Calculate deck boards and total linear feet for a deck build.
Fasteners
Estimated cost
Board coverage reference
| Gap | Coverage (5.5 in board) |
|---|---|
| 1/8 in | 5-5/8 in |
| 3/16 in | 5-11/16 in |
| 1/4 in | 5-3/4 in |
| 5/16 in | 5-13/16 in |
Usage Tip
Order boards in lengths that span the deck with the fewest butt joints, and stagger any joints you cannot avoid.
linear feet = boards across × length × (1 + waste ÷ 100)
The result is total linear feet of decking to buy.
How many deck boards do I actually need?
It looks like a simple multiplication, and then you meet the lumber industry. A board sold as 5/4x6 is not 6 inches wide, your gaps eat into the coverage, and a diagonal layout quietly wastes a stack of material. This calculator handles all of that and gives you the number that matters at the checkout: how many boards to buy, adjusted for waste, plus fasteners and an estimated cost.
The method is the same one the big-box estimators use: divide your deck area by the real coverage of one board (actual width plus gap, times board length), then add a waste factor for cuts and offcuts.
Actual vs nominal size: the goblin math
Lumber is named by its rough-sawn size, not its finished size. After milling and drying it shrinks, so the number on the tag is bigger than the board in your hands.
| Sold as | Actual size | Notes |
|---|---|---|
| 5/4 x 6 | 1 in x 5-1/2 in | The classic pressure-treated deck board |
| 2 x 6 | 1-1/2 in x 5-1/2 in | Thicker, stiffer, spans farther |
| 1 x 6 | 3/4 in x 5-1/2 in | Thinner, often porch flooring |
| Composite 5.5 in | about 5-1/2 in | Varies slightly by brand; check the spec |
Notice the pattern: almost every common deck board is about 5-1/2 inches wide regardless of the name. Always size your deck on the actual width, never the nominal.
Board coverage with a gap
Boards are spaced apart so water drains and wood can move. Each board therefore covers its own width plus one gap. That combined figure is the coverage.
| Board | Actual width | Coverage at 1/4 in gap |
|---|---|---|
| 5/4 x 6 | 5-1/2 in | 5-3/4 in |
| 2 x 6 | 5-1/2 in | 5-3/4 in |
| Composite | 5-1/2 in | 5-3/4 in |
Which gap should you use?
Pressure-treated boards are often installed tight (1/8 in) because they shrink as they dry; kiln-dried and composite boards use a fixed 3/16 to 1/4 in gap. A wider gap means fewer boards but more visible space between them.
Board direction and waste
Boards usually run perpendicular to the house so the run sheds water away from the wall, but parallel and diagonal layouts are common for looks. Direction changes how much you waste on cuts.
| Layout | Suggested waste |
|---|---|
| Simple rectangle | 5% |
| Normal deck | 10% |
| Diagonal or complex | 15% |
| Picture frame or breaker boards | 20% |
Picture frame borders
A picture frame runs border boards around the deck perimeter, hiding the cut ends of the field boards. It looks great and adds material: a single frame adds one board-width border on all sides, a double frame adds two. Budget extra boards and bump your waste to 15 to 20 percent, because every field board now needs a clean cut at the border.
Fasteners: screws or hidden clips
Each board is fastened at every joist it crosses. With face screws that is two screws per joist per board; with a hidden clip system it is roughly one clip per joist per board. Joist count depends on your spacing, so the totals climb fast on a big deck. A 16 by 20 deck can easily need 1,000 to 1,500 screws.
Joist spacing
Spacing matters for fasteners and for which decking you can use. 16 in on center is standard for most lumber; 12 in on center is required for many composite boards run perpendicular, and even tighter for diagonal layouts. Always check your decking maker spec.
Common deck sizes and board counts
Approximate boards for 5/4 x 6 at a 1/4 in gap, using 16 ft boards and a 10 percent waste factor.
| Deck size | Area | Boards (waste-adjusted) |
|---|---|---|
| 10 x 10 ft | 100 sq ft | about 16 |
| 12 x 12 ft | 144 sq ft | about 21 |
| 12 x 16 ft | 192 sq ft | about 29 |
| 16 x 20 ft | 320 sq ft | about 47 |
Tips before you buy
1. Size everything on actual board width, not the nominal name. 2. Pick your gap and stick to it across the whole deck. 3. Add waste for cuts before you leave the yard, not after. 4. Order picture-frame and stair boards separately. 5. Buy fasteners in bulk; you will use more than you think.
Frequently asked questions
How wide is a 5/4 x 6 deck board really?
About 5-1/2 inches. The 6 is the nominal rough-sawn size; the finished board is narrower. Use 5.5 in for planning.
Does the gap really change my board count?
Yes. A 5-1/2 in board at a 1/8 in gap covers 5-5/8 in; at 1/4 in it covers 5-3/4 in. Over a large deck that difference is several boards.
Why does a diagonal deck need more boards?
Angled cuts at the edges produce offcuts that are often too short to reuse, so waste rises to about 15 percent or more.
How many screws per deck board?
Two face screws at every joist the board crosses. A 16 ft board over joists at 16 in on center crosses about 13 joists, so roughly 26 screws per board.
Should boards run parallel or perpendicular to the house?
Perpendicular is most common so water drains away from the wall and boards are fully supported. Parallel and diagonal are chosen mainly for appearance and may need closer joist spacing.
How do I estimate cost?
Multiply your waste-adjusted board count by the price per board, or use price per linear foot or per square foot. The calculator does all three.
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Estimates are for planning only. Board coverage, composite specs, joist spacing requirements, and local code vary; always confirm against your decking manufacturer instructions and a final on-site measurement before purchasing.