What This Calculator Does
Enter a tube diameter, the depth you're filling, and how many tubes, and this gives you the concrete each pier holds — in cubic feet, in cubic yards for a ready-mix order, and in 60 lb and 80 lb bag counts. The math is the cylinder volume π r² h and nothing else. A 12" tube filled 4 ft deep works out to π × 0.5² × 4 = 3.14 cubic feet, which is about six 80 lb bags or a little over a tenth of a yard. Four of those tubes is roughly 0.47 cubic yards total.
Bags or a Ready-Mix Truck?
The number that decides your whole day is total volume. Under about half a cubic yard, bagged mix poured dry into the tube and hydrated is practical. Above it — say six or more piers, or any 16"-plus columns — the bag count climbs past 22 and hand-mixing turns into hours of work, so a ready-mix short load is usually cheaper per yard and gives every footing the same consistent strength. Add about 10% to your total for cardboard bulge and the concrete you spill screeding the tops flush.
Get the Depth Right, Not the Tube Length
The one input people fumble is depth. Sonotubes ship in 4 ft and 12 ft lengths and you cut them to size, so the tube you buy is longer than the concrete you pour. What matters for volume is the fill depth — typically your frost-line depth plus the 6" of form left standing above grade. Put the actual concrete depth in the calculator, and confirm the footing diameter and depth against your local building code before you dig.
Sonotube Calculator
Quick Presets
Sonotube Concrete Volume Reference
Cubic feet per tube by diameter and height.
| Diameter | 36" Tall | 48" Tall | 60" Tall | 80-lb Bags (48") |
|---|---|---|---|---|
| 6" | 0.07 | 0.10 | 0.12 | 1 |
| 8" | 0.12 | 0.17 | 0.22 | 1 |
| 10" | 0.19 | 0.27 | 0.34 | 1 |
| 12" | 0.28 | 0.39 | 0.49 | 1 |
| 14" | 0.37 | 0.53 | 0.67 | 1 |
| 16" | 0.47 | 0.70 | 0.87 | 2 |
| 18" | 0.59 | 0.88 | 1.10 | 2 |
| 20" | 0.73 | 1.09 | 1.36 | 2 |
| 24" | 1.05 | 1.57 | 1.96 | 3 |
How to Use This Calculator
- Pick Your Tube Diameter: Choose the stamped tube size — 12" for typical deck posts, 8"–10" for fence posts, 16"–24" for structural columns.
- Enter the Fill Depth: Put in the depth of concrete you're actually pouring in inches — frost-line depth plus about 6" above grade — not the length of tube you bought.
- Set the Tube Count and Price: Enter how many piers you're pouring and your local ready-mix price per cubic yard to get a total cost alongside the volume.
- Read Your Volume and Bag Count: Check the cubic yards for a ready-mix order and the per-tube bag count, then add about 10% before you buy.
How It Works
A Sonotube is a waxed cardboard tube you set in the ground and fill with concrete to form a round pier or footing. Because the pour is a cylinder, the concrete it holds is fixed by two numbers: the tube's inside diameter and how deep you fill it.
The basic rule:
- Volume in cubic feet = π × (diameter in feet / 2)² × height in feet. A 12" tube is 1 ft wide, so the radius is 0.5 ft.
- Standard Sonotube diameters run 6", 8", 10", 12", 14", 16", 18", 20", and 24" — the diameter is stamped on the tube.
- Fill depth is what counts, not tube length. If you buy a 12 ft tube and cut it to a 4 ft footing, you only pour 4 ft of concrete.
- Set the base below your local frost line and leave 6" of tube above grade so the post bracket sits clear of standing water.
Add about 10% to whatever the tube volume works out to. Cardboard forms bulge slightly under a wet pour, you always spill some off the top, and running short mid-pour on a footing means a cold joint. For two or three tubes, bagged mix in a wheelbarrow is fine. Past about half a cubic yard total, price a short-load ready-mix delivery — mixing 25+ bags by hand is a long afternoon.
Tips & Considerations
- Enter the depth of concrete you're pouring, not the length of the tube — a 12 ft tube cut to a 4 ft footing only holds 4 ft of concrete.
- Round bag counts up for each tube separately; six tubes each needing 5.2 bags is 36 bags, not 31.
- Once your total passes roughly half a cubic yard, price a ready-mix short load against the bag count — around 22 bags of 80 lb is the break-even.
- Order about 10% over the calculated volume so form bulge and spillage don't leave a footing short and cold-jointed.
- Drop one #4 rebar down the center of each tube — it's cheap, common practice, and often required by code for pier footings.
Frequently Asked Questions
How many bags of concrete does one Sonotube take?
Work out the tube volume in cubic feet, then divide by the yield per bag: an 80 lb bag makes about 0.6 cu ft and a 60 lb bag about 0.45 cu ft. A 12" tube filled 4 ft deep holds π × 0.5² × 4 = 3.14 cu ft, so that's roughly 6 bags of 80 lb or 7 bags of 60 lb per tube. Multiply by your number of tubes and round each tube up — you can't buy a partial bag.
Should I buy 60 lb or 80 lb bags?
80 lb bags cost less per cubic foot and mean fewer bags to open, so they're the better buy if you can lift them — an 80 lb bag is heavy and there's about a third fewer of them. Go with 60 lb bags if you're working alone or your back would rather carry lighter loads. For the same 3.14 cu ft tube that's 6 bags of 80 lb versus 7 bags of 60 lb.
At what point should I order ready-mix instead of bags?
Bags stop making sense somewhere around half a cubic yard, or roughly 22 bags of 80 lb. Four 12" tubes at 4 ft deep total about 0.47 cu yd — right at the line. Six or more piers, or anything 16" and up, is usually cheaper and far faster as a ready-mix short load, and every tube gets identical, consistently mixed concrete instead of batch-to-batch hand mixing.
How much extra should I add for waste?
Add 10%. The cardboard flexes and bellies out a little when it's full, you lose some concrete screeding the top flush, and a footing that comes up short creates a weak cold joint where the first pour has already started to set. On a 0.47 cu yd job, 10% is about 0.05 cu yd — cheap insurance against a second trip to the store.
What diameter tube do I need for a deck or fence?
Deck posts are typically 12", stepping up to 16" for a tall or heavily loaded deck. Fence and pergola posts usually run 8"–10". A mailbox or lamp post is fine in a 6"–8" tube. Porch and structural columns land at 16"–24". Your footing size is set by the load and your local code, not by the post — check the permit requirement before you dig.
Do I fill the whole tube or just the footing depth?
You fill to whatever depth your footing calls for, which for most decks and fences is the frost-line depth plus 6" of tube left above grade. The tube length is just the form — cut it to size. The calculator uses your entered height, so put in the actual concrete depth, not the length of tube you bought.
Can I leave the cardboard on the concrete?
Strip the cardboard from the part that shows above grade once the concrete sets — it looks better and won't wick water against the post base. The buried portion can stay; it rots away underground and does no harm. Some inspectors specifically want the above-ground form peeled before final sign-off.