Fence calculator

DIY complexity

Intermediate

Setting posts and attaching panels is achievable with common tools (post-hole digger, spirit level, concrete mix), though getting posts truly vertical and evenly spaced over a long run takes patience.

Introduction

This calculator estimates the posts, bays, boards, rails, fixings and post-hole concrete needed to build a timber or panel fence, from a short garden boundary to a long close-board privacy run. It works from the total fence length and the maximum post spacing allowed for the chosen post and panel system, because a straight run is divided into whole bays, and a fence always needs one more post than it has bays. Every straight run, corner, gate opening and change of direction should be measured and entered separately, since corner, end and gate posts often need to be counted and sized differently from mid-run posts. Beyond posts, the tool sizes board width and gap, the number of horizontal rail rows, and the concrete volume for each post hole from its diameter and depth. Solid privacy fencing catches significantly more wind load than an open picket or ranch-style fence, which affects post size and hole depth. Results are rounded up to whole posts, boards, rails and bags of concrete after your chosen waste allowance. It cannot detect buried services, verify wind-load design, or replace a site survey.

Quick answer

For an 18 m straight fence with a 2.4 m maximum post spacing, that is 8 bays and 9 posts (bays + 1), plus the boards, rail rows and post-hole concrete sized from the panel height and hole dimensions you choose.

Calculation methodology

01Split the fence into separate straight runs at every corner, gate and change of direction, and measure each run's length independently.
02For each run, divide its length by the maximum allowable post spacing for the chosen post and panel system, rounding up to a whole number of bays.
03Add one post per run (posts = bays + 1) to account for the post needed at both ends of that run; identify and flag corner, end and gate posts, which are shared between runs or need different sizing.
04Calculate board quantity per run from board width and gap, and rail quantity from the number of rail rows specified for the panel height and fence style.
05Calculate concrete volume per post hole from hole diameter and depth, multiply by the total post count, then convert to whole bags of pre-mix or a delivered ready-mix volume.
06Apply the waste percentage to boards and rails for cutting losses, and round every purchasable item up to a whole unit.

Formulas

bays = ceil(length ÷ max_post_spacing); posts = bays + 1

A straight fence run is divided into the fewest whole bays that keep every gap at or under the maximum post spacing, and since posts sit at both ends of every bay, the run always needs one more post than it has bays.

Variables

length = total length of one straight fence run (m or ft)
max_post_spacing = maximum allowable distance between posts for the chosen system (m or ft)
bays = whole panel/board sections in that run
posts = whole posts required for that run, before accounting for shared corner or gate posts

Rounding rule: Bays always round up, never down — a bay wider than the maximum spacing is not a valid option, so the actual bay width ends up slightly narrower than the maximum once evenly divided.

Limitation: Applies to one straight run only; corners, gates and direction changes must be measured as separate runs, and shared posts at their junctions should not be double-counted when totals are combined.

concrete_per_hole = π × (hole_dia ÷ 2)² × hole_depth; total_concrete = concrete_per_hole × posts

Each post hole is treated as a cylinder; its volume from diameter and depth is multiplied by the total number of posts to get the total concrete needed, which is then converted to whole bags.

Variables

hole_dia = diameter of the post hole (m or ft)
hole_depth = depth of the post hole, driven by post height, frost depth and soil type (m or ft)
posts = total post count across all runs
total_concrete = concrete volume required (m³ or cubic ft)

Rounding rule: Total concrete volume is not rounded itself; it is divided by the yield of one bag and rounded up so no post hole runs short of concrete.

Limitation: Hole depth and diameter needed for adequate wind resistance depend on fence height, solidity (solid privacy panels versus open pickets), local wind exposure and soil type — these are not universal defaults and should follow local practice or an engineer's guidance for taller or more exposed fences.

Inputs that matter

Maximum post spacingSet by the post and panel system's own strength; using a wider spacing than specified risks sagging panels and overloaded posts, especially on solid fencing exposed to wind.
Corner, end and gate postsOften need a larger post size or different footing than mid-run posts, and are shared between two runs at a corner rather than counted twice.
Number of rail rowsTaller and solid privacy fences typically need three rail rows instead of two to resist wind load and prevent panel sag over time.
Post-hole diameter and depthDetermines the concrete volume per hole and how much wind and frost resistance the finished post has; too shallow a hole is a common cause of post failure.
Board width and gapA solid privacy fence with no gap behaves very differently from a picket fence with wide gaps, both in material quantity and in the wind load the structure must resist.
Buried services along the fence lineNot something the calculator can detect — utilities, drains and cables must be located and marked before any post holes are dug.

Worked examples

Garden fence, 12 m, 1.8 m high

A single straight 12 m garden fence run, 1.8 m panel height, 2.4 m maximum post spacing, 2 rail rows, 5% waste.

bays = ceil(12 ÷ 2.4) = 5 bays. posts = 5 + 1 = 6 posts. Rails = 5 bays × 2 rows = 10 rail lengths. Actual bay width = 12 ÷ 5 = 2.4 m exactly, so no adjustment needed.

6 posts and 5 bays is the whole-run structural count; because 12 m divides evenly by 2.4 m here, the actual bay width equals the maximum exactly — on a length that does not divide evenly, the actual bay width would come out slightly under the maximum instead.

⚠ This calculation assumes no corner, gate or end condition beyond the two run-end posts already counted; a gate opening within the run would need its own post sizing and should be entered as a distinct segment.

Close-board privacy fence with corner, 25 m

A 25 m close-board privacy fence made of two runs meeting at one 90° corner, 2.2 m maximum post spacing, 3 rail rows for wind resistance, 7% waste, post hole diameter 0.32 m and depth 0.7 m.

Say the runs are 15 m and 10 m. Run A: bays = ceil(15 ÷ 2.2) = 7, posts = 8. Run B: bays = ceil(10 ÷ 2.2) = 5, posts = 6. Raw total posts = 8 + 6 = 14, but the corner post is shared by both runs, so actual total = 14 − 1 = 13 posts. Rails = (7 + 5) bays × 3 rows = 36 rail lengths. Concrete per hole = π × (0.32 ÷ 2)² × 0.7 ≈ 0.056 m³; total ≈ 13 × 0.056 = 0.73 m³.

The corner post appears in both runs' raw counts but physically exists once, so it must be subtracted once when combining runs — forgetting this subtraction is a common source of over-ordering posts and concrete on any fence with a corner or return.

⚠ A 3-rail, closely spaced-post design like this is an editable example, not a universal wind-load solution — taller or more exposed solid fencing may need closer post spacing, larger posts or deeper footings than shown here; check local wind-load guidance.

Post and concrete purchase for the 18 m straight fence

The 18 m, 2.4 m spacing fence (8 bays, 9 posts) from the introduction, post-hole concrete needed is 0.4 m³ total, bought as 25 kg bags each yielding about 0.0125 m³, posts priced individually.

Bags needed = ceil(0.4 ÷ 0.0125) = 32 bags. If one post costs P and one bag costs Q in the selected currency, total post-and-concrete cost = 9 × P + 32 × Q, before boards, rails and fixings are added as separate line items.

32 bags rounds up from a theoretical 32.0 bags with little surplus in this case, but on a smaller job it is common to see a larger relative surplus purely from bag-size rounding — that surplus is expected, not a sign the input was wrong.

⚠ Changing the selected currency only changes the displayed symbol — it does not convert a price you already entered. Enter current local supplier prices per post and per bag in the currency you have selected, and confirm applicable sales tax separately.

Reading the results

The results separate post count from bay count, and board and rail quantities from post-hole concrete, because each follows a different rule tied to spacing, panel style or hole geometry rather than one blended total. When a fence has more than one run, check whether shared corner or junction posts have already been de-duplicated in the combined total, since raw per-run counts naturally overlap at shared posts. Post-hole concrete is shown both as a raw volume and as a rounded bag count so it stays clear which figure is the calculated requirement and which is the actual purchase quantity.

Common mistakes

Forgetting the +1 post beyond the bay count, which under-orders posts by exactly one on every straight run.
Double-counting a shared corner or junction post as belonging fully to two separate runs instead of subtracting the overlap once.
Treating a fence with a gate as one uninterrupted run instead of measuring the gate opening and its posts as a distinct segment.
Using the same post spacing and hole depth for a solid privacy fence as for an open picket fence, ignoring the much higher wind load a solid panel catches.
Skipping a check for buried utilities, drains or cables before digging post holes, which the calculator has no way to detect from the entered dimensions.
Assuming two rail rows are always enough regardless of fence height or solidity, when taller or solid fencing commonly needs a third row to prevent sag.

Adapting to your country

Post sizes, panel heights, board widths and rail systems vary significantly by manufacturer and region — treated softwood, hardwood and composite fencing each have their own recommended maximum post spacing and fixing method from the product's data sheet. Metric markets typically size posts and boards in millimetres and sell panels in fixed widths such as 1.8 m; imperial markets commonly sell 6 or 8 ft panels and posts. Concrete for post holes is sold as bagged pre-mix with a stated yield per bag, or as ready-mix by volume for larger jobs, and packaging sizes differ by supplier and country. Selecting a different currency changes only the displayed symbol — enter current local supplier prices for posts, panels, rails, fixings and concrete, and confirm applicable sales tax separately since it is not built into the calculator's cost output.

Limitations and verification

This calculator produces a planning estimate, not a structural design, engineering certification or contractor quotation. It does not verify post strength against wind or soil loading, boundary ownership or party-wall rules, local height restrictions, or compliance with building regulations. It cannot detect buried utilities, drains or cables along the fence line — these must be located and marked before digging. Corner, gate and junction post counts require manual review to avoid double-counting shared posts across runs. Always confirm post spacing, footing depth, boundary rules and any permit requirements with local regulations and a qualified professional before construction.

Last reviewed: 2026-08-24

Frequently asked questions

Why does the calculator always give one more post than bays?

Because a continuous straight run needs a post at both ends, not just between panels — n bays need n+1 posts along that run.

How should I handle a corner in the fence line?

Measure and calculate each straight run on either side of the corner separately, then subtract one from the combined post total because the corner post is shared by both runs, not duplicated.

Does the calculator know where a gate should go?

No. You need to identify the gate location and treat it as its own segment with its own posts, since gate posts are typically sized differently from mid-run posts.

Why would a solid privacy fence need deeper post holes than an open fence?

A solid panel catches far more wind load than an open picket or slatted design, which increases the overturning force on each post — deeper, wider footings help resist that additional load.

Is 2 rail rows always enough?

Not necessarily. Taller panels and solid privacy fencing commonly need 3 rail rows to stay rigid and resist wind-driven sag; check the panel manufacturer's recommendation for the chosen height and style.

Can the calculator detect buried pipes or cables?

No. It works only from the dimensions you enter. Buried services must be located and marked by the appropriate local service before any post holes are dug.

Does changing the currency convert prices I already entered?

No. Changing currency only changes the displayed symbol and formatting; enter a current local supplier price in the currency you select.

Is the result enough to order materials or start digging?

No. It is a planning estimate. Confirm post spacing, footing depth, wind-load requirements and any boundary or permit rules with local regulations before ordering or digging.

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