How it works
The formula
with waste = one footing × footings × 1.10
bags = total cu ft ÷ bag yield, round up once
tubes = footings × (length ÷ 4 ft, round up)
Assumptions
- You give the diameter, depth and number of footings. We do not size footings: that depends on your deck load, soil and frost depth, so use your plan or local code.
- Each tube is filled solid from the bottom of the hole to the height above ground you enter. Diameters are inside the tube: 8, 10 or 12 in.
- Bag yields, from the volume printed on the bag: 80 lb makes 0.60 cu ft, 60 lb 0.45, 50 lb 0.375, 40 lb 0.30.
- Bags are rounded up once, on the total for all footings. Unlike fence posts, you pour a set of footings in one session, so the leftover mix from one tube goes into the next. If you pour them on different days, round each footing up instead: for the example below that is 5 bags each.
- Form tubes are sold in 4 ft sections. Each footing needs its own tube, cut to length, so each one is rounded up to whole sections. A leftover piece is not counted for another footing.
- 10% extra for waste by default, since tubes bulge a little and some mix is spilled. Use 5% for a careful pour.
- A pour of 1 cu yd or more (about 45 bags of 80 lb) shows a ready-mix order, rounded up to the quarter yard with a 1 yard minimum. This is a rule of thumb: ask your plant.
Worked example
6 footings, 10 in tubes, 4 ft deep and 2 in above ground
- Length of one footing: 4 ft + 2 in (0.17 ft) = 4.17 ft
- One footing: π × (10 ÷ 2 ÷ 12 ft)² × 4.17 ft = 2.27 cu ft
- All footings with 10% extra: 2.27 × 6 × 1.10 = 15.00 cu ft
- In 80 lb bags: 15.00 ÷ 0.60 = 25.0, round up to 25 bags
- Tubes: 4.17 ft ÷ 4 ft = 1.04, round up to 2 per footing. 2 × 6 = 12 tubes
- At the middle of our typical prices ($6.50 a bag, $15.88 a tube): $353
Bags of 80 lb per footing
| Tube | 3 ft deep | 4 ft deep | 5 ft deep |
|---|---|---|---|
| 8 in | 2.0 | 2.7 | 3.3 |
| 10 in | 3.2 | 4.2 | 5.2 |
| 12 in | 4.6 | 6.0 | 7.4 |
Each footing sticks up 2 in above ground, with 10% extra. Not rounded: add up the footings first, then round up once. For 6 footings the bags are the total, rounded up.
Questions people ask
How many bags of concrete for a sonotube?
A 10 in tube, 4 ft deep and 2 in above ground holds 2.27 cu ft. With 10% extra that is 4.2 bags of 80 lb, or 5.6 bags of 60 lb. For 6 footings it is 25 bags of 80 lb, counted together.
Should I use 8, 10 or 12 inch tubes?
That comes from your plan or local code, not from this page. A larger tube holds more: at 4 ft deep, 8 in takes 2.7 bags of 80 lb, 10 in takes 4.2 and 12 in takes 6.0 per footing.
How deep should a sonotube footing be?
At least below your local frost line, so the ground cannot heave it, and as deep as your plan says. Frost depth ranges from a few inches in the South to 4 ft or more in the far North. Ask your building department. Type the depth into the calculator: we do not choose it for you.
Do I need rebar in a sonotube?
Some codes and plans call for it and some do not. We do not work it out here. Check your plan or ask your local building department or an engineer. If it is required, rebar is not in the list above, so add it.
Is a sonotube better than digging a hole and filling it?
A tube keeps the concrete to the size you planned and gives a clean, round pier above ground. A bare hole is usually dug wider than planned and fills with more concrete: a 14 in hole 4 ft deep takes 4.45 cu ft, against 2.27 cu ft in a 10 in tube. Some inspectors accept a hole with no tube where the soil is firm, so check your local code.