Concrete footing calculator
Round tube or square footings, with or without a wider pad at the bottom: the concrete volume, how many pre-mixed bags of each size, and how much they weigh.
Concrete to buy
6 × 10 in round footings, 4 ft of concrete, plus 10% extra.
- Concrete0.53 yd³ (14.4 ft³)6 × 2.18 ft³ + 10%
- 80 lb bags24about 1,920 lb to carry
- Ready-mix0.75 yd³rounded up to ¼ yd³
| Part | Volume |
|---|---|
| Pier, 10 in round × 4 ft | 2.18 ft³ |
| Each footing | 2.18 ft³ |
| All 6, +10% | 0.53 yd³ (14.4 ft³) |
| Bag | Bags | Yield each | Total weight |
|---|---|---|---|
| 80 lb bag | 24 | 0.6 ft³ | 1,920 lb |
| 40 lb bag | 48 | 0.3 ft³ | 1,920 lb |
| 50 lb bag | 39 | 0.38 ft³ | 1,950 lb |
| 60 lb bag | 32 | 0.45 ft³ | 1,920 lb |
| 90 lb bag | 22 | 0.68 ft³ | 1,980 lb |
Yields are the mixed volume printed on the bag. Use your own bag's figure if it differs; "scaled" bags are worked out from the labelled ones.
How to use the footing calculator
Choose round for cardboard tube forms or augered holes, square for boxed forms. Enter the diameter or side, and the concrete depth: the depth of the hole plus any tube standing above the ground. Add the number of footings and an allowance for spills and holes dug wider than planned.
If the footing has a wider pad at the bottom, as DCA 6 shows for decks, tick the box and give its shape, width and thickness; the pier above it then needs only its own height. The DCA 6 helper looks up the pad size for your beam and joist spans and fills it in.
Bags and ready-mix
The bag table uses the mixed volume printed on each bag: 0.60 ft³ for an 80 lb bag of QUIKRETE Concrete Mix, for example, and about 10 L for a 20 kg Easy Mix bag. If your bag says something else, choose "Other" and copy its label. The weight column is what you will lift and haul; past a cubic yard or so, compare it with a ready-mix delivery.
Worked example
Six 10 in tubes 48 in deep: each holds π × 5² × 48 = 3,770 in³, or 2.18 ft³. Six of them are 13.1 ft³, and with 10% extra 14.4 ft³ (0.53 yd³). At 0.60 ft³ per 80 lb bag that is 24 bags, which weigh 1,920 lb.
Deck footing sizes and depth
For US decks, AWC's DCA 6 Table 4 sizes footings by beam span and joist span, assuming soil that bears 1,500 psf, a 40 psf live load and 2,500 psi concrete; corner posts may use 90% of the listed size. DCA 6 and the 2021 IRC (R507.3) both require footings to sit at least 12 in below undisturbed ground, and below the frost line where that is deeper. Frost depth is set locally: ask your building department, and call 811 before digging in the US.
| Beam span | Joist span ≤ 10 ft | ≤ 14 ft | ≤ 18 ft |
|---|---|---|---|
| 6 ft | 18 / 16 / 7 | 21 / 18 / 8 | 24 / 21 / 10 |
| 8 ft | 20 / 18 / 8 | 24 / 21 / 10 | 27 / 24 / 11 |
| 10 ft | 23 / 20 / 9 | 27 / 24 / 11 | 31 / 27 / 13 |
| 12 ft | 25 / 22 / 10 | 30 / 26 / 13 | 34 / 30 / 15 |
| 14 ft | 27 / 24 / 11 | 32 / 29 / 14 | 37 / 33 / 16 |
| 16 ft | 29 / 26 / 12 | 35 / 31 / 15 | 40 / 35 / 18 |
| 18 ft | 31 / 27 / 13 | 37 / 33 / 16 | 42 / 37 / 19 |
These are a planning aid. Soil that bears less, heavy snow, hot tubs and anything outside the tables need a design, and the building department and inspector decide what is acceptable.
Sources
- American Wood Council, DCA 6: Prescriptive Residential Wood Deck Construction Guide: Table 4 footing sizes and the footing depth rule.
- International Code Council, 2021 International Residential Code, R507.3 Footings: at least 12 in below undisturbed ground and frost protection.
- QUIKRETE, Concrete Mix (No. 1101) data sheet: bag yields from 40 to 90 lb.
- Easy Mix, Concrete Supremo 32MPa data sheet: about 10 L (0.01 m³) per 20 kg bag. The 25 and 30 kg yields are scaled from these labels.
Questions
How many bags of concrete for a 12 in tube 4 ft deep?
A 12 in tube 48 in deep holds about 3.14 ft³ (0.089 m³). That is 6 bags of 80 lb at 0.60 ft³ each, 7 bags of 60 lb, or 9 of 20 kg at about 10 L each, before any allowance for spills.
How many 80 lb bags make a cubic yard?
A cubic yard is 27 ft³. At the 0.60 ft³ printed on QUIKRETE's 80 lb bag, that is 45 bags, which weigh 3,600 lb. A cubic metre takes about 100 bags of 20 kg at 10 L each.
How deep do deck footings need to be?
DCA 6 and the 2021 IRC require at least 12 in below undisturbed ground, and below the local frost line where that is deeper. The frost depth comes from your building department; in cold regions it can be several feet.
Should I add extra concrete?
Yes. Holes are rarely dug to the exact size and some concrete is always spilled or left in the mixer. The 10% default covers ordinary jobs; dug holes in loose or rocky soil can take more, and a spare bag is cheaper than a second trip.