Proper Estimate

Deck footing calculator

Enter the deck area each post holds up. You get the load on the footing, the smallest footing the building code allows, the hole to dig and the bags of concrete to buy.

Footings

Half the joist span × the post spacing. See how below.

Usually one per post. The table below has typical counts.

Hole diameter

By load picks the next size up from the code minimum.

Below your frost line. Add any form height above ground.

Bag size

Bags to buy

27

80 lb bags

Footing size

  • Load on the footing2,400 lb
  • Smallest footing (IRC)18 in round
  • Hole 18 inBig enough

Concrete

  • Each hole5.3 cu ft, 9 bags
  • All holes15.9 cu ft (0.59 cu yd)

Sizes assume average soil (1,500 lb per sq ft) and a 40 psf deck. Bags are rounded up for each hole.

How big should deck footings be?

Each footing carries the part of the deck its post holds up. Find that area, turn it into a load, and spread the load over enough soil:

  1. Area per post. A beam holds the joists halfway to the next support on each side, and a post holds the beam halfway to the next post. On a 16 × 12 ft deck attached to the house, the beam carries 6 ft of deck; the middle post holds 8 ft of beam, so 6 × 8 = 48 sq ft, and each corner post 6 × 4 = 24 sq ft.
  2. Load. Decks are designed for 40 lb per sq ft of people and furniture plus 10 lb of deck: 50 × 48 = 2,400 lb on the middle footing.
  3. Soil. Unless you know better, use 1,500 lb per sq ft, the lowest value in IRC Table R401.4.1 (clay and silt). 2,400 ÷ 1,500 = 1.6 sq ft of footing, a circle about 17 in across.
  4. Code minimum. IRC Table R507.3.1 gives the same sizes: 16 in round at 40 sq ft and 19 in at 60 sq ft. Round up to a hole you can dig: 18 in for the middle post, 14 in at the corners.
Area per post Load Smallest footing Hole to dig 80 lb bags
20 sq ft 1,000 lb 12 in 12 in 4
30 sq ft 1,500 lb 14 in 14 in 6
40 sq ft 2,000 lb 16 in 16 in 7
50 sq ft 2,500 lb 18 in 18 in 9
60 sq ft 3,000 lb 19 in 20 in 11
80 sq ft 4,000 lb 22 in 24 in 16
100 sq ft 5,000 lb 25 in 30 in 25
120 sq ft 6,000 lb 27 in 30 in 25

Round footings 36 in deep in soil bearing 1,500 lb per sq ft, for a 40 psf live load (IRC Table R507.3.1, interpolated). Better soil, such as sand or gravel, allows smaller footings, and heavy snow needs bigger ones; your building department can tell you which applies.

How many footings does a deck need?

Usually one under each post. The posts sit under the beams, no more than about 8 ft apart, with one at each end of every beam. An attached deck has a beam along its outer edge, a freestanding deck has a beam on each side, and a deck wider than its joists can span gets another beam in the middle, which carries twice as much.

Deck size Attached Freestanding Footing sizes, attached
8 × 8 ft 2 4 2 × 12 in
10 × 10 ft 3 6 2 × 12 in, 1 × 14 in
12 × 12 ft 3 6 2 × 12 in, 1 × 16 in
16 × 12 ft 3 6 2 × 14 in, 1 × 18 in
16 × 16 ft 3 6 2 × 16 in, 1 × 20 in
20 × 12 ft 4 8 2 × 12 in, 2 × 16 in
20 × 16 ft 4 8 2 × 14 in, 2 × 18 in
24 × 16 ft 4 8 2 × 16 in, 2 × 20 in

Length along the house × width out from it, joists 16 in apart, posts at most 8 ft apart. Stair footings or pads are extra.

How the concrete is worked out

A round hole holds π × radius² × depth. An 18 in hole 36 in deep holds 3.1416 × 0.75 ft × 0.75 ft × 3 ft = 5.3 cu ft. An 80 lb bag of premixed concrete makes about 0.6 cu ft, so each hole takes 9 bags.

Bag size Makes about Bags per 12 × 36 in hole Bags per 18 × 36 in hole
40 lb 0.3 cu ft 8 18
50 lb 0.38 cu ft 7 15
60 lb 0.45 cu ft 6 12
80 lb 0.6 cu ft 4 9

A footing doesn't have to be a straight hole. A wide pad at the bottom with a 12 in tube above it bears on the same area of soil with less concrete; many builders use a plastic footing form for that.

Footing depth

Footings must sit on undisturbed soil and reach below the frost line, which runs from under a foot in the South to 4 ft or more in the far North (IRC Section R507.3.2). Your building department will tell you the frost depth and often has a standard footing detail.

Questions

How big should deck footings be?

Big enough for the deck area its post holds up. On average soil, a post carrying 24 sq ft needs a 13 in round footing and one carrying 48 sq ft needs 18 in (IRC Table R507.3.1). That's the corner and middle posts of a 16 × 12 ft deck, so dig 14 in and 18 in holes.

How much weight does a deck footing hold?

Its share of the deck, at 50 lb per sq ft: 40 lb for people and furniture plus 10 lb for the deck itself. The middle post of a 16 × 12 ft deck holds up 48 sq ft, so its footing carries 2,400 lb. A hot tub is far heavier and needs its own design.

How many bags of concrete per deck post?

It depends on the hole. 36 in deep, an 80 lb bag fills: 4 bags for a 12 in hole, 6 for 14 in, 7 for 16 in and 9 for 18 in. Bigger holes take a lot more, because the volume grows with the diameter squared.

How deep should deck footings be?

Below the frost line where you live, on undisturbed soil (IRC Section R507.3.2). Frost depth ranges from under a foot in the South to 4 ft or more in the far North; your building department sets it.

Can I use deck blocks instead of footings?

Only for some low, freestanding decks, where the code allows footings that don't reach the frost line. Decks attached to the house need footings below the frost line so the deck and house don't move differently.

How many footings does a 12 × 12 deck need?

With posts no more than 8 ft apart: 3 for an attached deck (one beam at the outer edge) and 6 for a freestanding deck (a beam at each side).