Deck Board Calculator
Enter the deck size and your board width to get the board count, the linear footage to order, the joists underneath and the screws to fix it all down — with the gap between boards included, which is what most quick estimates forget.
How to use this calculator
- Decide which way the boards run. Boards normally run parallel to the house and perpendicular to the joists. Enter that dimension as the deck length — the calculator lays rows across the other one.
- Use the actual board width. A "6 inch" deck board is 5.5 in actual. Composite boards are usually 5.5 in as well but check the profile.
- Set the gap for your material. 1/4 in for kiln-dried lumber and most composite. Wet pressure-treated lumber shrinks as it dries, so it is often laid nearly tight and the gap appears on its own.
- Pick the board length that minimises butt joints. If the deck is 16 ft long, 16 ft boards mean every row is one piece — no joints to align over joists. Mixed lengths work but every butt joint must land on a joist.
- Waste at 10% covers cuts and culled boards; go 15% for diagonal decking, which also needs 12 in joist spacing.
The formula
Decking is rows across the width, each a full length of the deck:
rows = ceiling( deck width ÷ (board width + gap) )
linear feet = rows × deck length × (1 + waste%)
boards = ceiling( linear feet ÷ board length )
joists = floor( deck length ÷ spacing ) + 1
screws = rows × joists × 2
| Decking | Actual width | Max joist spacing |
|---|---|---|
| 5/4 x 6 wood | 5.5 in | 16 in |
| 2 x 6 wood | 5.5 in | 24 in (16 in is better) |
| Composite, most brands | 5.5 in | 16 in (12 in diagonal) |
Deck screws run about 75 to the pound at 2-1/2 in. Hidden fastener systems use one clip per board-per-joist crossing instead of two screws.
Worked example
A 16 × 12 ft deck, 5.5 in boards with a 1/4 in gap, 16 ft boards, joists at 16 in, 10% waste:
- Rows: 12 ft = 144 in; 144 ÷ (5.5 + 0.25) = 25.04 → 26 rows
- Decking: 26 × 16 = 416 lf; with 10% = 458 linear ft
- Boards: 458 ÷ 16 = 28.6 → 29 boards of 16 ft
- Joists: 16 ft = 192 in; 192 ÷ 16 + 1 = 13 joists, each 12 ft
- Screws: 26 × 13 × 2 = 676, about 9 lb
Note the boards run the 16 ft direction so every row is a single board — no butt joints anywhere on the surface. Running them the other way would need 35 rows of 12 ft boards and look busier.
What this does not count
| Item | How it is sized |
|---|---|
| Beams and posts | By span and load — structural design |
| Footings | Per post; frost depth applies (see the concrete calculator) |
| Rim joists, hangers, blocking | Per the framing plan |
| Ledger and flashing | Per the attachment detail — the most failure-prone part of a deck |
| Railings and stairs | Linear feet of rail; see the stair calculator for the steps |
Frequently asked questions
How many deck boards do I need for a 16x12 deck?
With standard 5.5 inch boards and a quarter inch gap, a 16 by 12 foot deck takes 26 rows of decking — 416 linear feet, or about 458 with 10 percent waste. That is 29 boards of 16 feet.
What gap should I leave between deck boards?
About a quarter inch for kiln-dried lumber and most composite. Wet pressure-treated boards shrink as they dry, so they are often laid nearly tight and the gap opens on its own within a season.
How far apart should deck joists be?
Sixteen inches on centre for most decking. Diagonal decking and some composite profiles require 12 inches; 24 inches only suits full 2x thick wood boards and is on the bouncy side even then. Check the span rating printed by your decking manufacturer.
How many screws per deck board?
Two per board at every joist crossing. A 16 foot board over joists at 16 inch centres crosses 13 joists, so 26 screws per board. Deck screws run about 75 to the pound at 2-1/2 inches.
Should deck boards run parallel or perpendicular to the house?
Usually parallel to the house, which means perpendicular to the joists — boards must always cross the joists. Choosing the direction that matches your board length also eliminates butt joints, which are the first place a deck surface looks worn.