How to use the deck footing calculator calculator
- 1
Take the footing size from your drawings
Diameter for a round footing, or length and width for a rectangular pad. These come from the deck design, because they depend on the loads each post carries and the soil beneath it.
- 2
Enter the depth of concrete
Enter the depth that will actually be concrete. If your design places gravel beneath the footing, that is not part of the pour.
- 3
Set how many footings you are pouring
Enter the count of identical footings. If the deck has footings of two different sizes, run the calculation twice and add the results.
- 4
Add an allowance
A hole drilled or dug in soil is rarely a perfect cylinder, and the sides often slough. Most people order somewhat above the geometric figure, so choose a percentage of your own.
- 5
Add a bag yield or ready-mix price
Enter the yield printed on the bag for a bag count. For a larger deck, enter a price per cubic yard to see whether delivery makes more sense than bags.
Formula
Round footing cubic feet = pi x (diameter(ft) / 2)² x depth(ft) x count
A rectangular pad uses length times width times depth instead. Divide the total by 27 for cubic yards.
Worked example
Nine round footings, each 12 inches in diameter and 36 inches deep
- 1Convert the diameter: 12 in = 1 ft, so the radius is 0.5 ft.
- 2Find the area: pi x 0.5² = 0.785 square feet.
- 3Volume per footing: 0.785 x 3 ft = 2.36 cubic feet.
- 4Total for nine: 2.36 x 9 = 21.21 cubic feet.
- 5Convert: 21.21 / 27 = 0.79 cubic yards.
The nine footings need about 21.21 cubic feet, or 0.79 cubic yards. Those dimensions are the ones entered for this example, not recommended sizes.
Worked examples
Bags for the same job
The 21.21 cubic feet above at a 0.6 cubic foot bag yield
- 1Divide by the yield entered: 21.21 / 0.6 = 35.35 bags.
- 2Round up: 36 bags.
- 3That is 4 bags per footing across nine footings.
Thirty-six bags at that example yield. Read the yield off the bag you actually buy, since it differs by product.
Stepping up one footing size
The same nine footings at 16 inches in diameter instead of 12
- 1Radius: 16 in / 2 = 8 in, which is 0.667 ft.
- 2Area: pi x 0.667² = 1.396 square feet.
- 3Volume per footing: 1.396 x 3 = 4.19 cubic feet.
- 4Total for nine: 37.70 cubic feet, up from 21.21.
Going from 12 inch to 16 inch footings raises the concrete by 78 percent, because volume scales with the square of the diameter. A modest change on paper is a large change in material.
Why diameter matters more than depth
A round footing is a cylinder, so its volume depends on the square of the radius but only linearly on the depth. That asymmetry surprises people who change a footing size late in planning.
Going from a 12 inch footing to a 16 inch one at the same depth raises the concrete by 78 percent. Adding a foot of depth to a 12 inch footing raises it by a third. Both changes look similar on a drawing; they are not similar on the order.
It also means diameter is the number worth measuring carefully. A hole augered slightly oversize, or reamed out at the top, holds noticeably more than the nominal figure suggests, which is part of why an allowance is normal on footings poured against soil.
Round holes, form tubes, and rectangular pads
The three common cases behave differently on site even though two of them share a formula. A hole augered directly into soil is roughly cylindrical but not precisely, and its walls can slough, so the concrete placed usually exceeds the calculated volume.
A cardboard form tube gives a known, consistent diameter and a clean result, which makes the arithmetic reliable. If you are filling tubes rather than open holes, the form tube calculator is set up for that case and the numbers should match closely.
A rectangular pad footing uses length times width times depth. Some deck designs use a pad beneath a pier, in which case the pad and the pier are two separate volumes that both need calculating and adding.
Bags or ready-mix for a deck
Deck footings sit right at the crossover point. The worked example above comes to under a cubic yard across nine footings, which is a comfortable bag job for most people. A larger deck with bigger footings can easily reach several cubic yards, where mixing by hand becomes a serious amount of work.
The calculator prices both routes from figures you enter, because that is the only honest way to compare: bag prices and ready-mix rates both vary, and ready-mix carries a delivery charge that weighs heavily on a small order.
One practical point the arithmetic does not show: footings are separate pours rather than one continuous placement, so a delivered load has to be distributed around the site before it starts setting. Access matters more here than on a slab.
Common mistakes
Where deck footing estimates go wrong:
- Entering the radius in the diameter field, which changes the answer by a factor of four.
- Using the excavated hole depth rather than the depth of concrete.
- Treating footings of two different sizes as one uniform set.
- Forgetting a pad beneath a pier, which is an additional volume.
- Ordering the exact geometric figure for holes augered in soil, where the walls slough.
- Assuming a bag yield instead of reading it from the bag.
- Reading a volume result as confirmation that the footing size is adequate.
Sources and methodology
Every figure on this page comes from geometry, not from a product or materials database. Areas use the standard area formulas, volumes are area multiplied by depth, cylinders use pi times the radius squared times the height, and unit conversions use exact factors: 12 inches to a foot, 27 cubic feet to a cubic yard, and 9 square feet to a square yard.
YardCalc publishes no material density, coverage rate, product dimension, soil swell factor, compaction factor, or price. Those vary by product, by supplier, by soil, and by region, and a figure that is right in one case is wrong in another, so the calculator asks you for the values that apply to your job. Weight estimates appear only when you supply a density.
Nothing here is engineering, safety, or code guidance. Results are planning estimates. Read how every value on the site is derived and checked in the YardCalc calculation methodology.
Round footing volume by diameter and depth
Concrete in one footing at each size, with the number of footings one cubic yard would fill. These sizes demonstrate the arithmetic and are not footing design recommendations.
| Diameter x depth | Cubic feet each | Cubic yards each | Number that fit in 1 cubic yard |
|---|---|---|---|
| 8 in x 24 in | 0.698 | 0.0259 | 38 |
| 10 in x 30 in | 1.36 | 0.0505 | 19 |
| 12 in x 36 in | 2.36 | 0.0873 | 11 |
| 12 in x 48 in | 3.14 | 0.116 | 8 |
| 16 in x 36 in | 4.19 | 0.155 | 6 |
| 16 in x 48 in | 5.59 | 0.207 | 4 |
| 20 in x 48 in | 8.73 | 0.323 | 3 |
| 24 in x 48 in | 12.57 | 0.465 | 2 |
Frequently asked questions
- How much concrete do I need for deck footings?
- Find the volume of one footing, then multiply by the count. Nine round footings 12 inches across and 36 inches deep need about 21.21 cubic feet, or 0.79 cubic yards.
- How many bags of concrete per deck footing?
- Divide the footing volume by the yield printed on your bag. A 12 inch by 36 inch footing is 2.36 cubic feet, which is about 4 bags at a 0.6 cubic foot yield or 6 bags at a 0.45 yield.
- What size should my deck footings be?
- This calculator does not size footings. Diameter, depth, and embedment depend on the loads each post carries, the soil, the frost depth, and local code, so take them from stamped drawings, a qualified designer, or your building department.
- How deep do deck footings need to be?
- Not something this page determines. Depth is usually driven by frost depth and bearing requirements, both of which are set locally. Decks are commonly permitted and inspected, so confirm with your building department.
- Why does a slightly larger footing need so much more concrete?
- Because volume grows with the square of the diameter. Going from a 12 inch to a 16 inch footing at the same depth raises the concrete by 78 percent, while adding a foot of depth to a 12 inch footing raises it by only a third.
- Should I use bags or order ready-mix?
- It depends on the total and your site access. Under a cubic yard is usually comfortable with bags; several cubic yards is a lot of mixing. Enter both prices above to compare, remembering that footings are separate pours rather than one placement.
Footing volume uses the exact cylinder formula for round footings and length times width times depth for rectangular pads, from the dimensions you enter. No footing size, depth, embedment, bag yield, or price is assumed, and nothing here constitutes footing design. Quantities come from exact geometry using the dimensions you enter. Confirm product dimensions, installation requirements, local codes, site conditions, safety requirements, and final order quantities with the appropriate manufacturer, supplier, contractor, or professional.
Results are planning estimates. Last reviewed 2026-07-28.