How to use the board spacing calculator
- Measure the clear opening and the actual width of each equal slat.
- Enter the number of slats.
- Choose whether the outer edges also get a full equal gap, then mark from the first center and the center-to-center pitch.
The formula
Gap = (available span − slat count × slat width) ÷ number of gaps
Worked example
Twelve 40 mm slats in a 1000 mm opening leave 520 mm. With matching outer gaps, divide by 13 to get 40 mm per gap.
Before you measure and cut
This is geometric spacing only. It does not verify guardrail opening limits or other construction requirements.
Small width variations accumulate. Check actual stock and the final opening before fastening every slat.
Frequently asked questions
Why are there more gaps than slats?
With a gap at both ends, n slats create n + 1 gaps. Without edge gaps, they create n − 1 gaps.
Can I calculate a fence bay?
Yes, if all pickets are equal width and you use the clear width of the bay.