Deck Board Calculator
A decking calculator that tells you how many boards to buy by stock length, where the butt joints go, how wide the last row is, and how many screws or clips you need. It also checks your joist spacing against the IRC.
Checks
- Joist spacingPass
16" o.c. ≤ 16" for 1 1/4" wood decking (IRC Table R507.7).
- Last rowPass
Ripped row 3 3/8" wide.
- IRC R507.7: at least two 8d threaded nails or two #8 screws at each joist.
Deck layout
Scroll →
- R — Last row ripped to 3 3/8"
- Board
- Ripped row
- Joist line
Cut list
| 26 rows · 16' 0" · cut from 16 ft |
| Last row: rip to 3 3/8" |
How to measure
Length is the direction the boards run; width is across the boards — out from the house on a typical deck. Measure the finished deck surface. Note the joist spacing on center and the board you plan to use.
How it works
Rows and the last row
Rows = width ÷ (board + gap). The leftover is the last row's rip width. If it's thinner than half a board, rip the first and last rows equally ("Balance").
Board lengths and joints
When one board covers the deck length, it's cut from the shortest stock length that fits. Longer decks need a butt joint, and every joint must land on a joist.
On short decks several rows are cut from one board — the calculator picks the stock length with the least total footage.
Joint stagger
Joints in neighboring rows are kept at least two joists apart, in a pattern that repeats every 3 rows, choosing the stock lengths with the least waste.
Screws and clips
Two fasteners per board at each support — the IRC R507.7 minimum for wood is two 8d threaded nails or two #8 screws. Hidden clips: one per board edge at each joist; for composite, follow the manufacturer.
Picture frame
The border boards shrink the field. The field board ends sit on blocking, which needs its own fasteners.
Joist spacing for deck boards (IRC R507.7)
| Decking | Perpendicular | Diagonal |
|---|---|---|
| 1 1/4" wood | 16" | 12" |
| 2" wood | 24" | 16" |
| Plastic composite | Per label (ASTM D7032) | |
Diagonal boards: max 45° from perpendicular. Most composite boards are rated 16" o.c. Source: IRC Section R507 (ICC).
Worked example
A 16 ft × 12 ft deck with 5/4x6 pressure-treated boards (5 1/2"), a 1/8" gap and joists 16" o.c. → 13 joists and 26 rows (25 full + the last row ripped to 3 3/8"). Buy 26 boards at 16 ft + 2 spares = 28 — 416 linear ft and 676 screws. No joints: each row is one 16 ft board.
When the deck is longer than your boards
A 24 ft × 10 ft composite deck, 3/16" gap, 16" o.c., stock 12/16/20 ft → 19 joists, 22 rows. The last row would be only 9/16", so rip the first and last rows to 3 1/16". Joints fall at 12' 0", 9' 4" and 14' 8", repeating every 3 rows. Buy 32 × 12 ft + 15 × 16 ft, 418 hidden clips, with about 56 ft of offcuts.
FAQ
How many deck boards do I need for a 12x16 deck?
With 5/4x6 boards (5 1/2" wide), a 1/8" gap and joists 16" on center, a 16 ft × 12 ft deck takes 26 rows: 25 full boards plus a last row ripped to 3 3/8". Each row is one 16 ft board, so you need 26 boards, or 28 with 2 spares — 416 linear feet and about 676 screws.
What gap should I leave between deck boards?
Wet pressure-treated wood: about 1/8" — it shrinks and the gap opens to about 1/4" as it dries. Composite and PVC: follow the manufacturer, often 3/16", with end gaps set by the temperature at install.
What joist spacing do deck boards need?
IRC Table R507.7: 1 1/4" wood decking — 16" on center perpendicular, 12" diagonal. 2" wood decking — 24" perpendicular, 16" diagonal. Plastic composite — per the label (ASTM D7032); most composite boards are rated 16" on center.
Where should deck board joints be?
Always centered on a joist, with a doubled joist or blocking so both board ends have full bearing and room for fasteners. Stagger the joints so neighboring rows don't share a joist.
How many screws per deck board?
Two at every joist the board crosses, including the joists at each end. A 16 ft board over joists 16" on center crosses 13 joists, so it takes 2 × 13 = 26 screws.
What if the last deck board is too narrow?
If the last row comes out thinner than half a board, it's hard to fasten and looks off. Rip the first and last rows equally instead — the calculator's "Balance first and last row" option splits the leftover width between both edges.
What is a picture frame deck border?
Border boards run around the edge of the deck, framing the field boards, with mitered corners. The field boards end against the border, so you need blocking under those ends. A frame adds cuts and some waste.