Fence Slat Calculator: How Many Slats Do You Need?

This calculator works out the number of fence slats (pickets or boards) you need for a given fence length and slat spacing, and it also estimates the total linear footage of slat material. You enter the total fence length, slat width, and the gap between slats (or choose zero for a solid board-on-board fence), and it returns the exact count and total material length. The result includes a built-in waste allowance so you can order confidently.

Formula

The basic formula for the number of slats is:

Number of slats = ceil( (Fence length in inches) / (Slat width + Gap) )

Where:

• Fence length – the total horizontal distance the fence covers, measured in feet (or meters, but keep units consistent).
• Slat width – the face width of one slat, usually in inches (typical pressure-treated pine pickets are 5.5 in wide).
• Gap – the space between adjacent slats, in inches. For a solid privacy fence, gap = 0. For a spaced picket fence, gap is typically 0.5 to 1 inch.
• ceil() – round up to the next whole number, because you can’t buy a fraction of a slat.

The total slat length is simply the number of slats multiplied by the slat width, converted to feet (divide by 12). This tells you the linear footage of lumber to buy before adding waste.

Worked Example

Using the default values from the calculator above: fence length = 100 ft, slat width = 5.5 in, gap = 0.5 in, waste = 10%.

1. Convert fence length to inches: 100 ft × 12 = 1200 in.
2. Add slat width and gap: 5.5 + 0.5 = 6.0 in (this is the center-to-center spacing).
3. Divide: 1200 / 6.0 = 200 slats exactly.
4. Apply waste: 200 × 1.10 = 220 slats (round up, but it’s already whole).
5. Total slat length: 200 × 5.5 / 12 = 91.67 ft (before waste). With waste: 220 × 5.5 / 12 = 100.8 ft.

So for a 100-ft fence with 5.5-in slats and 0.5-in gaps, you need about 220 slats (including 10% waste) and roughly 101 ft of slat material.

How to Choose Slat Width and Gap

Your choice of slat width and gap depends on the look and function you want:

• Solid privacy fence – gap = 0, slats flush against each other. Use wider slats (5.5–6 in) for fewer pieces and faster installation.
• Spaced picket fence – gap of 0.5–1.5 in. A common rule: gap should not exceed the slat width, or the fence looks too open. For a 5.5-in slat, a 1-in gap gives a 5.5:1 ratio, which is a classic look.
• Board-on-board (shadowbox) – slats are placed on both sides of the rails, overlapping. The visible gap is zero, but the effective spacing is the slat width (each slat covers half of the one behind it). Use the calculator with gap = 0, but double the number of slats if you want both sides covered.
• Horizontal slat fences – the same math applies, but the “lengthâ€� is the vertical height of the fence, and slat width is the horizontal board width. Measure the total vertical span.

Common Mistakes and How to Avoid Them

• Mixing units – always convert everything to inches before dividing. A fence length in feet must be multiplied by 12.
• Forgetting the gap – many people divide the fence length by slat width alone, which overestimates the count for spaced fences.
• Not rounding up – you can’t buy 199.3 slats; you need 200. The calculator rounds up automatically.
• Ignoring waste – you will cut slats at posts, corners, and gates, and some boards will have defects. A 10% waste allowance is typical; 15% is safer if you’re new to fencing.
• Assuming all slats are the same width – pressure-treated lumber can vary slightly. Measure a few boards and use the average width.

Limits of the Formula

This formula gives a precise count for a straight, level fence line. It does not account for:

• Posts and rails – the slat count is independent of post spacing, but you’ll need to cut slats to fit around posts, which adds waste.
• Gates – if you have a gate, the slat count for that section is different (the gate frame is narrower or wider). Measure the gate opening separately.
• Sloped ground – on a slope, you may need to cut slats at an angle or step the fence, which increases waste.
• Corners – at a 90-degree corner, you need an extra slat or a post to cover the joint, depending on the design.

For most residential fences, the formula is accurate to within a few slats. For complex layouts, draw a sketch and count slats per section manually.

Frequently Asked Questions

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

With 5.5-in slats and 0.5-in gaps, you need 200 slats before waste. Add 10% for waste and you get 220 slats. If you use 6-in slats with no gap, you need 200 slats (100 ft × 12 / 6 = 200).

What is the standard gap between fence pickets?

There is no official standard, but common gaps are 0.5 to 1 inch for a spaced picket fence. For a privacy fence, the gap is 0. For a semi-private look, many builders use a gap equal to the slat width (e.g., 5.5-in slat with a 5.5-in gap), but that leaves large openings.

How do I calculate slats for a board-on-board fence?

For a board-on-board (shadowbox) fence, you place slats on both sides of the rails, overlapping. The visible spacing is zero, but each slat covers half of the one behind it. Use the calculator with gap = 0 to get the number of slats for one side, then double it for both sides. For a 100-ft fence with 5.5-in slats, that’s 218 slats per side, or 436 total (before waste).

Should I buy extra slats for waste?

Yes. You will cut slats at posts, corners, and gates, and some boards may have knots or cracks. A 10% waste allowance is typical for a straight fence; 15% is safer if you have corners, slopes, or are a beginner. For a 100-ft fence needing 200 slats, 10% extra means buying 220.

Does the calculator work for horizontal slat fences?

Yes, the math is the same. For a horizontal fence, the “fence length� is the total vertical height of the wall (e.g., 6 ft), and the “slat width� is the horizontal board width (e.g., 5.5 in). Enter the height in feet and the board width in inches, and the calculator gives the number of horizontal boards needed.

What if my fence has a gate?

Measure the gate opening separately. The slat count for the gate section is based on the gate frame width, not the fence line. For example, a 4-ft gate with 5.5-in slats and no gap needs 4 × 12 / 5.5 = 8.7, so 9 slats. Add that to your main fence count, and remember to account for the fact that the gate frame is narrower than the opening.

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in len | Fence length | ft | 100 | 0
in sw | Slat width | in | 5.5 | 0
in gap | Gap between slats | in | 0.5 | 0
in waste | Waste allowance | % | 10 | 0
out count | Number of slats | slats | ceil(len * 12 / (sw + gap)) * (1 + waste / 100) | 0
out total | Total slat length | ft | ceil(len * 12 / (sw + gap)) * sw / 12 | 1