Fence calculator
A hundred-foot fence at eight-foot spacing is twelve and a half sections, so thirteen sections and fourteen posts, not thirteen. Every run needs one more post than it has sections, because the last section still has to end somewhere. Corners and gates each add another, and a gate post is a heavier section than a line post because it carries a swinging load.
A fence run needs one more post than it has sections. Divide the length by the post spacing, round up for sections, then add one: a 100 ft run at 8 ft spacing is 13 sections and 14 posts. Each hole goes a third of the post height deep and swallows about 0.03 cubic metres of concrete.
How to count fence materials
Spacing is set by the fence type, not by preference. Panel fencing is fixed at the panel width, usually 1.83 m, and the posts must land exactly there. Rail-and-picket fencing tolerates anything up to about 2.4 m before the rails start to sag. Closeboard with heavy arris rails can stretch to 3 m. Going wider than the timber wants saves posts and costs you a fence that visibly bows within two winters. On a slope, panels either step down in fixed jumps or the fence is built raked with rails following the ground — the two need different post lengths, and stepping wastes the most timber.
What holds the fence up is the hole rather than the post. The rule of thumb, bury a third, exists because a fence post is a lever: wind pushes at the top and the buried section resists in two directions at once, so shallow posts do not fail by pulling out, they fail by rocking until the hole widens. That is also why a hole should be a straight-sided cylinder about three times the post width rather than a cone, because a cone-shaped plug of concrete works its way loose. A gravel base under the post lets water out of the end grain, which is the difference between a post that rots in six years and one that lasts fifteen.
What people use it for
- Pricing materials for a garden fence
- Working out posts for a paddock or field fence
- Checking a fencing quote line by line
- Planning a picket fence with an even gap
- Working out how much postcrete each hole takes
- Setting posts for a gate, a pergola or a carport
- Replacing a run of posts that have rotted at ground level
Questions
Eight feet or 2.4 m for rail fencing, exactly the panel width for panel fencing, and up to 3 m for heavy closeboard.
Fourteen at 8 ft spacing: thirteen sections plus the closing post. Add one for each corner and two for each gate.
Round the sections up and make the last bay narrower, or spread the difference across every bay. Never stretch one bay past what the rails can span.
One per section, and the post spacing has to equal the panel width — usually 1.83 m — because a panel cannot be cut narrower without losing its frame.
A third of the above-ground height. A 1.8 m fence wants 600 mm buried; a 2.4 m fence wants 800 mm.
About three times the post width: 300 mm across for a 100 mm post, and straight-sided rather than tapered.
About 0.03 cubic metres of mixed concrete for a standard 100 × 100 mm post, which is four 20 kg bags of postcrete — a 20 kg bag yields roughly 9 litres once mixed, so one bag fills under a third of the hole. Six to eight bags for a gate post or an exposed corner.
Yes. A gate post carries a constantly moving load and should be a heavier section, set deeper, ideally with a concrete collar.
Both. Gravel at the base for drainage, concrete around the sides for stability, and a slight dome on top so water sheds away from the timber.
For a light fence in firm ground, yes. For anything over 1.2 m or on an exposed site, concrete is the only thing that holds.
Two for a fence up to 1.2 m, three above that, four for closeboard over 1.8 m. Too few rails and the boards cup.
Either step the panels down in fixed jumps, which needs longer posts and wastes timber at each step, or rake rail fencing so the rails follow the ground.