LocalServiceHQ
Cost Guides
Get quotes
Tools
Directory
State Guides
For Pros

Deck boards & joists calculator

Board rows, 12 ft boards, joists, posts, fasteners and lumber cost for a deck frame.

By the LocalServiceHQ editorial team
Updated Oct 29, 2026
Formulas and sources listed below
Prices checked Oct 2026
Your job
Decking pattern
Rim and blocking
Your estimate
12 ft boards
—
decking boards, rounded up
Lumber cost
—
decking, joists, posts and fasteners
Typical range for this job: —
How it adds upTypical
Board rowswidth × 12 ÷ board face plus gap
—
Boardsrows × length ÷ 12, rounded up
—
Joistsceil(length × 12 ÷ spacing) + 1
—
Postspost rows every 8 ft
—
Fastenersscrews, hangers and joist tape
—
Lumberboards, joists and posts at retail
—
US lumber prices for October 2026: 5/4 × 6 decking about $1.30–$2.20 per linear ft, 2 × 8 joists $1.40–$2.10 per linear ft and 4 × 4 posts $12–$22 each. Fasteners and hangers add about $1 per sq ft.
Board rows assume a small gap between boards. Add 10 percent waste for cuts and stairs.
Photo via Pexels

How the math works

Board rows
Layout
width in inches ÷ (board face + gap)
A 6 in board has a 5.5 in face; allow a 1/4 in gap.
12 ft boards
Purchase
rows × length ÷ 12
Longer boards mean fewer butt joints and a better deck.
Joists
Frame
ceil(length × 12 ÷ spacing) + 1
Spacing depends on the decking and load.
Posts
Frame
max 8 ft between post rows
Every deck needs footings below the frost line.

Common starting points

Common US deck sizes with 5/4 decking at 16 in joist spacing.

Tap a preset in the calculator above to load a typical job, then adjust the numbers to match your site.
Starting points: 10 × 10 ft deck · 12 × 12 ft deck · 16 × 12 ft deck · 16 × 12 ft, 12 in spacing · 20 × 16 ft deck

Rather hire it out?

See installed prices and permit rules for deck boards & joists calculator.

Service costs

Embed this calculator

Free for blogs and contractor sites. Keep the credit link.

Embed this calculator on your site with the credit link intact.

How to use the deck boards & joists calculator

Measure the deck, pick the board and joist spacing, then add waste before ordering.

1
Measure the deck
Length and width of the decking surface, not the house wall.
2
Pick the board
5/4 × 6 is standard; 2 × 6 is stronger and more expensive.
3
Set the joist spacing
12 in for diagonal or composite, 16 in for standard wood.
4
Plan posts and footings
Posts every 8 ft maximum, with footings below the frost line.

Mistakes that cost homeowners money

!
Setting joists 24 in apart under wood decking that needs 16.
!
Butting too many short boards instead of ordering longer lengths.
!
Skipping blocking between joists, which lets the deck flex.
!
Footings that are too shallow for the frost line.

Deck boards & joists calculator FAQ

How many deck boards do I need?

Divide the deck width in inches by the board face plus gap to get rows, then multiply by length. A 12 × 12 ft deck with 5.5 in boards needs about 17 boards of 12 ft.

At 16 in on centre, divide the deck length in inches by 16 and add one. A 16 ft deck needs 13 joists at 16 in spacing, or 17 at 12 in.

Sixteen inches on centre for standard wood decking, 12 in for diagonal layouts and most composite products. Follow the decking manufacturer's table.

Eight feet maximum between post rows for most residential decks, with footings sized to the load and below the frost line.

Pressure-treated decking and framing run roughly $12 to $25 per sq ft in materials, before labour. Composite decking roughly doubles the material cost.

Framing and decking are realistic DIY work, but footings, ledger flashing and guardrails need to meet code. Many cities require plans and inspections.

Using the calculator defaults, 10 × 10 ft deck gives 12 ft boards: 18 and Lumber cost: $825, with a typical range $624 to $1,037. Change the waste allowance, material or thickness to match your site.

Using the calculator defaults, 20 × 16 ft deck gives 12 ft boards: 57 and Lumber cost: $2,077, with a typical range $1,599 to $2,622. Change the waste allowance, material or thickness to match your site.