, and brace it to at least two faces with stakes and 1× or 2× lumber. Backfill and tamp around the outside in lifts so it cannot drift or float. Hang the rebar with about 3 in of cover, then place concrete in 12 to 18 in lifts, rodding or vibrating each one. Overfill slightly, screed the top, set the anchor while the mix is still plastic, and leave it 24 to 48 hours before loading.

A standard 3,000 to 4,000 psi mix is right for most residential piers. Bagged high-strength or "concrete mix" with 3/8 in aggregate flows into a narrow tube better than a stiff mix with large stone; fast-setting mix is convenient for fence and mailbox posts but gives you very little working time in a deep pier. Keep the water at the low end of the printed range — extra water makes placing easier and the finished pier substantially weaker. For anything over about 1.5 cubic yards, order ready-mix and specify the strength.

No. It is single-use formwork. The waxed liner bonds to the concrete as it cures and the fibre tears as you strip it, so what comes off is ruined. The portion below grade is normally left in place, where it rots away harmlessly over time; the exposed portion should be peeled off once the concrete has set rather than left to absorb water and delaminate. Plastic and composite column forms, by contrast, are designed to be stripped and reused.

Clean, well-graded gravel or the excavated soil, either one placed in 6 to 8 in lifts and tamped as you go. Gravel drains better and compacts more predictably; native soil is free and fine in firm ground. Do the backfilling before the pour, not after, so the tube is supported against the pressure of wet concrete. Avoid dumping the hole full of loose spoil and calling it done — loose backfill settles, opens a gap at the surface, and funnels water straight down the side of the pier.

Below the local frost line, onto undisturbed bearing soil. That is a code number set by your building department and it ranges from essentially nothing in the warm South to 4 ft or more in the northern states and much of Canada. Depth is not optional: a pier that stops short is lifted by frost every winter and settles unevenly afterwards, which shows up as a deck that has gone out of level. Add whatever height you want above grade to the buried depth — that total is the pour height this calculator asks for.

The form itself is the cheap part — roughly $10 to $30 for a 4 ft length in 8 to 12 in diameter. The concrete dominates: a 12 in pier 4 ft deep is about 6 bags of 80 lb mix, so $35 to $50 in material, plus rebar and a post base. A contractor-installed pier including digging, forming, rebar and pour typically runs a few hundred dollars each, which is why the cost panel above prices bags, ready-mix and forms as separate lines you can fill in with your own supplier’s numbers.