Deck Board Calculator
Count deck boards, courses, linear feet, and joists from your deck size and board width, with a waste factor you control. Everything runs in your browser.
How to use
- Measure the deck's length and width in feet, outside of frame to outside of frame.
- Enter the board width in inches and the gap you plan to leave. A common 5/4 by 6 composite or pressure-treated board measures 5.5 inches wide, with a 0.25 inch gap.
- Pick the board direction and the stock length you will buy, then choose a joist spacing of 12, 16, or 24 inches on center.
- Read the results: boards to buy with the waste factor applied, board courses, linear feet, and joist count. Buy about 10 percent extra unless the count already includes your own margin.
How the math works
The deck is divided into board courses by the side the boards run across: that side, in inches, divided by the effective board width of board plus gap, rounded up so the far edge stays covered. Each course fills with whole boards of your stock length, again rounded up, and the waste factor lifts the final board count. Joists run perpendicular to the boards, spaced on center, with one at each edge. Deck dimensions are feet, board dimensions inches, and results are rounded to two decimals.
The board count assumes a plain rectangle with straight runs. Stair stringers, fascia, rim boards, and breaker boards are separate material.
Lumber yards hold good stock and warped stock side by side, and end cuts are never zero. Keep about 10 percent spare boards beyond the count, even with the waste factor applied.
Frequently asked questions
Is my deck plan uploaded or saved?
No. The calculator runs entirely in your browser tab, and your numbers never leave the page. Nothing is stored, nothing is sent to a server, and the page sets no cookies. Ad slots on the page are served by third-party networks, which follow their own privacy policies.
Why does board direction change the count?
Boards run in one direction and are divided across the other. A 16 by 12 deck with boards running along the length needs boards to cross 12 feet, so the 12 foot side is divided by the effective board width into courses. Run the boards the other way and the 16 foot side becomes the one divided, which usually changes the count and the cut plan.
What board width should I enter?
The dressed width, not the nominal one. A 2 by 6 is actually 5.5 inches wide, and a 5/4 by 6 deck board is also about 5.5 inches. Composites vary by brand, so measure a board with a tape rather than trusting the label.
How is the joist count figured?
Joists run perpendicular to the boards, spaced evenly on center along the board run, with one joist at each edge. A 16 foot deck framed at 16 inches on center takes floor(192 / 16) + 1 = 13 joists. The count includes both rim joists but assumes a simple rectangular frame; cantilevers, beams, and stair cutouts are on you.
What waste factor should I use?
The default 10 percent covers end cuts, checking, and one mis-measured row on a straightforward rectangular deck. Diagonal layouts, picture frames, and lots of notching eat more, so 15 percent is a calmer number there. You can set it to 0 if you already plan your own margin.