Wondering how many joists your deck needs? Enter the deck size and the spacing. You get the joist count, the lumber to buy and the exact spacing you end up with.
Your shopping list
13Joists
15Joist lumber to buy
16 inSpacing you get
- Total joist length156 ft
Joists run across the deck width. Check a span table for the lumber size you plan to use. Extra of 10% is included.
How to use the Deck Joist Calculator
- Enter the deck length and width. Joists span the width.
- Pick the joist spacing and the lumber length you can buy.
- Choose straight or diagonal boards. Diagonal boards need closer joists.
The math behind it
Joists = deck length x 12 / spacing + 1, rounded up
Diagonal spacing = spacing x 0.75
Total length = joists x deck width
A worked example
What we typed in
- Deck length: 20 ft
- Deck width: 14 ft
- Joist spacing: 12 in apart
- Joist lumber length: 12 ft
- Board layout: Straight boards
- Extra for mistakes and cuts: 10%
What we got
- Joists: 21
- Joist lumber to buy: 28
- Spacing you get: 12 in
- Total joist length: 294 ft
Tips from the job site
- The spacing shown is the real spacing after the joist count is rounded up.
- Joist size and span depend on your lumber and local code, so check a span table.
Common questions
How far apart should deck joists be?
16 in on center is standard for most wood decking. Use 12 in for diagonal boards.
How many joists for a 12 x 16 deck?
With joists 16 in apart along the 16 ft side you need 13 joists, each 12 ft long.
See all Deck Calculators.