Calculator Methodology

Fence Posts

The number of posts depends on fence length and post spacing (on-center).

posts = ceil(length / maxSpacing) + 1

For a 100-foot fence with 8-foot spacing: ceil(100 / 8) + 1 = 14 posts.

Corner posts, gate posts, and end posts are counted in this total — the formula treats every span endpoint as a post. If your design requires double gate posts, add two manually.

Default spacing: 8 feet on-center for most wood and vinyl styles; 10 feet for chain link.

Rails

Rails span between posts. Each span (post-to-post segment) requires one rail per rail row.

spans = posts − 1
rails = spans × rails_per_span

A 100-foot fence with 13 posts has 12 spans. At 3 rails per span, that's 36 rails.

Standard rail counts by style:

• Privacy / shadowbox: 3 rails
• Picket: 2 rails
• Split rail: 2 or 3 (user selects)
• Chain link: top rail only (1), plus tension wire at bottom

Pickets

Picket count uses the desired gap between pickets and the picket face width.

pickets = ceil(length / (picket_width + gap))

For 100 feet with 3.5-inch pickets and 0-inch gap: ceil(1200 / 3.5) = 343 pickets.

A 3-inch gap with 3.5-inch pickets: ceil(1200 / 6.5) = 185 pickets.

All length values are converted to inches before calculation. The result is always rounded up — you never install a fraction of a picket.

Waste factor: Calculators that include a waste option add the selected percentage to the raw picket count before rounding.

Picket Spacing

When you know how many pickets you have and want even gaps, the spacing calculator distributes remaining space equally.

total_picket_width = pickets × picket_width
remaining_space = length − total_picket_width
gap = remaining_space / (pickets + 1)

For 20 pickets (3.5 in wide) on a 10-foot (120 in) span: (120 − 70) / 21 = 2.38 inches per gap.

This formula places a gap at both ends of the span (before the first picket and after the last). If your design butts pickets against posts with no end gap, subtract 2 from the denominator.

Concrete per Post

Concrete volume fills the cylindrical hole around the post.

hole_volume = π × (hole_radius)² × depth
post_volume = post_width² × depth   (square cross-section approximation)
concrete_volume = hole_volume − post_volume

Result is converted to 60 lb or 80 lb bags using published yields:

• 60 lb bag: 0.45 cubic feet
• 80 lb bag: 0.60 cubic feet

Default hole diameter: 3× the post width (e.g., 12-inch hole for a 4×4 post). This matches common contractor practice and most local codes, though some jurisdictions specify different ratios.

Post Depth

The recommended burial depth follows the one-third rule with a frost-line floor.

depth_one_third = fence_height / 2   (post total = height × 1.5, buried third = height × 0.5)
depth_frost = local_frost_depth + 6 inches
recommended_depth = max(depth_one_third, depth_frost)

For a 6-foot fence in a 24-inch frost zone: max(36 in, 30 in) = 36 inches.

Frost depth values are sourced from USDA frost-depth maps and rounded to the nearest inch. We provide the recommendation but cannot account for local soil conditions, wind load, or specific building codes — always verify with your local permit office.

Cost Estimates

Cost is calculated as material quantity multiplied by a unit price range.

low_cost  = quantity × low_unit_price
high_cost = quantity × high_unit_price

We show a low-to-high range rather than a single number because material prices vary significantly by region, supplier, and season.

Price data sources: Unit prices are based on national retail averages compiled from publicly available home improvement retailer pricing, updated annually. The last update was January 2026.

What's excluded: Labor, permits, equipment rental, delivery, and disposal fees are not included in any estimate. These can add 50–100% to material cost depending on your region and project complexity.

Rounding Policy

• Whole units (posts, rails, pickets, bags): Always rounded up (ceiling). You cannot buy a partial post.
• Dimensions (depth, spacing, gap): Rounded to two decimal places in feet, or nearest ⅛ inch in imperial display.
• Volume (cubic feet, cubic yards): Rounded to two decimal places.
• Cost: Rounded to the nearest dollar.

Intermediate calculations are done in full floating-point precision; rounding is applied only to displayed output.

Limitations

• Estimates only. All outputs are starting estimates. Actual material needs depend on site conditions, installation method, and local code requirements.
• Straight fence lines. Calculators assume a single straight run. For L-shaped or curved fences, calculate each segment separately.
• Standard post sizes. Calculators default to 4×4 posts (wood) or standard 1⅝-inch round posts (chain link). Non-standard post sizes will affect concrete volume.
• No engineering calculations. Post depth and concrete recommendations are general guidelines. Wind load, soil bearing capacity, and seismic requirements require a licensed engineer.
• Prices change. Material costs are updated annually but can shift significantly between updates. Always get a current quote from your supplier before budgeting.

Questions about the formulas? Contact us and we'll clarify.