About the Fence Calculator
A wooden fence is one of the most common outdoor building projects, and it is straightforward once the materials are clear. Most fences share the same structure: posts set in concrete at regular intervals, horizontal rails between them, and vertical boards or pickets fixed to the rails. Panel fences replace the rails and boards with ready-made panels, but the posts and concrete are the same.
The quantities all follow from the fence's length, its height and the spacing of the posts. This fence calculator works out the posts (including gate posts), the rails, the boards or pickets, the length of post to buy, and the concrete needed to set them.
How to Use the Fence Calculator
Choose imperial or metric.
Enter the fence length — the total run, including any gates.
Enter the height and the post spacing. 8 ft (2.4 m) is common; closer spacing makes a stiffer fence.
Enter the number of gates. Each adds a post, because a gate needs a post on both sides.
Enter the actual post width, the board width and the gap between boards — zero for a closed privacy fence.
How the Materials Are Worked Out
bays = length ÷ post spacing, rounded up
posts = bays + 1 + gates
rails = bays × (2, or 3 if taller than about 5 ft / 1.5 m)
boards = length ÷ (board width + gap), rounded up
hole = 3 × post width across, max(height ÷ 3, 24 in / 600 mm) deep
concrete = posts × (hole volume − buried post)Step-by-Step Example
A 100 ft fence, 6 ft high, posts at 8 ft, no gates, 4×4 posts (3½ in), 5½ in boards with ½ in gaps.
Bays: 100 ÷ 8 = 12.5 → 13 bays
Posts: 13 + 1 = 14 posts, 8 ft long
Rails: 13 × 3 = 39 rails
Boards: 1200 in ÷ 6 in = 200 boards
Holes: 10.5 in wide × 24 in deep
Concrete: 14 × 1.03 ft³ = 14.5 ft³ → 25 × 80 lb bagsPosts: Length and Depth
A common rule is to bury a third of the post's height above ground, with a minimum of about 2 ft (600 mm). A 6 ft fence therefore needs posts about 8 ft long, set 2 ft deep. In frost-prone areas, the bottom of the hole should reach below the frost line so the posts are not heaved out of the ground.
Holes are typically about three times the post's width across — 10 to 12 in for a 4×4 — so there is a good collar of concrete all round. A few inches of gravel in the bottom helps drainage, and sloping the concrete away from the post at the top sheds water.
Rails and Boards
Fences up to about 5 ft (1.5 m) usually have two rails per bay; taller fences need three to stop the boards bowing. Rails are typically 2×4s (38 × 89 mm) laid flat or on edge between the posts.
Boards can be spaced for a picket fence, butted tight for a privacy fence, or overlapped in a shadowbox or board-on-board style, which needs about half as many boards again because each one overlaps its neighbours. For a board-on-board fence, run the calculator with a gap of zero and add about 50% to the boards.
Gates
Each gate needs a post on each side, and gate posts take more load than the others, so they are often a size larger and set deeper. Gates also need hinges, a latch and often a diagonal brace running up from the bottom hinge side to stop them sagging.
Planning and Permission
Before building, check where the boundary is — ideally from the deeds or a survey — and whether your neighbour needs to agree. Many areas limit fence heights without permission: in England, 1 m next to a road and 2 m elsewhere; many US towns allow about 4 ft at the front and 6 ft at the back. Always have buried services located before digging post holes.
Panels Instead of Boards
Panel fencing, common in the UK, uses ready-made panels — usually 6 ft (1.83 m) wide — slotted between concrete or timber posts. The posts are one per panel plus one, so the count is the same as above with a post spacing equal to the panel width plus the post; concrete gravel boards along the bottom keep the panels off the soil. For a panel fence, ignore the rails and boards and use the number of bays as the number of panels.
Treating and Finishing
Use posts rated for ground contact, whatever the rest of the fence is made of, because the buried section is where rot starts. Cut ends of treated timber should be re-treated with an end-grain preservative. A stain or paint applied once the wood has dried out protects it from the weather and can be renewed every few years.
Setting Posts Straight
Run a string line between the end posts first, and set every post to it, checking each one for plumb on two adjacent faces with a level. Brace the posts while the concrete sets — usually a day for ordinary mixes — before hanging rails or boards.
Understanding Your Result
Materials gives the posts, rails and boards.
Posts gives the bays, post length and burial depth.
Rails gives the rails per bay.
Post concrete gives the hole size, volume and bags.
Worth knowing covers permission and buried services.
When Should You Use This Calculator?
Planning a new fence or replacing an old one.
Comparing post spacings or fence heights.
Ordering concrete for setting posts.
Pricing a fencing job.
Common Mistakes
Forgetting gate posts. Each gate adds one.
Posts too short. Bury a third of the height.
Two rails on a tall fence. Use three above about 5 ft.
Ignoring the frost line. Posts can heave.
Digging without locating services. Call before you dig.