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Deck Footing Calculator
Figure out how much concrete your deck footings need. Enter the number of footings, the tube diameter or square size and the depth, and get the volume, the number of 40, 50, 60 or 80 lb bags and how many form tubes to buy.
Last updated:
Footings
Footing shape
in
in
in
At least your local frost depth; check local code.
in
Tube stick-up so the post base stays dry.
in
in
Concrete
%
$
Enter your values to see the result.
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How to use the Deck Footing Calculator
- Choose round or square footings and enter how many you need.
- Pick the tube diameter (or square width), the depth below grade and how far the tube sticks up.
- Add a base pad if your plan calls for a wider spread footing under each pier.
- Choose the bag size and a waste allowance, and optionally the price per bag.
- Read the bags and tubes to buy, and compare other diameters in the table.
Deck footing concrete formulas
- Round footing: volume (ft³) = π × (diameter ÷ 2)² × height ÷ 1,728, with all sizes in inches.
- Square footing: volume (ft³) = width × width × height ÷ 1,728.
- Height = depth below grade + stick-up above grade.
- Bags = total volume × (1 + waste %) ÷ bag yield, rounded up.
Example: nine 10 in tubes, 42 in deep with 2 in above grade: π × 25 × 44 ÷ 1,728 = 2.0 ft³ each, 18.0 ft³ in total, 19.8 ft³ with 10% extra, or 33 bags of 80 lb concrete.
Concrete per foot of form tube
| Tube diameter | ft³ per foot of depth | 80 lb bags per foot | 60 lb bags per foot |
|---|---|---|---|
| 6 in | 0.196 | 0.33 | 0.44 |
| 8 in | 0.349 | 0.58 | 0.78 |
| 10 in | 0.545 | 0.91 | 1.21 |
| 12 in | 0.785 | 1.31 | 1.75 |
| 14 in | 1.069 | 1.78 | 2.38 |
| 16 in | 1.396 | 2.33 | 3.10 |
| 18 in | 1.767 | 2.95 | 3.93 |
| 24 in | 3.142 | 5.24 | 6.98 |
Tips for pouring deck footings
- Call before you dig (811 in the US) so buried utility lines are marked.
- Get the size from your plan: footing diameter and depth depend on post loads, soil and frost depth, which your local code or deck plan specifies.
- Set the tube height 1 to 2 in above grade so water drains away from the post base.
- Place anchors while wet: set J-bolts or post bases in the fresh concrete and check them for line and level.
- Let it cure: wait the time on the bag, usually at least a day or two, before loading posts.
Frequently asked questions
How many bags of concrete do I need for a deck footing?
Find the volume of one footing, then divide by the bag yield. A 10 in tube 44 in tall holds π × 5² × 44 ÷ 1,728 = 2.0 ft³, which is about 3.3 bags of 80 lb concrete (0.60 ft³ each), so plan on 4 bags per footing, or fewer when you total all footings before rounding.
How much concrete does an 80 lb bag make?
A typical 80 lb bag of premixed concrete yields about 0.60 cubic feet. A 60 lb bag yields about 0.45 ft³, a 50 lb bag about 0.375 ft³ and a 40 lb bag about 0.30 ft³. Check the yield printed on your brand's bag.
How deep should deck footings be?
Footings usually have to reach below the local frost line, which ranges from a few inches in warm climates to 48 in or more in cold regions, and often a minimum of 12 in below undisturbed soil. Your local building department sets the requirement, so check before digging.
What diameter sonotube should I use for a deck?
Common sizes are 8, 10 and 12 in. The required size depends on the load each post carries and your soil's bearing capacity, so it comes from your deck plan, span tables or local code rather than a single rule. Larger loads often use a wider base pad or bell footing under a smaller tube.
How do I calculate concrete volume for a round footing?
Volume = π × radius² × height. Work in inches and divide by 1,728 to get cubic feet, then by 27 for cubic yards. For a 12 in tube 48 in tall: 3.1416 × 6² × 48 ÷ 1,728 = 3.14 ft³.
Should I add extra concrete for waste?
Yes. Holes are rarely perfectly sized, some concrete spills, and the hole below the tube is often wider than the tube. Adding about 10% avoids a second trip to the store. If you dig a bell-shaped bottom, increase the allowance or add a base pad.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.
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