Board on Board Fence Calculator: Estimate Materials

A board on board fence calculator estimates the boards, posts, rails, and concrete needed for a fence, based on its length, height, and material dimensions. Enter the project measurements and spacing to get a practical starting estimate; the result is not a substitute for checking the fence layout, local requirements, or material specifications.

Formula

The board estimate assumes boards on both faces of the fence, with the front and back boards offset to cover the gaps. It treats the entered gap as the spacing used to estimate the number of boards on each face. For a rough continuous-run estimate:

• Boards = ceiling[(fence length in inches ÷ (board face width + gap)) × 2].
• Posts = ceiling(fence length ÷ post spacing) + 1.
• Rails = ceiling(fence length ÷ post spacing) × rails per section.
• Concrete = estimated number of posts × 0.6 cubic feet per post.

Here, fence length and post spacing are in feet; board width and gap are in inches; and the concrete result is in cubic feet. The ceiling function rounds up to the next whole item. The concrete figure is only a planning allowance of 0.6 cubic feet per post, not a hole-size calculation. Actual concrete volume depends on the post-hole dimensions and site conditions.

Worked Example

Using the calculator’s default values—100 feet long, 6 feet high, 5.5-inch board faces, a 1-inch gap, 8-foot post spacing, and 3 rails per section:

1. Boards: 100 × 12 = 1,200 inches. The estimated number per face is 1,200 ÷ (5.5 + 1) = 184.62. Rounding up gives 185 boards per face; two faces require 185 × 2 = 370 boards.
2. Posts: ceiling(100 ÷ 8) + 1 = 13 + 1 = 14 posts.
3. Rails: ceiling(100 ÷ 8) × 3 = 13 × 3 = 39 rails.
4. Concrete allowance: 14 × 0.6 = 8.40 cubic feet.

These values are estimates for a straight, uninterrupted fence run. They do not include extra boards for cuts, rejected lumber, gates, corners, or unusual end details.

How a Board on Board Fence Works

A board on board fence has boards attached to both sides of the rails, with the second face covering the gaps in the first. This overlapping arrangement gives the fence a more consistently screened appearance than a single row of spaced pickets, although the exact amount of privacy depends on board width, gap, alignment, and how the wood moves over time.

The calculator doubles the board count because it estimates two faces. That does not mean the same number of boards is required for every design. Some layouts use different board widths, intentional gaps, or a decorative top. A fence-height input is useful for planning, but height does not change the number of boards in this calculator: each board is assumed to run vertically for the fence’s full height. It does affect the length of each board you need to buy.

What to Measure Before You Buy

Measure the intended fence line rather than relying only on a property map or a rough visual estimate. For an irregular boundary, divide the route into straight sections, measure each one, and add the lengths. Plan corners, gates, and changes in height separately; a single straight-run calculation may place posts or rails incorrectly around them.

• Board face width: Use the actual face width, not a nominal lumber label. Lumber sold by a stated nominal size can have different finished dimensions.
• Gap: Enter the intended spacing between boards on a face. For a true board on board design, overlap must be sufficient to cover the opposite face’s gaps.
• Post spacing: This is the distance between posts along the fence. The result estimates sections and posts, but final spacing should suit the rail length, materials, ground, and design.
• Rails: Choose the number of horizontal rails appropriate for the fence height and construction plan. This calculator does not determine structural adequacy.
• Height and board length: Check the required finished height and allow for any intended ground clearance, slope, or top detail when selecting board lengths.

How to Read the Estimate

Board, post, and rail outputs are whole-item estimates, so they round upward rather than reporting fractions. “Rails” means individual rail pieces for the estimated sections; it does not mean the total length of rail stock. For example, 39 rails could mean 39 pieces approximately one section long, but the required cut length depends on post layout and the fence design.

Concrete is shown in cubic feet because it is calculated as a simple allowance per post. To compare with a supplier’s bag yield or a bulk order stated in another unit, convert units and use the actual yield or measured hole volume. One cubic foot is about 0.0283 cubic metres. Do not assume that every post hole needs the same volume: diameter and depth vary with post dimensions, soil, frost conditions, and construction requirements.

Common Estimating Mistakes and Limits

The most common source of error is mixing dimensions: the fence length and post spacing are entered in feet, while board width and gap are in inches. Convert measurements before entering them. Another frequent misunderstanding is treating the estimate as a cut list. It does not account for gates, terminal and corner posts, changes in terrain, board overlap at ends, waste, or exact rail lengths.

For a more realistic order, calculate each straight section separately, note the number of faces and boards in the chosen layout, then add a reasonable allowance for cuts and damaged pieces. The right allowance depends on the lumber, workmanship, and layout; there is no universal percentage that fits every project. Confirm post and rail dimensions, fasteners, ground contact suitability, and any applicable local requirements with relevant product instructions and authorities. Locate underground services before digging, and use appropriate protective equipment and safe tool practices.

Frequently Asked Questions

How many boards do I need for a board on board fence?
Estimate the number of boards on one face from the fence length, board face width, and spacing, then double it for the second face. The calculator rounds up, but add material for gates, corners, cuts, and waste that it does not model.

Does a board on board fence use twice as many boards?
It generally uses boards on both faces, so a simple estimate is about twice the one-face count. The precise total depends on the overlap, layout, and whether the boards on the two faces are offset as intended.

How far apart should fence posts be?
There is no single spacing suitable for every fence. The estimate lets you enter a planned spacing; choose one compatible with the rail lengths, fence design, materials, and ground conditions, and check the construction guidance for your project.

How much concrete does each fence post need?
This calculator uses a rough allowance of 0.6 cubic feet per post. Actual volume is determined by the hole’s diameter and depth, so calculate it from the planned hole dimensions rather than relying on the allowance for ordering.

Does this calculator account for gates or slopes?
No. It estimates a straight run with uniform spacing. Treat gate openings, corners, steps, and sloped sections separately, since they can change post placement, rail lengths, and board counts.

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in length | Fence length | ft | 100 | 0
in height | Fence height | ft | 6 | 0
in board | Board face width | in | 5.5 | 0
in gap | Gap between boards on each face | in | 1 | 0
in post | Post spacing | ft | 8 | 0
in rails | Rails per section | - | 3 | 0
out boards | Fence boards | boards | ceil(length * 12 / (board + gap) * 2) | 0
out posts | Fence posts | posts | ceil(length / post) + 1 | 0
out rails | Rails | rails | ceil(length / post) * rails | 0
out concrete | Concrete volume | ft^3 | posts * 0.6 | 2