Concrete calculator for deck footings

What it counts

A deck footing is a volume of concrete in the shape of the hole: a cylinder for a tube or an augered pier, a box for a pad, a box plus a cylinder for a pad with a pier on it, and a cone with its top cut off for a flared bell. The tool works each piece out in cubic inches, converts to cubic feet (1,728 cubic inches to the foot), multiplies by the number of footings, adds your waste allowance and divides by what one bag yields.

Unlike a fence post, a deck post normally does not stand in the concrete. It sits on a post base fastened to the top of the pier, so nothing is subtracted for the post. If your plan does embed the post, that is the fence post sum instead.

How to use it

1. 01Count the footings. One under every post. The beam span decides how many posts there are, and the beam calculator works that out.
2. 02Pick the shape. A plain tube is the common case. A pad with a pier on it, or a flared bell, puts more bearing area at the bottom without a fat tube all the way up.
3. 03Enter the diameter or pad size from the size calculator. Do not guess this one; it comes from the load and your soil.
4. 04Enter the depth below grade and any tube left above it. Depth is to the bottom of the footing. Tube left above grade is still concrete, so it counts.
5. 05Set waste and the bag you are buying. Ten percent covers a ragged augered hole and a spilled barrow. A hand-dug hole that belled out on its own deserves more.

Worked example: one deck, four ways

Six footings, 42 inches below grade, 10 percent waste. Same deck, same depth, and the shape changes the bag count a lot:

Diameter is what costs you. A cylinder's volume goes with the square of its diameter, so going from a 10-inch to a 12-inch tube puts about 44 percent more concrete in every foot of depth. When the size calculator asks for a big bearing area, a pad or a bell at the bottom usually gets there with less concrete than a fat tube all the way up.

Bag yields, and which one to buy

QUIKRETE's data sheet for its standard Concrete Mix gives 0.6 cu ft per 80 lb bag, 0.45 cu ft per 60 lb bag, 0.375 cu ft per 50 lb bag, 0.3 cu ft per 40 lb bag. Sakrete lists the same yields for its 40, 60 and 80 lb High Strength mix. Those are the numbers the tool uses. Fast-setting and specialty mixes are different products, so read the yield off the bag you are actually buying.

For the same volume, 80 lb bags mean fewer bags to open and mix. 40 and 60 lb bags are easier to carry down a slope or through a side gate. The "lb to haul" line in the result is the total weight either way, and it adds up fast.

Before the first bag is opened

The holes get inspected open. Under the 2021 IRC (R109.1.1) the foundation inspection happens after the piers are dug and any forms and reinforcing are in place, before concrete goes in. Depth is at least 12 inches below undisturbed ground, and deeper where your frost line is deeper — the footing depth guide covers how to find yours. The full build sequence is in how to build deck footings.

In Table R507.3.1, the code's minimum footing sizes, the thickness column never drops below 6 inches, and a pad you size yourself should not either. Simpson Strong-Tie also notes that a cast-in-place post base needs enough concrete around its embedded part to reach its published loads. That can call for a bigger footing than the code minimum.

Sources

• QUIKRETE Concrete Mix No. 1101 data sheet — bag sizes, yields, curing read 2026-10-18
• Sakrete High Strength Concrete Mix — bag sizes and yields read 2026-10-18
• 2021 IRC § R507.3 and Table R507.3.1, Deck Footings (UpCodes) read 2026-10-18
• 2021 IRC § R403.1.4 Minimum Depth and § R403.1.4.1 Frost Protection (UpCodes) read 2026-10-18
• 2021 IRC § R109.1.1, Foundation Inspection (UpCodes) read 2026-10-18
• Simpson Strong-Tie, Deck Connection and Fastening Guide (F-DECKCODE25), Footings, p. 26 read 2026-10-18