The circle divides into whole bays or it does not divide at all
Post spacing on a straight fence is a division: take the run, divide by the maximum spacing, round up, and the posts land wherever they land. On a circle the same logic applies but the geometry is different, because the spacing is a chord across the circle rather than a distance along it.
For a ring of n posts on a circle of diameter D, the chord between neighbouring posts is D times the sine of 180 divided by n degrees. Turn that around: the smallest n whose chord stays inside your maximum spacing s is the next whole number above pi divided by the arcsine of s over D. For a 60 ft circle with an 8 ft maximum, that expression gives 23.49, so 24 posts, and the actual chord is 60 times the sine of 7.5 degrees, which is 7.832 ft, or 7 ft 10 in.
The arc between the same two posts, measured along the circle, is pi times 60 divided by 24, which is 7.854 ft. The chord is a quarter of an inch shorter. That gap is small at 24 posts and grows fast as posts are removed: at 12 posts the arc is 15.71 ft and the chord 15.53, nearly two and a quarter inches apart.
Straight rails cut inside the circle
This is the part that surprises people who set the posts carefully and then find the pen measures short. The posts are on the circle. The rails between them are straight, so they fall inside it everywhere except at the posts, and the distance at the middle of a bay is the middle ordinate: the radius times one minus the cosine of 180 over n degrees.
At 24 posts on a 60 ft circle that is 30 times one minus the cosine of 7.5 degrees, which is 0.257 ft, or about 3 and 1/16 in. Both sides together take 6 and 3/16 in off the diameter, so the clear space across the rails at mid-bay is 59.49 ft rather than 60. The enclosed area follows: the 24-sided polygon holds 2,795 sq ft against 2,827 for a true circle.
| Posts on a 60 ft circle | Chord | Middle ordinate | Clear across rails |
|---|---|---|---|
| 12 | 15.53 ft | 1 ft 1/4 in | 57.96 ft |
| 16 | 11.70 ft | 6-15/16 in | 58.85 ft |
| 24 | 7.83 ft | 3-1/16 in | 59.49 ft |
| 32 | 5.88 ft | 1-3/4 in | 59.71 ft |
The middle ordinate falls roughly as the square of the bay count: double the posts and it drops to about a quarter. If the clear diameter matters, either add posts or set the post circle slightly larger than the pen you want and let the rails land on the number.
Board lengths and where the joints go
Rail take-off is the bay count times the chord times the number of rows, and the only subtlety is what the stock length does with it. A 16 ft board spans two 7.83 ft bays with 0.33 ft to spare, so joints land at every second post and the offcut is small. A 12 ft board spans one bay and wastes over four feet, or gets cut and jointed mid-bay, which is a joint with no post behind it.
Where the board length allows two or more bays, stagger the rows so the joints do not all stack over the same post. It costs nothing and it is the difference between a fence line that reads continuous and one with a visible seam every thirty feet.
A ring has no corners, and that is not entirely good news
On a rectangular fence, the corners are where the bracing goes, because that is where the wire or rail changes direction and where the tension is resisted. A closed ring has no corner posts at all — every post is a line post, and there is nothing in the geometry that acts like a brace assembly. How that is dealt with depends completely on the material and the system, and it is a question for the fence supplier and whoever is setting the posts. This page counts posts; it does not know what holds them up.
For a straight run instead, the fence post calculator does posts, rails and the concrete per hole, and the livestock fence calculator covers braced assemblies, wire rolls and gates on a perimeter. If the enclosure is built from fixed-length panels rather than posts and rails, the ring snaps to the panel instead of to a spacing, and the kennel and pen panel layout calculator handles that case. A wide gate leaf that wants to droop is the gate sag brace calculator. For the footing inside the pen, the arena footing calculator takes a circle directly.
Questions people ask
How many posts does a 60 ft round pen need?
It depends entirely on the spacing you will accept, because a circle has no natural post count. At a maximum of 8 ft between centres, 24 posts, giving an actual chord of 7.83 ft. At 10 ft maximum, 19 posts at 9.88 ft. At 12 ft maximum, 16 posts at 11.70 ft. The rule is that the smallest workable count is the next whole number above pi divided by the arcsine of the spacing over the diameter, and the actual spacing that results is always at or below the limit you set, never above it.
Why is the chord shorter than the arc, and which one is the spacing?
The chord is the straight line between two post centres and the arc is the curved distance along the circle between the same two points, so the arc is always longer. The spacing that matters for a rail is the chord, because a straight board spans the straight distance. Laying out with a tape around the circumference measures arcs, so posts set that way come out fractionally further apart than intended. On a 60 ft pen at 24 posts the difference is a quarter of an inch and nobody will notice; at 12 posts it is over two inches, and across twelve bays that adds up to a bay that will not close.
What is the middle ordinate and why does it matter?
It is how far a straight rail sits inside the circle at the middle of a bay, calculated as the radius times one minus the cosine of 180 divided by the post count, in degrees. It matters because it is the difference between the diameter you set out to the post centres and the clear space you actually get between the rails. On a 60 ft circle with 24 posts it is about three inches each side, so the clear diameter at mid-bay is 59.49 ft. With 12 posts it is over a foot each side and the clear diameter drops to 57.96 ft, which is a noticeably smaller pen than the one on the plan.
Can I use a spacing wider than the calculator allows?
The calculator does not impose a limit; it takes yours. What it does is find the smallest number of posts whose chord is at or under the figure you enter, so a wider entry means fewer posts and a longer unsupported rail span. What span a rail can carry, and what happens to it when something leans on it, is a property of the material and the fence system and comes from the supplier. The geometry consequence is visible in the output though: wider bays mean a larger middle ordinate, so the pen gets smaller and more obviously angular as the post count comes down.
Does the gate change the post spacing?
Only if the leaf is wider than a bay. A gate that fits inside a bay leaves a gap beside it, which the output prints as filler, and the ring itself is unaffected — the posts stay where the geometry put them. A leaf wider than the bay forces those two posts apart, which means the ring is no longer a regular polygon and the diameter grows on that side. The tidiest arrangement is to make the gate the full bay width, which removes the filler entirely and keeps every post on the circle. Whether that suits the opening you actually need is a separate question.