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How our numbers are worked out

Every formula, default value and rounding rule used by the CalculateFence calculators is written out on this page. Nothing is hidden in the code. If a number on this site looks wrong to you, this page is where you check our working.

Written by

Farah Jameel

Home improvement project coordinator · Co-owner

Reviewed by

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Last reviewed 3 September 2026 · Next review 3 March 2027

The formulas

Worked examples use the calculator's default project: a 150 ft run, 6 ft tall, 8 ft post spacing, wood privacy style, one gate.

Posts

sections = ceil(length ÷ spacing) posts = sections + 1 + gates

Each section needs a post at one end, and the run needs one more post to close it. Every gate adds one further post, because a gate opening is framed by a post on each side. Worked: ceil(150 ÷ 8) = 19 sections. 19 + 1 + 1 = 21 posts.

Rails

rails per section = 2 if height ≤ 4 ft 3 if height 4–8 ft 4 if height > 8 ft rails = sections × rails per section

The convention is roughly one rail per 24 inches of height. A third rail is added above 4 ft because an unsupported span of boards will bow. Worked: 19 × 3 = 57 rails.

Boards, pickets, panels and mesh

privacy boards = ceil(length × 12 ÷ 5.5) pickets = ceil(length × 12 ÷ (3.5 + 2.5)) vinyl panels = sections chain-link = ceil(length ÷ 50)

A nominal 1×6 board measures 5.5 in across the face, and a nominal 1×4 picket measures 3.5 in — nominal lumber sizes are larger than actual sizes. Privacy boards butt together with no gap. Pickets are spaced at 2.5 in, which is a common decorative gap rather than a rule. Vinyl arrives as one prefabricated panel per section. Chain-link fabric is sold in 50 ft rolls. Worked: 150 × 12 ÷ 5.5 = 327.3, rounded up to 328 boards.

Post holes and concrete

hole depth = max(24 in, height × 12 ÷ 3) bags per post = 2 if hole depth < 30 in 3 if hole depth ≥ 30 in concrete bags = posts × bags per post

One third of the post's length goes underground, with a 24 in floor. Bag counts assume a 4×4 post centred in a 10 in diameter hole and 50 lb bags of post-setting concrete. Worked: 6 × 12 ÷ 3 = 24 in, so 2 bags per post. 21 × 2 = 42 bags.

Default values and when to change them

Default	Value	Why this value	Change it when
Post spacing	8 ft	Standard residential spacing for wood fences to 6 ft; matches an 8 ft rail with no waste	High wind, fences over 6 ft, or heavier panels — drop to 6 ft
Board face width	5.5 in	Actual width of nominal 1×6 lumber	You are buying a different board size
Picket width	3.5 in	Actual width of nominal 1×4 lumber	You are buying a different picket size
Picket gap	2.5 in	Common decorative spacing; convention, not code	You want a tighter or wider look
Hole depth	⅓ of height, min 24 in	Standard rule of thumb for post stability	Frost line is deeper than the hole — see below
Hole diameter	10 in	Roughly three times the width of a 4×4 post	6×6 gate posts, or soft soil
Concrete bag	50 lb	Most common post-setting bag size in US stores	You buy 60 lb or 80 lb bags — recalculate by weight
Waste allowance	10%	Covers offcuts, warped boards and corner cuts	Sloped or irregular runs — go to 15%

Frost line overrides our hole depth. In cold regions, posts must be set below the depth at which ground freezes, or they will lift out over successive winters. That depth varies from a few inches in the far south to well over 40 inches in the northern states, and it is set by local building code — not by us. Our ⅓-of-height rule does not account for it. Check your municipal building department before you dig.

Rounding rules

Quantity	Rule	Reason
Sections	Always rounded up	A partial section still needs its posts and rails
Boards and pickets	Always rounded up	You cannot buy part of a board
Order quantity	Exact count × 1.1, rounded up	The 10% waste allowance shown beside each result
Unit conversion	1 ft = 0.3048 m exactly	Switching units converts your entered values, it does not re-round them

Rounding up at every stage means our counts lean slightly generous. That is deliberate — a second trip to the lumber yard costs more than one spare board.

What the calculator deliberately does not do

Knowing a tool's limits matters as much as knowing its formulas.

It does not price anything. Material prices move with lumber and vinyl markets and vary by region and yard. A stale price is worse than no price.
It does not handle slopes. Every calculation assumes level ground. Stepped or racked fencing needs longer posts and more of them.
It does not account for corners. A run with corners wastes more material than a straight run of the same length.
It does not read local code. Height limits, setbacks, permits and frost depth are all local and all override us.
It does not size gate hardware or the heavier posts a drive gate needs.

Where these numbers come from, and how confident we are

Being straight about this matters more than sounding authoritative.

Figure	Basis	Confidence
Nominal vs actual lumber sizes	Standardised across US lumber suppliers	High — these are fixed dimensions
Posts, rails, boards formulas	Arithmetic; no judgement involved	High
8 ft post spacing	Widespread residential convention	High as a default, but not a code requirement
⅓ height hole depth	Common trade rule of thumb	Medium — frost line and soil override it
Bags of concrete per post	Derived from hole volume at a 10 in diameter	Medium — varies with bag size and hole width
2.5 in picket gap	Aesthetic convention	Low — entirely a matter of preference

Found an error? These formulas are published precisely so they can be challenged. If a number here disagrees with your local code, your supplier or your own experience, we would rather hear it than not. Corrections are logged below.

Date	Change
2026-09-03	Corrected the worked example on the calculator page: it omitted the extra post required by a gate, understating posts by 1 and concrete by 2 bags. The calculator itself was always correct.
2026-09-02	Methodology page published alongside the fence material calculator.