Deck Board Calculator
Enter your deck size and get the number of deck boards to buy, the linear feet, how many joists you need and roughly how many screws — with a check against the IRC decking spans.
How to calculate how many deck boards you need
Work out the number of rows first, then how many boards go in each row. Everything else follows from those two numbers.
The row count comes from the deck width and the pitch of the decking — the width of one board plus the gap. A 5½-inch board with a ⅛-inch gap takes up 5⅝ inches of deck width, so a 12-foot-wide deck takes 144 ÷ 5.625 = 25.6 rows, which rounds up to 26. You always round up: a part-row at the edge still has to be cut from a whole board.
The boards per row come from the length of the deck divided by the length of board you're buying. A 16-foot deck built from 12-foot boards is two boards per row, with a butt joint somewhere over a joist. This is the step people skip, and it is why the honest number is always higher than the quick area estimate: a 12 × 16 foot deck has 192 square feet, which at 5½ inches wide is about 419 linear feet of decking — but you cannot buy 419 linear feet out of 12-foot boards without a lot of unusable offcuts. Counting row by row gives you what you actually have to put in the cart.
Add a waste allowance on top. Ten percent covers the offcuts from squaring the ends and cutting around posts. Diagonal layouts need more, because every board ends in a triangular offcut that usually cannot be used anywhere else — budget 15% for a 45° layout.
Laying out the butt joints
If a row needs more than one board, the joint between them must land on a joist — never in mid-air between two joists. Stagger the joints from one row to the next so they don't all line up in a visible seam, and keep them at least two joist bays apart. On a 16-foot run of 12-foot boards, plan on alternating the joint between the middle and third joists from each end.
The gap between deck boards
The ⅛-inch gap is one of those numbers everybody repeats, so it's worth being clear about where it comes from: it is not in the code. Section R507.7 of the International Residential Code covers decking, and it sets the maximum joist spacing and the fastening — but it does not publish a gap dimension at all. It defers to the decking manufacturer's installation instructions instead.
The ⅛-inch figure is trade practice, and it has a physical reason behind it. A 5½-inch-wide board moves roughly 1/16 to ⅛ inch across its width as moisture changes through the year. If the boards are tight together on a wet day, they will push against each other as they dry and cup, and the deck will hold water. Carpenters used to carry a 16d nail — about ⅛ inch in diameter — and use it as a spacer.
| Decking | Typical gap | Why |
|---|---|---|
| Pressure-treated, still wet | Butt them tight, or about 1/16 in | They shrink as they dry out to roughly ⅛ in on their own |
| KDAT (kiln-dried after treatment) | ⅛ in | Already at its dry width, so it will not close the gap for you |
| Cedar, redwood, ipe | ⅛ in to 3/16 in | Natural wood movement; ipe moves less but wants drainage |
| Composite | 3/16 in side, ¼ in end | Manufacturer's spec — expands more than wood, and wider in cold weather |
Should the boards run parallel or perpendicular to the house?
Boards almost always run parallel to the house, which means they run perpendicular to the joists and across the deck's width. This is ordinary framing practice, and it also looks right from the house. Running them the other way, parallel to the joists, usually means adding a second layer of blocking and is a lot of extra work for a look most people don't want.
What people do choose is straight versus diagonal. A 45° layout costs more material and a lot more cutting, but it makes a plain rectangular deck look bigger and hides the fact that the house wall isn't square. On a deck under about 12 feet wide, a diagonal layout can look busy; on a large deck it looks deliberate. Either way, remember that diagonal decking changes the joist spacing you're allowed to use — see the table below.
Code requirements
These are the decking limits from the International Residential Code (IRC), Chapter 5, Section R507.7. Decks are covered separately from the rest of the house in the IRC, and R507.7 is the part that governs the boards you walk on. Your local jurisdiction may amend these, so confirm with your building department before you order material.
| Item | Requirement |
|---|---|
| Joist spacing — 5/4 decking, perpendicular | 16 in on center max |
| Joist spacing — 5/4 decking, diagonal | 12 in on center max |
| Joist spacing — 2× decking, perpendicular | 24 in on center max |
| Joist spacing — 2× decking, diagonal | 24 in on center max (2021 IRC — was 16 in the 2018 edition) |
| Fastening | Not less than two 8d threaded nails, or two No. 8 wood screws, at every supporting member |
| Gap between boards | Not specified — per manufacturer's instructions |
| Composite decking | Per manufacturer's instructions (R507.7 refers out) |
Notice how much the diagonal row matters — and how much it depends on what you're laying. Switch from straight to 45° on 5/4 decking and your allowed spacing drops from 16 inches to 12, which on a 16-foot deck means going from 13 joists to 17. Switch on 2-inch decking and nothing changes: the 2021 edition of the IRC raised the diagonal allowance for 2-inch boards from 16 to 24, so it now matches the straight figure. Either way it's a framing decision, so it has to be made before you build the frame, not after.
One caveat the table above hides: the 2021 IRC also split these figures into multi-span and single-span columns. The numbers quoted here are the multi-span ones — deck boards resting on three or more joists, which is what you get on any normal deck. If you're laying short boards that land on only two joists, the allowance is lower and you need the full table. It matters most when you're using up offcuts.
Worked example: a 12 × 16 ft deck
Take a deck 16 feet long and 12 feet wide, using 5/4 × 6 decking (5½ inches wide), a ⅛-inch gap, 12-foot boards, joists at 16 inches on center, and 10% waste. The boards run straight.
The pitch is 5.5 + 0.125 = 5.625 inches. The deck is 144 inches wide, so it takes 144 ÷ 5.625 = 25.6 rows, which rounds to 26 rows. Each row is 16 feet and the boards are 12 feet, so that's 2 boards per row with one butt joint each.
26 rows × 2 boards = 52 boards, or 624 linear feet. Add 10% and you're at 686 linear feet, which is 58 boards. That's the number to buy.
The joists run the other way, across the 16-foot length, so 192 inches ÷ 16 = 12 bays, plus one for the end = 13 joists. Screws work out at two per board at every joist: 26 rows × 13 joists × 2 = 676 screws, or about 3½ per square foot, which is the rule of thumb contractors use.
If you switched this deck to a 45° layout, the code allowance for 5/4 decking drops to 12 inches on center — so you'd need 17 joists, not 13. And you'd want to budget 15% waste instead of 10%, which pushes the board count to about 41 boards. The decking itself costs more in cut-offs; the framing costs more in lumber.
Frequently asked questions
How many deck boards do I need for a 12 × 16 deck?
About 58 boards if you're using 5/4 × 6 decking, 12-foot lengths, a ⅛-inch gap and 10% waste. That's 26 rows across the 12-foot width, two 12-foot boards per row (52 boards), plus waste. Buy 58 and you'll have a few left over; buy 52 and you'll be short once you've cut around the posts.
What is the correct gap between deck boards?
⅛ inch is the standard figure for wood decking, and 3/16 inch for composite, but the IRC does not actually specify a gap — R507.7 defers to the decking manufacturer's instructions. The gap exists because boards move as moisture changes: a 5½-inch board grows and shrinks about 1/16 to ⅛ inch across its width over a year. If you're installing pressure-treated lumber that's still wet from the yard, butt it tight and let it shrink open; if it's kiln-dried after treatment, leave the full ⅛ inch from day one.
How many screws do I need per deck board?
Two fasteners at every joist the board crosses, which is what the IRC requires in R507.7 — two 8d threaded nails or two No. 8 wood screws. In practice that works out to roughly 3½ screws per square foot of deck, so a 192-square-foot deck takes about 670. Buy them by the box and round up; running out mid-row is worse than having a box left over.
Should deck boards run parallel or perpendicular to the house?
Parallel to the house, which puts them perpendicular to the joists. That's standard practice and it's what the joist spacing tables assume. Running boards parallel to the joists needs extra blocking and isn't worth it. The choice that actually matters is straight versus 45° diagonal — diagonal looks better on a large deck but costs more material and drops your allowed joist spacing by one step.
How far apart should deck joists be?
It depends on the decking, not on the joist itself. Under IRC Table R507.7, 5/4 decking needs joists at 16 inches on center or less when the boards run straight, and 12 inches when they run diagonally. Two-inch decking allows 24 inches either way — the 2021 IRC raised the diagonal figure from 16 to 24, so boards running at 45° no longer cost you anything in framing if you're using 2× material. Most decks are framed at 16 inches because it works for both 5/4 and 2× decking and it feels solid underfoot.
How much waste should I allow for decking?
10% for a simple rectangular deck with boards running straight. 15% for a 45° diagonal layout, because every board ends in a triangular offcut you usually can't reuse. Go to 15–20% if the deck has angled corners, a picture-frame border, or a lot of posts and cutouts to work around. Waste on decking is not waste in the usual sense — it's the cost of the cuts the design requires.
Can I use 2 × 6 for decking instead of 5/4?
Yes, and it's a common choice for a deck that takes a beating. A 2 × 6 is 1½ inches thick instead of 1 inch, which is stiffer and lets you space joists at 24 inches instead of 16 — that's fewer joists and less framing. The trade-offs are cost, weight, and a slightly chunkier look. On the calculator, switch the decking type to 2 × 6 and the allowed joist spacing changes with it.
Does a deck need a permit?
Almost always. Most jurisdictions require a permit for any deck more than 30 inches above grade, and many also require one above 200 square feet regardless of height. The 12 × 16 deck in the example above is 192 square feet — just under that line, but at 16 × 16 you'd be at 256 and over it. Rules vary by state and county, so call your building department before you order material; it's a five-minute phone call that can save you a rebuild.