How Much Concrete For A Sonotube

Before you order a truck or stack bags in the driveway, you need a realistic quantity for how much concrete for a sonotube. Too little concrete means a cold joint halfway through the pour, a second trip, and a weak seam; too much means bags or a short-load fee you did not need. This guide walks through a 10 in diameter sonotube, 3 ft deep with the volume formula, the typical bag yields from our material data, and a 10% waste allowance, and it ends with a range you can actually order against. Everything here is an estimate — measure twice, check the bag label, and buy a little extra for the parts of the job you cannot see until the forms come off.

What makes a 10 in diameter sonotube, 3 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 10 in tube the radius is 5 in, or about 0.42 ft. The square of the radius is what makes diameter matter so much: a 10 in tube holds about 70% more concrete than an 8 in tube of the same depth, and a 12 in tube holds more than twice as much. Only the inside of the tube counts — the ring of soil or gravel around the outside is not concrete.

Worked example: a 10 in diameter sonotube, 3 ft deep

Here is the full arithmetic for a 10 in (25 cm) diameter sonotube, 3 ft (0.9 m) deep.

Step 1 — radius in feet. Diameter 10 in, so the radius is 5 in, or 0.417 ft.

Step 2 — cross-section area. Pi × 0.417² = 3.1416 × 0.174 = 0.545 square feet.

Step 3 — volume. 0.545 × 3 ft deep = 1.64 cubic feet, or 1.64 ÷ 27 = 0.06 cubic yards.

Step 4 — bags, low end. A 60 lb bag yields about 0.45 cubic feet, so 1.64 ÷ 0.45 = 3.6 bags, rounded up to 4 bags. With 80 lb bags at about 0.6 cubic feet each: 1.64 ÷ 0.6 = 2.7, rounded up to 3 bags.

Step 5 — bags, high end with the 10% waste allowance. 1.64 × 1.10 = 1.80 cubic feet, or 4 60 lb bags (3 80 lb bags). Order 4–4 60 lb bags per tube and multiply by the number of tubes — for six tubes, that is 24–24 bags. Keep one spare bag for the last tube.

Cost guidance

Budgeting for how much concrete for a sonotube means pricing the range, not a single number. Take the worked-example quantity (0.06–0.07 cubic yards, or about 4–4 60 lb bags) and price it twice: once at the low end of the range and once at the high end, using the current shelf price per bag at the store where you will actually buy, or the per-cubic-yard ready-mix quote from a local supplier. The difference between the two totals is your contingency, and it is usually smaller than the cost of running short mid-pour. Two cost traps catch first-timers: ordering the low end and then paying a second delivery charge for the few bags that finish the job, and forgetting the extras — formwork, stakes, rebar or mesh, a float, and curing compound or plastic sheeting. Compare the total delivered cost, not the per-bag price, and keep one or two unopened bags for patching; an opened bag that gets damp is wasted money.

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 how much concrete for a sonotube 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 sonotube?

Use pi times radius squared times depth. A 10 in tube, 3 ft deep, holds about 1.64 cubic feet, or 0.06 cubic yards — about 4–4 60 lb bags or 3–3 80 lb bags with the typical 10% waste allowance. Multiply by the number of tubes for the whole job.

How many bags of concrete fill a sonotube?

It depends on the tube size — there is no single per-tube number. A 10 in tube 3 ft deep needs about 4–4 60 lb bags, while a 12 in tube 4 ft deep needs about twice as many. Measure the diameter and the depth, work out the cylinder volume, and divide by the bag yield of about 0.45 cubic feet per 60 lb bag.

What diameter sonotube do I need for a deck post?

The diameter is set by the load and by local code, not by the concrete calculator — a common residential choice is a 10 in or 12 in tube for a deck post. Larger posts and heavier decks need larger footings. Check the code and the deck design for the required footing size, then use the calculator to work out the concrete for that size.

How do I keep a sonotube straight while pouring?

Brace the tube plumb with stakes and a level before the pour, and backfill around the outside of the tube so it cannot shift. Pour in layers and rod the concrete to remove air pockets, checking the level again after the first layer. A tube that leans while the concrete is wet will set leaning, and it cannot be straightened afterwards.

Should the sonotube stick above ground?

Yes — the top of the tube usually finishes a few inches above grade so the post base sits above standing water and soil. Measure the depth from the bottom of the hole to the finished top of the tube, not just to ground level, because the part above grade still needs concrete.

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.