FitAndFillA deck estimate comes down to three counts: the rows of deck boards, the joists underneath them and the fasteners that hold one to the other. Once you see how the pieces cross, each count follows from a single measurement. This guide shows the idea with a 16 × 12 ft deck.
How the pieces fit
The joists are the frame. They run across the width of the deck, evenly spaced along its length. The deck boards lie on top of them at a right angle, so every board crosses every joist once, and each crossing needs a fastener or two.
Counting each piece
- Rows of boards. Each board and the gap beside it take up a strip of the width. Rows = width (in) ÷ (board width + gap), rounded up. Here that is 144 ÷ (5.5 + 0.125) = 25.6, so 26 rows.
- Boards to buy. Each row is 16 ft long, so 26 × 16 = 416 ft of boards. Add 10 % for cuts: 457.6 ft. In 12 ft boards that is 38.13, so 39 boards.
- Joists. Joists stand a fixed distance apart, measured center to center. For 16 in spacing along 192 in, that is 192 ÷ 16 = 12 spaces, and a row of 12 spaces has one more joist than spaces: 13 joists, each 12 ft long (the width of the deck).
- Fasteners. 26 rows × 13 joists = 338 crossings. With 2 fasteners at each, plus 10 % extra, that is 744 fasteners.
The Deck Calculator does these four steps with your own numbers.
Joist spacing and board length
Your plan and your decking maker tell you how far apart the joists must be. This is how the spacing changes the joist count on the same 16 ft deck:
| Spacing, on center | Joists | Fasteners (2 per crossing, 10 % extra) |
|---|---|---|
| 12 in | 17 | 973 |
| 16 in | 13 | 744 |
| 24 in | 9 | 515 |
The length of board you buy changes how many pieces you carry, not how many feet of board you need:
| Board length | Boards (10 % extra) |
|---|---|
| 8 ft | 58 |
| 10 ft | 46 |
| 12 ft | 39 |
| 16 ft | 29 |
| 20 ft | 23 |
These counts add up feet of board and divide by the board length, which assumes you can reuse offcuts. Board ends must land on a joist, so real waste can be higher. Raise the extra if your deck length does not match your board length.
What this does not cover
- The structure. Footings, posts, beams, the ledger and the allowed span of the joists depend on your plan, the lumber and your local code. The American Wood Council publishes a prescriptive deck construction guide (DCA 6) for that part, and your building department has the final say.
- Everything else you will buy: rim (band) joists, blocking, stairs, railings, flashing and fascia.
- Permits. Many places require one for a deck. Ask your local building department before you buy materials.
- Utilities. Before you dig for footings, know where buried utilities run. In the U.S. you can call 811.
Common mistakes
- Forgetting the gap. Counting each board at its width alone gives too few rows.
- Using the nominal width of a board. Measure one, or read the product page.
- Forgetting the last joist. A row of spaces always has one more joist than spaces.
- Mixing up the two directions. Length here is the way the boards run; width is the way the joists run.
Frequently asked questions
Should the boards run along the long side?
That is a design choice. The counts here work either way as long as you enter the side the boards run along as the length.
How many fasteners does one board need?
Follow your decking maker's instructions. The calculator lets you choose 2 fasteners at each joist (face screws) or 1 (hidden clips), and multiplies by the crossings.
How do I measure a deck that is not a rectangle?
Split it into rectangles, count each one and add the results. The square footage guide shows how to split a shape.