Deck Board Calculator: How Many Deck Boards Do I Need?
Enter the deck size, the width of the board you are using and the gap between boards. The count comes from the deck’s width and the board pitch, not from its area — which is why two decks with the same square footage can need different numbers of boards.
Deck and board sizes
Length runs along the boards; width runs across them.
This covers decking boards only — not joists, beams, posts, footings or fasteners. Board dimensions vary by product, so check the actual face width rather than the nominal size.
How many deck boards do I need?
Boards run in one direction, so what matters is how many rows fit across the deck. Divide the deck width by the pitch of one board — its actual face width plus the drainage gap — and that is your row count. Multiply by the deck length for linear feet, then divide by the board length you are buying.
Nominal sizes lie. A "5/4 × 6" board is about 5.5 inches wide, and a "2 × 6" is the same. Measure a sample or check the product listing rather than trusting the name.
Why the gap matters
Deck boards need a gap so water drains and the timber can move. An eighth of an inch is typical for boards installed dry; pressure-treated boards that are still wet will shrink, so they are often laid tight and open up on their own.
The gap is small but it compounds: over a 12 ft width, an eighth-inch gap on every board costs you most of a row. That is why it is an input here rather than an assumption.
How much waste to allow
| Waste | Good starting point for |
|---|---|
| 5% | A rectangular deck where boards span in one piece |
| 10% | Some cutting, a few butt joints |
| 15% | Angles, picture framing, or a diagonal layout |
Waste on a deck comes from butt joints and cuts. If your boards span the deck in one piece the waste is minimal; if every row needs two boards with a joint over a joist, the offcuts add up. A picture-framed border or a diagonal layout adds more again.
How to measure
The length is along the boards; the width is across them. Getting these the wrong way round changes the answer.
Not the nominal size. A 5/4 × 6 is about 5.5 inches.
A board length that divides the deck length neatly wastes far less than one that leaves a short offcut every row.
Common mistakes
- Dividing the deck area by the board area and ignoring the gap.
- Using the nominal board width instead of the actual face width.
- Swapping length and width, so the rows are counted the wrong way.
- Forgetting that this covers boards only — not the frame beneath them.
Building the frame first?
Work out the posts and rails if you are fencing the deck, or the concrete for the footings.
Open the Concrete Calculator →