How to Calculate Your Fence Posts in Five Simple Steps

Fence posts in one line

Fence posts are simple: divide the fence length by the post spacing, round up to whole intervals, add one post for the end of the run, multiply by the bags per post for concrete, and multiply by the price per post for cost. A 100 ft fence at 8 ft spacing is 13 intervals, 14 posts, 14 bags at 1 bag a post, and $168 at $12 a post.

Step 1: Measure the fence run

Measure the run on the ground, with a tape, from the first post position to the last. The worked example is 100 ft, a single straight run. Measure the slope length on a slope, not the flat length, since a fence follows the ground. If a gate breaks the run, measure each side separately, because each side is its own run with its own end posts. A run measured off a plan under-counts the ground it actually crosses.

Step 2: Divide by the spacing and round up

Divide the length by the post spacing for the intervals. The example spacing is 8 ft, a common timber fence spacing, and 100 ÷ 8 is 12.5 intervals. Round up to 13, because a fence cannot end part-way through an interval — the last interval is simply shorter than the rest, 4 ft on this run. Never round down: rounding down leaves the last stretch of fence with no post at its end.

Step 3: Add one post for the end

Add one post to the intervals for the post count. Thirteen intervals need 14 posts, because a run needs a post at both ends and a post between every pair of intervals. This is the step people forget, and it is the classic fence shortage: one post short, with the last rail hanging and a second delivery needed. Corners and gate posts add further posts beyond this count.

Step 4: Count the concrete bags

Multiply the posts by the bags per post for the concrete. The example uses 1 bag a post, so 14 posts need 14 bags. Ground conditions decide the real figure: soft or sandy ground takes more concrete per hole, firm clay takes less, and a post beside a gate takes more than a line post. Keep one spare bag beyond the count, since a hole dug wider than planned drinks concrete fast.

Step 5: Price the posts and order

Multiply the posts by the price per post for the cost. The example price is $12 a post, so 14 posts cost $168, before rails, boards and labour, which this guide does not count. Order the posts, the bags and the rails together, and check the delivery against the count before digging: a missing post found on delivery day costs a day, while a missing post found mid-run costs a weekend.

Fence posts in numbers

Suppose the fence is 100 ft long, the spacing is 8 ft, the bags are 1 per post, and the price is $12 a post. Intervals: 100 ÷ 8 = 12.5, rounded up to 13. Posts: 13 + 1 = 14. Bags: 14 × 1 = 14. Cost: 14 × $12 = $168. The last interval measures 4 ft, and the last post lands at 100 ft.

Common mistakes

The most common mistake is forgetting the end post, which leaves every run one post short. The second is rounding the intervals down instead of up, which leaves the last stretch unposted. The third is measuring flat length on a slope, which under-counts the run. The fourth is treating a gated run as one run, which misses the extra end posts. The fifth is ordering concrete to the exact count with no spare bag for a wide hole.

Worked example, start to finish

A straight 100 ft run, 8 ft spacing, 1 bag a post, $12 a post. Intervals: 13. Posts: 14. Bags: 14. Cost: $168. Result: buy 14 posts and 14 bags, set the first post at 0 ft and the last at 100 ft, and keep one spare bag for the hole that runs wide.

What changes the answer most

The spacing changes the answer most after the length: moving from 8 ft to 6 ft spacing lifts the count from 14 to 18 posts on the same 100 ft run. The length is second: moving from 100 ft to 120 ft lifts the count from 14 to 16 posts. The bags change only the bag total, and the price changes only the money — moving from $12 to $15 a post lifts the cost from $168 to $210 while the post count stays 14.

Frequently asked questions