Rounding happens at a different point
Let L be the remaining length in feet after gate openings and D the board width plus gap in inches. For positive dimensions, this tool uses ceil(L × 12 ÷ D). The division is performed for the whole run before the quantity is rounded upward.
An alternative calculation could divide the same length into intervals, apply ceil(interval length × 12 ÷ D) to each interval, and add those integers. Each interval can round a fractional quantity upward independently. Adding the lengths first and rounding only once can therefore produce a different total.
This comparison is about the order of arithmetic operations. It does not establish clear bay widths, position boards, account for post faces, or specify the gap at either end of a bay. A repeated interval in the examples is not a measured opening between posts.
Three controlled comparisons
All examples use hypothetical 5.5 in boards, zero gap, no gates, 6 ft height and 8 ft post spacing. Whole-run outputs were tested with this site's actual submit handler. The separate-interval column is independent comparison arithmetic, not another operating mode of the calculator.
| Total length | Whole-run calculation | Separate 8 ft intervals | Difference |
|---|---|---|---|
| 16 ft | ceil(192 ÷ 5.5) = 35 | 2 × ceil(96 ÷ 5.5) = 36 | 1 |
| 24 ft | ceil(288 ÷ 5.5) = 53 | 3 × ceil(96 ÷ 5.5) = 54 | 1 |
| 64 ft | ceil(768 ÷ 5.5) = 140 | 8 × ceil(96 ÷ 5.5) = 144 | 4 |
For the 64 ft example, every separate interval rounds up to 18. The whole-run calculation rounds up only once to 140. The extra four in the alternative arithmetic are not automatically four spare boards to buy: neither model describes an actual end-gap or cutting layout.
Matching totals do not prove a matching layout
A second test keeps the 64 ft length but uses a hypothetical 6 in board width and zero gap. The whole-run result is ceil(768 ÷ 6) = 128. The separate calculation is 8 × ceil(96 ÷ 6) = 128. The totals match because both divisions give whole numbers before rounding.
That agreement checks the arithmetic, not the fit. Post dimensions, actual openings and chosen end gaps still have not been supplied. Do not interpret a match as a completed panel design.
Post spacing is not a per-bay picket switch
Changing post spacing in this tool changes its straight-section count, but the picket branch still uses the full net run. In the tested 64 ft, 5.5 in, zero-gap case, changing spacing from 8 ft to 4 ft leaves pickets at 140. Posts change from 9 to 17 and rails from 24 to 48, with the same 6 ft height.
Those separate outputs cannot be multiplied together to reconstruct an exact board layout. In particular, dividing the picket total by the displayed section count gives an average, not an assigned quantity for each bay.
How to reconcile your own numbers
- Check whether the comparison rounds once for the full net length or separately for individual openings.
- Check that both calculations use the same board width, gap, units and gate deductions.
- Do not assume the selected post spacing equals every clear opening in a real layout.
- Separate a rounding difference from deliberate spare material, overlap or gate-leaf boards; this calculator adds none of those to its picket formula.
- Before ordering, reconcile the planning total with the actual fence system and measured layout.
The page does not add multi-bay inputs or a new calculator mode. Use the picket formula guide for the existing whole-run equation and the post-count guide for the distinct section formula.
Method and source
The whole-run and spacing checks execute this site's calculation code. The separate-interval comparisons are independently calculated hypothetical examples. They are not construction standards, product data or measurements from a real job, and no material prices are assumed.