Paddock and pasture fencing solves a different problem than a residential yard fence: the goal is safe containment and good visibility for a large animal, not blocking a sightline or marking a property boundary, which changes what kind of fence actually makes sense.
Why rails, not pickets
Solid picket infill can catch a hoof or leg if an animal gets a foot caught between boards, and it blocks the animal’s view of what is beyond the fence, which can startle a horse. Horizontal-rail styles like split rail or ranch fencing avoid both problems: clean sightlines through the rails, no picket edges close together for a hoof to catch on.
Two-rail versus three-rail
Two-rail is lighter and more often used as a decorative or light-containment boundary. Three-rail adds a middle rail, giving better containment and a more solid visual barrier, and is a common choice specifically for horse paddocks. Both use the same post spacing and post-count formula; only the number of rails per section changes.
Post spacing runs wider than a yard fence
Since rails, not infill panels, carry the design, paddock fencing commonly uses wider post spacing than a wood privacy fence, often 8 to 10 feet, though the right number depends on the animal and local practice more than a single fixed rule.
Working out pasture perimeter
A pasture is rarely a perfect rectangle, so perimeter length usually comes from an actual property survey or plat map rather than a simple formula. For a roughly rectangular section, add the length of each side that needs fencing; see the linear feet calculator for that basic arithmetic, and the acreage to fence calculator for a rough, square-lot estimate when no survey is available yet.
Plan the material list
The horse fence calculator defaults to a three-rail split rail layout and computes posts and rails for your perimeter. The split rail fence calculator covers the same material for a lighter, two-rail boundary fence instead.
Related reading: how many posts do you need for a fence, how far apart should fence posts be.