Sonotube Concrete Calculator

Figuring out sonotube concrete calculator starts with one honest admission: the answer is a range, not a single number. Concrete volume depends on the exact dimensions you measure, the depth you actually pour, and how much is lost to spillage and an uneven base — and the bag count depends on the bag size and the yield printed on the bag label. This page works through a 12 in diameter sonotube, 4 ft deep step by step, using the typical yields in our material data, and every result below is shown as a range. The product label governs: if the bag you buy yields a little less than the typical figure, the count goes up, and that is exactly why the range exists.

What makes a 12 in diameter sonotube, 4 ft deep different from a plain slab is the shape: a sonotube is a cylinder, so the volume comes from pi times radius squared times depth — and because the tube is round, a small change in diameter changes the volume a lot. That shape decides which formula applies and where the waste hides, so the worked example below does the arithmetic for this exact shape rather than quoting a generic per-square-foot figure. If your dimensions differ, the same steps still work — substitute your own length, width and depth, keep the 10% waste allowance, and round up to whole bags.

How the calculation works

The calculator needs the shape of the pour, the depth, and the bag size you plan to buy, and each one changes the answer. For a rectangular pour the volume is length times width times depth, all in feet, divided by 27 to get cubic yards — one cubic yard is 27 cubic feet (0.76 cubic metres). Depth is where most mistakes happen: 4 in (10 cm) is 0.333 ft, 6 in (15 cm) is 0.5 ft, so always convert inches to feet before multiplying. A slab that is 10 ft by 10 ft at 4 in thick is 100 times 0.333, or 33.3 cubic feet, which is about 1.2 cubic yards before waste. The same slab at 6 in thick is half as much again, about 1.9 cubic yards, which is why the thickness in the keyword matters as much as the footprint.

Once the volume is known, it is converted to bags. Our material data gives typical yields of about 45 80 lb (36.3 kg) bags per cubic yard and about 60 60 lb (27.2 kg) bags per cubic yard, because an 80 lb bag yields about 0.6 cubic feet and a 60 lb bag about 0.45 cubic feet — those are typical values, and the bag label on the exact product you buy governs. On top of that, add the typical waste allowance of about 10% for spillage, an uneven subgrade, and the concrete that stays in the mixer and the wheelbarrow. The worked example below shows the low end with no waste and the high end with the full 10% allowance, so you can see the range instead of a single number that pretends the pour will be perfect.

A sonotube is a cylinder, so the formula is pi times radius squared times height, with every dimension in feet. For a 12 in tube the radius is 6 in, or 0.5 ft, and the square of the radius is what makes diameter matter so much: a 12 in tube holds more than twice the concrete of an 8 in tube of the same depth. The hole around the tube is usually a little wider than the tube itself, and that extra ring is filled with soil or gravel, not concrete — only the inside of the tube counts.

Worked example: a 12 in diameter sonotube, 4 ft deep

Here is the full arithmetic for a 12 in (30 cm) diameter sonotube, 4 ft (1.2 m) deep.

Step 1 — radius in feet. Diameter 12 in, so the radius is 6 in, or 0.5 ft.

Step 2 — cross-section area. Pi × 0.5² = 3.1416 × 0.25 = 0.785 square feet.

Step 3 — volume. 0.785 × 4 ft deep = 3.14 cubic feet, or 3.14 ÷ 27 = 0.12 cubic yards.

Step 4 — bags, low end. A 60 lb bag yields about 0.45 cubic feet, so 3.14 ÷ 0.45 = 7.0 bags, rounded up to 7 bags. With 80 lb bags at about 0.6 cubic feet each: 3.14 ÷ 0.6 = 5.2, rounded up to 6 bags.

Step 5 — bags, high end with the 10% waste allowance. 3.14 × 1.10 = 3.46 cubic feet, or 8 60 lb bags (6 80 lb bags). Order 7–8 60 lb bags per tube and multiply by the number of tubes — for four tubes, that is 28–32 bags. Keep one spare bag for the last tube.

Cost guidance

Costs on this page are ranges, labelled typical retail range — check the current shelf price before budgeting. No single national price is quoted, because bagged concrete, ready-mix and delivery charges vary by region, retailer and supplier, and a price that is right in one town can be wrong in the next. The way to budget is to multiply, not to guess: take the quantity from the worked example (0.12–0.13 cubic yards, or about 7–8 60 lb bags), multiply it by the shelf price per bag or the per-cubic-yard price your local supplier quotes, then add delivery, formwork timber, rebar or mesh, and a finishing tool if you do not own one. For a small pour like this one, bagged mix bought by the bag is usually the practical route; once the volume passes roughly 1–2 cubic yards, a ready-mix short load often works out cheaper per yard even after the delivery charge, so price both options against the same quantity.

If the job grows while you measure — a footing that turns out longer than the plan, a sidewalk that needs an extra section — redo the volume from the new dimensions rather than guessing a percentage on top of the old total. Concrete quantities scale with volume, not with length alone, so a small change in depth changes the total more than most people expect. And if you are comparing bagged mix with ready-mix, remember the comparison only works when both quotes cover the same volume including waste, the same delivery distance, and the same extras; a cheap per-bag price with an expensive delivery is not a cheap job.

Step by step: measure, calculate, order

1. Measure twice. Measure the tube diameter in inches and the depth in inches or feet, from the bottom of the hole to the top of the tube. Measure each run separately — a footing that turns a corner is two runs, not one — and write every dimension down in feet and inches before you convert anything.

2. Convert and calculate. Convert inches to feet (divide by 12), then apply the volume formula for your shape: length times width times depth divided by 27 for a rectangle, or pi times radius squared times depth for a round tube or post hole. Use the calculator on this page to check your arithmetic, and run it twice with the low and high depth if your excavation is uneven.

3. Add waste and round up. Add the typical 10% waste allowance for spillage and an uneven subgrade, then round up to whole bags or to the nearest quarter cubic yard for a truck. A bag cannot be bought in fractions, and a pour that stops short is far more expensive than one spare bag.

4. Order and check the delivery. Order the high end of the bag range below, all the same mix, all the same product and mix, and check the bag labels against the yields the calculator assumed. If you are ordering ready-mix, confirm the slump, the mix strength and the delivery window, and make sure the truck can reach the pour.

5. Before you pour. Set and brace the forms, check the depth with a straightedge in several places, wet the subgrade lightly so it does not steal water from the mix, and have your screed, float and curing cover within reach. Concrete waits for nobody once it is mixed.

Common mistakes to avoid

The most common mistake with sonotube concrete calculator is measuring the top of the excavation and pouring to a different depth at the bottom. Footings and slabs are dug by hand or by machine, and the base is rarely flat — a trench that is 10 in deep at one end and 13 in at the other needs noticeably more concrete than the drawing says. Measure the depth in at least three places and plan on the deepest. The second mistake is forgetting the 10% waste allowance: spillage, the mix left in the barrow, and the uneven subgrade all eat volume. The third is ordering the low end of the range and then trying to stretch the last half-bag across a corner that still needs filling — that patch will always show. Order the high end, keep the receipt, and return unopened bags.

Frequently asked questions

How much concrete do I need for a 12 inch sonotube?

Use pi times radius squared times depth. A 12 in tube, 4 ft deep, holds about 3.14 cubic feet, or 0.12 cubic yards — about 7–8 60 lb bags or 6–6 80 lb bags with the typical 10% waste allowance. Multiply by the number of tubes for the whole job.

How deep should a sonotube footing be?

The depth is set by local code and by the frost line, not by a calculator — the tube must reach below frost depth and sit on undisturbed soil or compacted gravel. Typical residential deck footings run about 3–4 ft deep in cold climates and less in warm ones. Check the code for your area, and call the utility-locating service before you dig.

How many sonotubes do I need for a deck?

That depends on the deck size, the beam spans and the load, not on the concrete calculator — a typical small deck has one tube at each corner and one or two along each beam line, spaced about 6–8 ft apart. Work out the post layout first, then use the calculator once per tube and multiply. The tube count comes from the deck design; the concrete per tube comes from the diameter and depth.

Do I fill the whole sonotube with concrete?

Yes — the tube is filled to the top, and the concrete is consolidated so there are no voids against the tube wall. The post base or anchor bolt is set into the wet concrete at the top. The hole around the outside of the tube is backfilled with soil or gravel, not concrete.

Can I use ready-mix for sonotubes?

Yes, and for more than a few tubes it is usually easier than mixing bags — a small ready-mix load covers many tubes at once. For one or two tubes, bagged mix is the practical route. Either way, work out the total volume first: multiply the per-tube volume by the number of tubes, add about 10% waste, and order the high end.

Related calculators

Results are estimates — verify with the product label. Coverage, pack sizes, yields and prices vary by brand, product line, surface and region; the label on the product you buy governs. Cost figures are typical retail ranges — check the current shelf price before budgeting.