Deck framing
Deck joist & footing calculator.
Estimate joists at 12, 16, or 24 in spacing plus footing count from the deck size. A 16 x 12 ft deck at 16 in on center needs about 13 joists and 3 footings.
16 x 12, 16 in OC
13 joists
12 ft span each
Footings
3
posts every 8 ft
Spacing
16 in
on center, typical
Joists by spacing
Joist count for a 16 ft deck by spacing
Tighter spacing means more joists and a stiffer deck. Composite and diagonal decking often call for 12 in on center. Confirm the allowable spacing for your joist size and span.
| Spacing | Joists (16 ft deck) | Notes |
|---|---|---|
| 12 in OC | 17 | Stiffest; composite / diagonal |
| 16 in OC | 13 | Most common |
| 24 in OC | 9 | Fewest joists; check spans |
Footings need concrete
Each footing is a concrete pier or pad. Estimate the concrete with the post-hole calculator using your footing diameter and depth.
Formula shown
How the framing estimate works
1. Joists
joists = ceil(length x 12 / spacing) + 1
2. Posts per beam
posts = ceil(length / post spacing) + 1
3. Footings
footings = posts per beam x number of beams
Sources
- Decks.com framing and spans — joist spacing, beam, and footing layout norms
- IRC deck provisions reference — joist spacing and span guidance
Reviewed for estimating accuracy
Last updated: June 26, 2026
FAQ
Deck framing questions
Joist count, joist spacing, footing count, and the limits of a layout estimate.
How many joists do I need for a deck? + -
Divide the deck length in inches by the joist spacing and add one. A 16 ft long deck at 16 in on center needs about 13 joists, each spanning the 12 ft width, plus beams, ledger, and rim joists.
What joist spacing should I use? + -
Sixteen inches on center is the most common. Twelve inches is stiffer and is often used under diagonal decking or composite boards; 24 inches uses fewer joists but allows more bounce and is limited by board and span rules. Always check span tables and local code.
How many footings does a deck need? + -
Footings carry the posts under the beams. For a ledger-attached deck with one outer beam and posts every 8 ft, a 16 ft deck needs about 3 footings. A freestanding deck has a second beam, so it needs roughly double.
Does this size the beams and joists? + -
No. It counts joists and footings for layout and material planning. The actual joist size, beam size, and post spacing must come from span tables and local code for your lumber and load.