How even spacing is calculated
With n boards across a width, there are n − 1 gaps between them. To fill the width exactly:
gap = (deck width − n × board width) ÷ (n − 1)
The calculator tries the board counts closest to your target gap and shows the gap each one needs. It recommends the closest option within ±40% of your target and warns you when nothing whole-board lands that close.
Example: a 10 ft wide deck
A 10 ft (120") wide deck with 5-1/2" boards and a 3/16" target gap needs 22 boards, but the last one would be ripped to about 9/16" — a sliver. With 21 full boards, the gap works out to about 7/32" (0.225"), so every board stays full width.
It doesn't always work out. On a 12 ft wide deck, the closest whole-board layout needs a 9/32" gap — wider than most manufacturers allow. There you'd keep your gap and rip the last board, or adjust the overhang at the edge.
Before you commit to a gap
- Measure several boards. Treated lumber in particular can vary by 1/8" or more.
- Decide how the edge boards finish — flush with the rim, overhanging the fascia or tight to the house — and measure the width you actually need to cover.
- Check the gap range in your board’s installation guide. An even gap outside that range isn't worth it.
Once you know your spacing, use the deck board calculator to work out how many boards and which lengths to buy.
Frequently asked questions
How do I calculate deck board spacing?
Decide how many boards will fit, then share the leftover width between the gaps. With boards running edge to edge across the deck, gap = (deck width − boards × board width) ÷ (boards − 1). Start with the deck width divided by the board width plus your target gap, rounded down, to get the board count. The calculator above does this and shows the nearest gaps that fit whole boards.
What should I do when deck boards are too close together?
First check whether the boards are wet pressure-treated lumber: those are often installed tight and open up as they dry over the following weeks or months. If dry boards are touching or nearly so, water and debris can't drain and the wood stays damp, so it's worth fixing. Clear the gaps with a putty knife or gap-cleaning tool, then for a few problem boards, remove the fasteners and relay them with spacers. Some people widen gaps with a circular saw set just to the board depth, but it's easy to hit fasteners or wander, so relaying is usually cleaner. Composite boards that are too tight can buckle in heat, so follow the manufacturer's gap.
What is the standard gap between deck boards?
Most decking is installed with a side gap of 1/8" to 3/16" (about 3–5 mm). Composite brands publish exact figures — Trex specifies 3/16" between boards, and TimberTech allows 1/8" to 3/16". For wood, the right gap depends on moisture content: wet treated lumber shrinks after installation.
Is it better to rip the last board or adjust the gap?
A thin ripped board looks odd and can be weak, especially under 2" or so. If adjusting the gap by a few hundredths of an inch gives you whole boards, that usually looks better. Stay within your manufacturer’s allowed gap range.
Do end gaps (butt joints) work the same way?
No. End gaps at butt joints are about expansion along the board, and for composite they depend on the temperature when you install. Fiberon, for example, lists larger end gaps in cold weather and smaller ones in heat. This calculator handles side-to-side spacing only.
How do I keep the gap consistent?
Use spacers — a purpose-made gap tool, a nail of the right diameter or hidden clips that set the gap automatically. Check the remaining distance to the end every few rows and adjust slightly so you don’t end up with a sliver.