Deck Footing Spacing Calculator
Footing count, beam rows, posts per row, and even spacing for a deck plan.
What it does
Deck footings are placed under posts, and posts are placed under beams. The layout starts with the beam lines, not with the deck boards. Once the beam lines are set, the post spacing along each beam decides how many footings the excavator will mark.
The number of beam rows comes from the deck width and the maximum joist span. Joists run from the ledger or from one beam to the next. Each bay must be no wider than the joist span allowed for the lumber, so the width is divided by that span and rounded up. A ledger-attached deck counts only the beams away from the house. A freestanding deck needs one extra beam row because both edges need support.
Along each beam, the post spacing works the same way as fence posts. Divide the length by the maximum spacing, round up to get the number of spaces, then add one post for the far end. The exact spacing is the length divided by the number of spaces. That is why a 20 ft beam with an 8 ft limit gets four posts at 6.67 ft, not three posts at 8 ft. Three posts would create two 10 ft spaces, which breaks the limit.
The default plan is a 20 ft by 16 ft ledger deck with a 10 ft joist span and an 8 ft post limit. It returns 8 footings in 2 beam rows, 4 posts per row, and 6.67 ft on center. The metric defaults describe the same plan in meters and return the same counts.
This is a layout count, not a structural design. Footing diameter, thickness, depth, and reinforcement come from the prescriptive tables or from an engineer. Frost depth, soil bearing capacity, deck height, guard loads, and the ledger connection all need separate checks before anyone digs.
How to use
- Choose imperial or metric. The counts should not change when the same plan is entered in the other system.
- Select whether the deck is bolted to a house ledger or stands on its own.
- Enter the deck length along the beam and the deck width across the joists.
- Enter the maximum joist span from the lumber table or the local deck guide.
- Enter the maximum post spacing allowed by the beam size and the local code.
- Read the footing count, beam rows, posts per row, and exact spacing. Mark the lines, then call 811 before digging.
Formula
- length
- Deck dimension parallel to the house and along the beam.
- width
- Deck dimension from the house to the outer edge, across the joists.
- joist span
- Longest allowed unsupported joist run between supports.
- post spacing
- Largest allowed center-to-center distance between posts along a beam.
- beam rows
- Number of beam lines that need posts, not counting the house ledger.
- footings
- Total posts, with one concrete footing under each post.
The ceiling function is what keeps every bay at or below the limit. If the division is exact, the result stays exact; a 24 ft length with an 8 ft limit gives three spaces and four posts. The extra post at the end is easy to forget. The exact spacing is then spread evenly, so it is usually a little less than the maximum. This page assumes a rectangular deck with beams running parallel to the house and posts spaced evenly along each beam. It does not add stair, landing, or hot tub footings, and it does not choose the beam or joist size.
Worked example
Example: 20 ft by 16 ft ledger deck
A ledger-attached deck is 20 ft long and 16 ft wide. The joist span limit is 10 ft and the post spacing limit is 8 ft.
- Beam rows = ceil(16 / 10) = 2. The house ledger is not counted as a beam row.
- Posts per row = ceil(20 / 8) + 1 = 3 + 1 = 4.
- Footings = 2 rows x 4 posts = 8.
- Even spacing = 20 / (4 - 1) = 6.67 ft on center.
- Check the spacing: 6.67 ft is below the 8 ft limit, so the layout works.
- The metric defaults, 6.1 m by 4.88 m with a 3.05 m joist span and 2.44 m post limit, give the same 8 footings and 2.03 m spacing.
Reference table
| Deck length | Posts per row | Footings | Even spacing |
|---|---|---|---|
| 12 ft | 3 | 6 | 6.00 ft |
| 16 ft | 3 | 6 | 8.00 ft |
| 20 ft | 4 | 8 | 6.67 ft |
| 24 ft | 4 | 8 | 8.00 ft |
| 28 ft | 5 | 10 | 7.00 ft |
| 32 ft | 5 | 10 | 8.00 ft |
Limits
- Counts only. It does not size footings, beams, joists, posts, or connectors.
- Assumes a rectangular deck with evenly spaced posts and beams parallel to the house.
- Does not include stair, landing, ramp, or hot tub footings.
- Frost depth, soil bearing, slope, drainage, and the ledger attachment still need a local check.
Common mistakes
- Dividing the deck area by a footing spacing instead of working along the beam lines.
- Forgetting the end post, which turns three spaces into only three posts.
- Rounding the exact spacing up instead of down, which pushes the layout past the limit.
- Using the joist span as the post spacing, or the post spacing as the joist span.
- Digging before calling 811 or before checking the local frost depth and footing table.
Try it yourself
Quick checks. Answers sit tucked under each question.
A 24 ft by 12 ft ledger deck uses an 8 ft joist span and a 6 ft post limit. How many footings?
Beam rows = ceil(12 / 8) = 2. Posts per row = ceil(24 / 6) + 1 = 5. Footings = 10, spaced 6 ft on center.
A 10 ft by 10 ft freestanding deck uses a 10 ft joist span and a 5 ft post limit. What is the layout?
Beam rows = ceil(10 / 10) + 1 = 2. Posts per row = ceil(10 / 5) + 1 = 3. Footings = 6 at 5 ft on center.
A 30 ft beam has a maximum post spacing of 8 ft. What is the exact spacing?
Posts = ceil(30 / 8) + 1 = 5. Spaces = 4. Spacing = 30 / 4 = 7.5 ft on center.
Questions
How many footings does a 20 by 16 deck need?
With a 10 ft joist span and an 8 ft post limit, the layout has 2 beam rows and 4 posts per row, for 8 footings. A different span or spacing changes the count.
Does the house ledger need footings?
No. The ledger is fastened to the house foundation or framing. The beam rows counted here are the lines away from the house.
Why does 20 divided by 8 give four posts?
20 divided by 8 is 2.5, which rounds up to 3 spaces. Three spaces need 4 posts, one at each end and two in between. The even spacing is 6.67 ft.
Should post spacing be rounded up or down?
Down. The exact spacing after adding the end post is the number to use. Rounding up can exceed the maximum allowed spacing.
Does this include stair footings?
No. Stairs, landings, and ramps need their own posts and footings, and the stair stringer layout is separate.
What size footing should I pour?
Use the footing table in the local deck guide or the engineer's plan. The prescriptive tables depend on post height, beam span, soil bearing, and frost depth.
Sources
- American Wood Council: Prescriptive Residential Wood Deck Construction Guide, DCA 6
- Roanoke County, Virginia: Typical Deck Details