Picket Spacing Calculator: How to Use It

A picket spacing calculator works out the exact gap between fence pickets so the fence looks even and meets local building codes. You enter the total fence length, picket width, and number of pickets (or desired gap), and it returns the spacing, number of pickets needed, and total material length. This tool saves you from the trial-and-error of laying out pickets by hand.

Formula

The core formula for picket spacing is:

Gap = (Total length in inches – (Number of pickets × Picket width)) / (Number of pickets – 1)

Where:

• Total length – the horizontal distance between the two end posts, measured in feet or inches.
• Picket width – the face width of one picket, in inches (common sizes are 3.5, 5.5, and 7.5 inches).
• Number of pickets – how many vertical boards you plan to install.
• Gap – the open space between adjacent pickets, in inches.

The formula subtracts the total width occupied by all pickets from the overall length, then divides the remaining space by the number of gaps (one less than the number of pickets). This ensures the gaps are equal across the entire run.

Worked Example

Using the default values from the calculator above:

• Fence length = 100 ft = 1200 inches
• Picket width = 5.5 in
• Number of pickets = 20

Step 1: Convert fence length to inches: 100 × 12 = 1200 in

Step 2: Total picket width: 20 × 5.5 = 110 in

Step 3: Remaining space for gaps: 1200 – 110 = 1090 in

Step 4: Number of gaps = 20 – 1 = 19

Step 5: Gap = 1090 ÷ 19 ≈ 57.37 in

So the gap between pickets is about 57.4 inches. That’s a very wide gap—more like a spaced picket fence. For a privacy fence, you’d need many more pickets (or wider pickets).

Why Spacing Matters

Picket spacing isn’t just about looks. It affects privacy, airflow, and whether the fence meets local building codes. Many municipalities have rules about maximum gap size, especially for pools or front yards. For example, a common requirement is that gaps must be less than 4 inches to prevent small children from squeezing through. Always check your local code before setting the spacing.

Spacing also influences structural integrity. If gaps are too wide, the fence may look sparse and offer little privacy. If too narrow, you’ll use more material and increase cost. A calculator helps you balance these factors.

How to Choose the Right Spacing

There’s no universal “best” spacing—it depends on your goal:

• Privacy fence – gaps of 0 to 1 inch, or no gap at all (board-on-board).
• Semi-private – gaps of 1 to 3 inches.
• Decorative or airy – gaps of 3 to 6 inches.
• Pool safety – gaps must be less than 4 inches (often 3.5 inches or less).

If you have a specific gap in mind, you can work backward: divide the total length by (picket width + gap) to get the number of pickets, then round up. The calculator above lets you adjust the number of pickets to see what gap results.

Common Mistakes and How to Avoid Them

One frequent error is mixing units. If you measure the fence in feet but the picket width in inches, you must convert everything to the same unit. Another is forgetting that the number of gaps is one less than the number of pickets. For a 20-picket fence, there are only 19 gaps. Also, don’t forget to account for end posts: the first and last pickets are often placed flush with the posts, which can shift the spacing if not measured from the inside of the posts.

Rounding errors also creep in. If you round the gap to the nearest inch, small errors accumulate over a long run. Use at least one decimal place, and mark the spacing on a story pole or tape measure to keep it consistent.

Alternatives and Variations

If you’re not using standard pickets, the same formula works with any vertical board. For a board-on-board fence (where pickets overlap), the spacing is negative—meaning pickets overlap by a set amount. In that case, use the formula with a negative gap to find the overlap. Some calculators also let you input a desired gap and output the number of pickets needed. That’s a useful reverse calculation.

Another variation is a shadowbox fence, where pickets are placed on both sides of the rails. The spacing formula still applies, but you’ll need to calculate the layout for each side separately.

Frequently Asked Questions

What is the standard gap for a picket fence?

There’s no single standard. For privacy, gaps are usually 0–1 inch. For a classic spaced picket fence, 2–4 inches is common. For pool fences, check local code—often 4 inches maximum. The right gap depends on your needs and local regulations.

How many pickets do I need for a 100-foot fence?

It depends on picket width and gap. For example, with 5.5-inch pickets and a 2-inch gap, each picket covers 7.5 inches of fence length. 100 feet = 1200 inches, so you’d need about 160 pickets (1200 ÷ 7.5). Always round up and buy a few extra for waste.

Can I use this calculator for a vinyl or metal fence?

Yes, the math is the same. Just measure the actual width of the picket or panel and the total length. The material doesn’t affect the spacing calculation.

What if I have a gate or corner in the fence?

Measure each straight section separately and calculate spacing for each run. Gates and corners interrupt the layout, so treat them as separate segments. The calculator works for any straight length.

Why does my calculated gap seem too wide or too narrow?

Check your inputs. Make sure you’re using the same units for length and width. Also verify the number of pickets—if you’re using 20 pickets on a 100-foot fence, the gaps will be huge. Increase the picket count for a tighter fence.

Do I need to account for post width in the spacing?

Yes. The total length should be measured between the inside faces of the end posts, not the overall outside dimensions. If you measure from outside to outside, you’ll include the post widths, which throws off the spacing.

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in L | Fence length | ft | 100 | 0
in w | Picket width | in | 5.5 | 0
in n | Number of pickets | - | 20 | 1
out gap | Gap between pickets | in | ((L*12) - (n*w)) / (n - 1) | 2
out pickets | Total pickets needed | - | n | 0
out rail_len | Rail length (if 3 rails) | ft | L * 3 | 0