Deck Joist Calculator — How Many Joists Do I Need?
Enter your deck size and joist spacing to get the number of joists for the frame. No sign-up, no data stored.
Joists needed
How deck joist count is estimated
- Base joists = ceil(width x 12 / spacing) + 1 (one at each end).
- Doubled joists add one extra member per selected spot.
- Total pieces = base + doubled.
- Size hint comes from the span you entered: 2x6 to 8 ft, 2x8 to 11 ft, 2x10 to 14 ft, 2x12 to 17 ft at 16-in spacing.
Joist spacing is measured on center - the distance between the centers of two joists, not the clear gap. Rim joists and blocking are separate; the calculator covers the parallel members only. If your span exceeds the size hint, add a beam under the joists rather than upsizing the board.
Spacing → joist count reference
Joists across a width, not including doubled members.
| Run width | 12-in spacing | 16-in spacing | 24-in spacing |
|---|---|---|---|
| 8 ft | 9 | 7 | 5 |
| 10 ft | 11 | 9 | 6 |
| 12 ft | 13 | 10 | 7 |
| 16 ft | 17 | 13 | 9 |
| 20 ft | 21 | 16 | 11 |
Last updated: October 2026 · Spans assume Douglas fir or treated SYP framing at 40-psf live load. Confirm against your local code before ordering.
Deck joist FAQ
Joists run perpendicular to the house and to the beam that supports them, so the decking boards run parallel to the house for the classic look. On a square deck, run them the short way to save material.
Yes - rows of blocking (or bridging) every 4-6 ft keep joists from twisting and stiffen the frame. Over 12-ft spans, blocking becomes structural; don't skip it on long joists.
Only for short spans (under 8 ft at 16-in spacing) and light decks. Most deck codes want 2x8 or larger for ground-level and elevated decks. Check your local span tables before planning.
One hanger per joist at the ledger and beam. Nail each with the hanger manufacturer's pattern - usually 2 rows of nails into the ledger plus into the joist. Never use plain nails instead of approved hanger nails.
Angle-cut joists meet the rim at 45°, which shortens their effective span and needs careful notching. Count the same number as a straight run and add 2-3 extra for the angled cuts and doubled corner members.
Related calculators & guides
How Many Deck Joists Do I Need?
Spacing, span tables and blocking rules for deck framing.
Read the guide →