Sonotube Calculator
Deck footings, fence posts, mailbox posts: they're all the same problem, a cylinder of concrete you have to buy in rectangular bags. Enter the tube or hole diameter, the depth, and how many you're pouring. You'll get the bag count in 80s and 60s, per hole and total. If a 4x4 or 6x6 post is going into the hole, the tool subtracts the post's volume, which saves real money across a fence line.
Cylinders, bought in bags
Deck footings, fence lines, pergolas, mailboxes: round holes, rectangular bags. The conversion trips people because a cylinder's volume doesn't scale the way it looks. Doubling a tube's diameter quadruples the concrete, so a 12-inch tube isn't "a bit more" than an 8-inch, it's 2.25 times the bags. Enter your diameter, depth, and hole count, and the tool does the pi-r-squared honestly.
The post deduction matters on fence jobs. A 4x4 post (really 3.5 by 3.5 inches) displaces about a tenth of a cubic foot per foot of depth. Across 20 fence posts, subtracting it saves a real stack of bags.
Bags per tube, the quick table
80-lb bags per foot of depth, no waste. The tool adds your waste percentage and rounds up per the total, not per hole.
| Tube diameter | Cu ft per ft of depth | 80-lb bags per ft | Bags for 4 ft |
|---|---|---|---|
| 8 in | 0.35 | 0.6 | 2.4 |
| 10 in | 0.55 | 0.9 | 3.7 |
| 12 in | 0.79 | 1.3 | 5.3 |
| 16 in | 1.40 | 2.3 | 9.3 |
| 18 in | 1.77 | 2.9 | 11.8 |
| 24 in | 3.14 | 5.2 | 21 |
Sizing the tube itself
The rule most inspectors accept for deck footings: tube diameter three times the post width. A 4x4 post wants a 12-inch tube, a 6x6 wants 16 to 18. Depth is set by your frost line, not by preference; the footing has to bear below it or winter lifts the deck. Frost depth runs from nothing along the Gulf to 4 feet and more across the northern tier, and your building department publishes the local number. Most deck footings also need an inspection of the open hole before any concrete goes in, so dig, call, then pour.
A fence line, worked
Sixteen posts, 10-inch holes, 3 feet deep, 4x4 posts set in them. Each hole is 1.64 cubic feet, minus 0.26 for the post, so 1.38 net. All sixteen: 22 cubic feet, call it 24 with 10% waste, which is 41 bags at 80 lb. At two bags a post you'd have guessed 32 and run out on post thirteen; at three a post you'd have bought 48 and returned seven. The right answer sits between the rules of thumb, which is the point of doing the geometry.
Field notes
- Bell the bottom of frost-zone holes slightly wider than the top, or keep tube sides smooth. Frost grabs rough, tapered holes and lifts them.
- Set posts in wet concrete crowned at the top so water sheds away from the wood. Flat tops pool water and rot posts at the collar.
- Mix to the bag's water spec. Soupy concrete is weak concrete, and post holes hide it until the fence leans.
- For gates and corners, upsize the hole one diameter. Those posts carry the swing loads.
Pouring a pad too? Size it in the concrete calculator and add both to your project list.
Common questions
A 12-inch tube takes about 1.3 bags (80 lb) per foot of depth. So 4 feet deep is 6 bags after rounding up. An 8-inch tube takes about 0.6 bags per foot. Enter your exact size above for the real count.
A typical 10-inch hole, 2.5 to 3 feet deep with a 4x4 post, takes 1 to 1.5 bags at 80 lb, call it 2 to be safe. The rule of thumb is a third of the post's height underground.
Below your frost line, which ranges from 12 inches in the South to 48 or more in the northern states. Your local building department publishes the number. Deck footings also usually need inspection before the pour.
Bag makers say mix it. Dry-pour setting works for a mailbox in a pinch, but it cures weaker and unevenly. For anything structural, decks especially, mix to a wet pour.
Estimates for planning. Verify against your supplier's units and your local building codes before ordering or building. For anything structural, follow your engineer or local code.