How the fence calculator works
A straight fence is a repeating pattern – post, section, post, section – with one extra post to close the run. The calculator divides the length by the post spacing, rounds up to whole sections, adds one post, and multiplies the sections by the number of rails. For the concrete it treats each hole as a cylinder, subtracts the space the post occupies and adds 10%.
Sections = ⌈length ÷ spacing⌉ · Posts = sections + 1 · Concrete per hole = (π × (diameter ÷ 2)² − post area) × depthThe post is taken as 100 × 100 mm in metric mode and a nominal 4×4 (3.5 × 3.5 in actual) in imperial. Bag yields are 10 litres per 20 kg bag of post mix, and 0.375 ft³ per 50 lb bag of fast-setting concrete. If you use 80 lb bags of general concrete mix, each yields about 0.6 ft³.
Worked example
A 25 m boundary with posts every 1.8 m: 25 ÷ 1.8 = 13.9, so 14 sections and 15 posts. With three rails per section that is 42 rails. Each 25 cm × 60 cm hole holds π × 0.125² × 0.6 = 0.0295 m³, of which the post fills 0.006 m³, leaving 0.0235 m³ of concrete. Fifteen holes need 0.352 m³, or 0.387 m³ with the margin: 39 bags of 20 kg. In feet, an 80 ft run at 8 ft centres gives 10 sections and 11 posts; each 10 in × 24 in hole takes 0.92 ft³ once the post is subtracted, 11.1 ft³ in total with the margin, or 30 bags of 50 lb.
Standard fence sizes
| Item | Europe / UK | North America |
|---|---|---|
| Panel or section width | 1.83 m (6 ft) lap and closeboard panels; 2.0–2.5 m rigid mesh | 6 ft and 8 ft pre-built panels; 8 ft maximum for stick-built wood |
| Post size | 75 × 75 mm light panels, 100 × 100 mm most fences, 125 × 125 mm gates | 4×4 standard, 6×6 for gates and tall fences |
| Hole diameter | 25–30 cm | 10–12 in for a 4×4 |
| Buried depth | About 60 cm for a 1.8 m fence (2.4 m post) | 24–36 in, below the frost line in cold regions |
| Rails | 2 up to about 1.2 m tall, 3 above | 2 up to 4 ft, 3 for 5–6 ft privacy fences |
Corners, gates and slopes
Run the calculator once per straight run. An L-shaped fence of 12 m + 13 m at 1.8 m centres needs 8 + 9 posts minus the shared corner, so 16 – one more than a single 25 m run. Treat a gate opening as a gap: calculate the fence on each side separately, and fit heavier, deeper-set posts either side of the gate. On sloping ground, measure the horizontal distance; stepped panels keep their width, while racked fences follow the slope.
Common mistakes
- Using the panel width as the spacing. Panels sit between posts, so the centre-to-centre distance is the panel plus one post width. Forgetting this leaves the last panel short.
- Flat concrete tops. Finish the concrete slightly above ground and slope it away from the post so rain runs off instead of pooling against the timber.
- Digging without checking for services. Ask your utility locating service (811 in the US) before digging post holes.
- Ignoring wind and height rules. Tall solid fences act like sails. Local rules often limit height, and exposed or retaining fences deserve a professional's design.
Frequently asked questions
How many posts do I need for a 100 ft fence?
At 8 ft spacing a 100 ft run has 13 sections and 14 posts. At 6 ft spacing it has 17 sections and 18 posts.
How many bags of concrete per fence post?
A 4×4 post in a 10 in × 24 in hole needs about 0.92 ft³: roughly 2.5 bags of 50 lb fast-setting mix or 1.5 bags of 80 lb concrete. A 100 mm post in a 25 × 60 cm hole needs just under 2.5 bags of 20 kg post mix.
How far apart should fence posts be?
Wood privacy fences are usually built with posts 6–8 ft (1.8–2.4 m) apart. UK panel fences use 1.83 m panels, so posts end up about 1.93 m apart with 100 mm posts.
How deep should fence posts be set?
Bury a quarter to a third of the post: about 60 cm for a 1.8 m fence, or 24–36 in in the US. In cold regions go below the frost depth set by local code.
Does a fence need 2 or 3 rails?
Two rails are enough up to about 1.2 m (4 ft). Taller board fences need three rails so the boards stay straight and the middle does not bow.
Last reviewed: 2026-09-27 · How we calculate