Decking
Deck Board Spacing Calculator
Space your deck boards perfectly across any joist span. This calculator gives you two options: Even Spacing gently adjusts your target gap so you don't have to rip the final board, while Target Gap enforces your exact gap and calculates the width of the final ripped board.
Deck Dimensions
Total width of the deck framing.
Standard 5/4x6 is usually 5.5"
Preferred space between boards.
Edge Overhangs (Added to Span)
Option 1: Even Spacing (No Rips)
Tweaks the gap slightly so all boards are full width.
Option 2: Target Gap (Rip Last Board)
Keeps your exact gap, leaving one ripped board at the end.
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How deck board spacing is calculated
Finding the perfect layout across a deck span comes down to calculating the exact number of boards and gaps that fit within the total surface area. First, we add the frame span and any overhangs together to get the Total Deck Surface. Then, we calculate the layout using two different jobsite methods.
Option 1: Even Spacing
The cleanest look with no ripped (cut) boards. We determine how many full boards roughly fit into the space, subtract the width of those boards from the total span, and divide the remaining space perfectly across the gaps.
Remaining Space = Total Span − (Number of Boards × Board Width)
Exact Gap = Remaining Space ÷ (Number of Boards − 1)
Option 2: Target Gap
The easiest method for installation. You use a physical spacer (like a framing square or a 16d nail) to enforce an exact gap between every board. The final board is ripped on a table saw to fit the remaining space.
Space Taken = (Full Boards × Board Width) + (Full Boards × Target Gap)
Rip Width = (Total Span − Space Taken) + Target Gap
Standard Deck Board Dimensions
Before you start laying boards, always measure your actual stock. Lumber dimensions are "nominal," meaning the actual size is smaller than the name suggests.
| Nominal Size | Actual Width | Actual Thickness |
|---|---|---|
| 5/4 × 6 Decking | 5-1/2"140 mm | 1"25 mm |
| 2 × 6 Lumber | 5-1/2"140 mm | 1-1/2"38 mm |
| 2 × 4 Lumber | 3-1/2"89 mm | 1-1/2"38 mm |
| Composite Decking | Varies (usually 5.4" or 5.5")Varies (137–140 mm) | Varies (~1")Varies (~25 mm) |
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Frequently Asked Questions
What is the standard gap between deck boards?
For pressure-treated wood, a gap of 1/8" to 3/16"3mm to 5mm is standard to allow for water drainage, debris clearance, and wood swelling. Wet, green lumber will shrink as it dries, so you can install it tighter. Dry wood will expand, requiring a larger gap. For composite decking, follow the manufacturer's spec (typically 3/16" or 1/4"5mm or 6mm).
How do I avoid ripping a deck board at the end?
By tweaking your gap spacing! If you divide the remaining space evenly across all the gaps on your deck, you can usually adjust the gap by less than 1/16"1.5mm to make sure the final board ends perfectly flush with the edge. Use "Option 1: Even Spacing" in the calculator above to find that exact gap.
What is the actual width of a 5/4x6 deck board?
A standard 5/4x6 wood deck board is actually 1" thick and 5.5" wide25mm thick and 140mm wide. Composite deck boards vary by manufacturer, but are typically around 5.4" or 5.5"137mm to 140mm wide. Always measure your actual boards before calculating your layout.
Why add an overhang?
Many builders prefer to let the deck boards overhang the outer rim joists by 1" to 1-1/2"25mm to 38mm. This covers the top edge of the fascia board, hiding the seam and providing a clean, finished look. If your deck attaches to a house on one side, you'll have 0 overhang there, and only overhang on the outer edges.
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