A tube is a cylinder, and nothing else
Fiber concrete forms — Sonotube is the brand name most people use for all of them — are open tubes of fixed diameter that you cut to length and set in the ground. The concrete you pour is the full round volume. There is no post standing in the middle of it, so unlike a fence post hole there is nothing to deduct: what the inside of the tube measures is what you buy mix for.
That is the whole calculation, and it is why the numbers on this page can be checked with a tape measure and one formula. The rest of the guide is the practical part — measuring the right diameter, working out the height you actually pour, and not being short a bag at the bottom of the last lift.
The formula, so you can check the chart
Volume of a cylinder is area of the circle times height. In feet, with the diameter measured in inches:
volume (cu ft) = π × (diameter ÷ 24)² × height (ft)
The 24 is doing two jobs: halving the diameter for the radius, and dividing by 12 to get from inches to feet.
Take the most common size there is, a 12-inch tube three feet tall. Diameter ÷ 24 is 0.5, so the radius squared is 0.25 and the area is 0.785 square feet. Times three feet of height is 2.36 cubic feet. Divide that by the 0.60 cubic feet an 80 lb bag yields and you get 3.93 bags — which rounds up to 4.
That rounding direction is deliberate. Every figure here rounds up, because a footing three percent short of its planned volume is not a footing you can top up later once the mix has started to set.
Chart: bags by tube diameter and height
Heights are the cut length of the tube, not the length printed on it in the store. A 48-inch tube cut down to 30 inches is poured as a 30-inch tube, and that is the row to use.
| Tube dia × height | Concrete (cu ft) | 40 lb | 60 lb | 80 lb |
|---|---|---|---|---|
| 8" × 24" | 0.70 | 3 | 2 | 2 |
| 8" × 36" | 1.05 | 4 | 3 | 2 |
| 8" × 48" | 1.40 | 5 | 4 | 3 |
| 10" × 24" | 1.09 | 4 | 3 | 2 |
| 10" × 36" | 1.64 | 6 | 4 | 3 |
| 10" × 48" | 2.18 | 8 | 5 | 4 |
| 12" × 24" | 1.57 | 6 | 4 | 3 |
| 12" × 36" | 2.36 | 8 | 6 | 4 |
| 12" × 48" | 3.14 | 11 | 7 | 6 |
| 14" × 24" | 2.14 | 8 | 5 | 4 |
| 14" × 36" | 3.21 | 11 | 8 | 6 |
| 14" × 48" | 4.28 | 15 | 10 | 8 |
| 16" × 24" | 2.79 | 10 | 7 | 5 |
| 16" × 36" | 4.19 | 14 | 10 | 7 |
| 16" × 48" | 5.59 | 19 | 13 | 10 |
| 18" × 24" | 3.53 | 12 | 8 | 6 |
| 18" × 36" | 5.30 | 18 | 12 | 9 |
| 18" × 48" | 7.07 | 24 | 16 | 12 |
These are the numbers the concrete calculator returns for the same tube, rounded up to whole bags. Counts are calculated from the exact volume, not from the rounded column beside them: a 14-inch tube 48 inches tall prints as 4.28 cu ft but the true figure is fractionally over eight bags of 80 lb, so the chart shows 8 rather than 7.
How much tube one bag fills
Working from the other end is often more useful on site. If you have six bags in the truck and need to know whether they will finish the pour, this is the answer: one 80 lb bag is 0.60 cubic feet, so it fills the tube to a fixed height that depends only on diameter.
| Tube diameter | Height per 80 lb bag | Bags for 48 in |
|---|---|---|
| 8 in | 20.6 in | 3 |
| 10 in | 13.2 in | 4 |
| 12 in | 9.2 in | 6 |
| 14 in | 6.7 in | 8 |
| 16 in | 5.2 in | 10 |
| 18 in | 4.1 in | 12 |
The pattern to notice: doubling the diameter quadruples the concrete per inch of height. An 8-inch tube climbs 20.6 inches on a bag while an 18-inch tube climbs 4.1 — same bag, one fifth the height. That is the single most expensive assumption people make when they size a tube by eye.
Height is a decision, not a product size
Tubes are sold in a few long lengths — 48 inches is the common one at a lumberyard — and cut with any handsaw. What you are deciding when you cut is how much footing you want, and there are three heights in play at once:
- How deep it is buried. The buried length has to reach below the frost line where the ground freezes, or the footing moves with the season. That depth is set by your region and your local building department, not by a number in a national guide.
- How high it finishes above grade. Two to four inches is typical for a footing that carries a post or a bracket, and it keeps the base of the wood out of standing water.
- What the cut length adds up to. Buried depth plus above-grade height is the number to enter in the calculator. A tube in a 3-foot hole finishing 4 inches proud of the ground is poured as 40 inches, not 36.
If the cut length falls between chart rows, use the next row up. A 40-inch pour is sized as 48 inches, because the alternative is being short. On a footing carrying a structural load, the plan or an engineer sets the dimension — this page only tells you how much mix it takes.
Which bag size to buy
The bag sizes are interchangeable in volume, not in effort. Pick the largest bag you can lift and mix comfortably, then buy that size.
| Bag | Yield | Bags per cu yd |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
A cubic yard is 27 cubic feet, which is where the 45 figure for 80 lb bags comes from. That number is the useful tripwire: six tubes at 6 bags each is 36 bags, still in bag territory, but a dozen 18-inch tubes crosses a yard and a half and stops being a wheelbarrow job. Past about a yard, a delivered mix is easier to place and easier to finish before it starts to set — the bags versus ready-mix break-even guide works through where that line falls.
Pouring the tube
The bags are only half the job. The details that decide whether a footing lasts are the ones around the pour:
- Brace before you pour. A tube full of wet mix is heavy and it will bulge and lean if nothing holds it. Stake it on two adjacent sides and check it with a level on both faces, not one.
- Fill in lifts and rod each one. Twelve to 18 inches at a time, working a rod or a scrap of rebar up and down through the full depth of the lift to release trapped air. Tapping the outside of the tube helps at the walls.
- Crown the top away from the post. Slope the finished surface so water runs off rather than sitting against whatever sits on the footing.
- Check the plan for reinforcement. Many tube footings carry a bar or a small cage. Whether yours does is an engineering question for the plan or your local code, not something to guess from a bag label.
- Keep heavy loads off it. Concrete gains strength over days, not hours. Setting a post or stacking material the next morning is asking the footing for strength it does not have yet.
Mix to the yield on the bag rather than to a consistency that feels right. A mix that pours like soup has more water in it than the mix designer intended, and water is the thing that costs strength. If a bag feels stiff, more mixing time usually fixes it before more water does.
The mistakes that cost a bag, or a footing
01
Sizing off the outside diameter
The tube wall adds thickness, so the outside measures wider than the hole the concrete fills. Wrap a tape around the tube for the circumference if you need to check it, then divide by pi — or measure across the opening's inside.
02
Forgetting that the buried part still holds concrete
A tube set 12 inches into the ground is poured for its full length, not just the above-grade part. Cutting a 48-inch tube down to 36 and then pouring only 24 inches leaves a footing half the size you planned.
03
Dumping a 48-inch tube in one lift
A single dump traps air against the tube wall, and the voids show up later as honeycomb. Fill in lifts of about 12 to 18 inches and rod each lift through its full depth before adding the next.
04
Adding water until it pours easily
Extra water makes a mix that places faster and cures weaker. Mix to the bag's stated yield, and if it needs to be wetter to work, that is a sign it needs more mixing time rather than more water.
05
Leaving the tube on forever, or stripping it in an hour
Fiber tubes are usually left in place below grade and stripped above grade. Either way, the concrete needs to hold its own shape first — a day at minimum before removing anything, and check the mix's own instructions.
Run your own tube count
The link below opens the concrete calculator in its cylinder mode, on the 12-inch tube 3 feet tall from this page — no post deducted — showing 2.36 cubic feet and 4 bags of 80 lb. Change the diameter and depth to your own tube and the bag counts follow, with the cubic yards shown alongside so you can see the moment the job stops being a bag job.
Setting posts in the ground instead of pouring a footing? Bag counts per fence post is the same arithmetic with the buried post subtracted, and the deck board counts by deck size covers what sits on top of the footing once it has cured.