๐ง Fence Picket Calculator
Any consistent unit works (e.g. cm) โ just use the same unit for all values. Assumes a layout flush with pickets at both ends.
Enter your fence's total length, the width of a single picket, and your target gap between pickets, to calculate how many pickets you need for a run that starts and ends flush with a picket, and the actual evenly-distributed gap.
How to use
- Enter the total length of the fence run.
- Enter the width of a single picket.
- Enter your target gap between pickets.
- The number of pickets needed and the actual evenly-distributed gap are calculated automatically.
How the calculation works
When pickets are spaced evenly along a fence run that starts and ends flush with a picket, the relationship is L = Nรw + (Nโ1)รg, where N is the number of pickets, w the picket width and g the gap. The key detail is that gaps only occur between pickets, so there is always one fewer gap than there are pickets. Solving for N gives N = (L + g) รท (w + g). Since you cannot fit a fraction of a picket, the largest whole number that fits is this value rounded down. Rounding down leaves a small amount of length unaccounted for. This tool therefore also redistributes that remainder evenly across the (Nโ1) gaps and reports the resulting actual gap. Widening every gap very slightly generally looks better than using the target gap exactly and leaving an odd offcut of space at one end.
Worked example
A 300cm run with 9cm pickets and a 3cm target gap 1. One picket plus one gap: 9 + 3 = 12cm 2. Number of pickets: (300 + 3) รท 12 = 25.25 โ round down to 25 3. Width taken by pickets: 25 ร 9 = 225cm 4. Length left for gaps: 300 โ 225 = 75cm 5. There are 24 gaps: 75 รท 24 = 3.125cm 25 pickets at a gap of about 3.13cm fills exactly 300cm. That is 0.13cm wider than the target, but it leaves no leftover space at either end.
Things to be aware of
- Any unit works, but use the same one for run length, picket width and gap. Mixing centimetres and millimetres will give wrong results.
- Post thickness is not included. If pickets run between posts, enter the clear inside dimension as the run length.
- Real timber varies in size and moves with humidity. For outdoor work especially, measure your actual boards before cutting.
- This formula assumes pickets at both ends. If you want a gap at each end too, subtract that margin from the run length before entering it.
FAQ
What formula is used?
Length = pickets ร picket width + (pickets โ 1) ร gap. This finds the largest number of pickets that fits within your target gap. Once the picket count is fixed, any leftover length is spread evenly across the (pickets โ 1) gaps to get the actual gap.
Why does the actual gap differ slightly from my target?
The total length rarely divides evenly into whole pickets and gaps, so the leftover length gets distributed evenly across the gaps โ which is why the actual gap can differ slightly from what you entered.
What if I want a gap at both ends too?
This tool assumes a layout that starts and ends flush with a picket. If you also want space at both ends, subtract that end margin from the total length before entering it.