A fence is a long run of identical boards, and the count looks like it should be simple: divide the fence length by the board width. But the gap between boards changes the answer, and forgetting the gap is how people end up short on the last section. Posts and rails change the shopping list too, and concrete for the posts is easy to forget entirely. This estimator counts everything at once: boards, posts, rail length, and concrete bags, from six simple inputs.
Boards are counted by pitch, not by width alone. The pitch is the board width plus the gap, and the fence length divided by the pitch gives the board count. With 15 cm boards and a 1 cm gap, the pitch is 16 cm, so a 20 metre fence needs 125 boards. With no gap at all, the same fence needs 134 boards. That difference of nine boards is exactly the shortage that sends people back to the timber yard, so measure the gap you actually plan to use, including the thickness of your spacer if you use one.
Posts are counted from the spacing, plus one for the end. A 20 metre fence with posts every 2 metres needs 11 posts, not 10, because both ends need a post. Shorter spacing gives a stronger fence but costs more posts and concrete, while wider spacing saves money and can let the fence sag or lean in wind. Two metres is a common spacing for timber fences; check local practice for tall fences or windy sites, and always add a post on each side of a gate.
Rails are the horizontal timbers the boards fix to, and they are bought by length. Two rails per section suit fences up to about 1.5 metres, while taller fences usually need three rails to stop the boards bowing. Our example fence with two rails needs 40 metres of rail timber in total. Rails are usually sold in fixed lengths, so divide the total by the length you can buy and round up, and allow a little extra for joints that must land on a post.
Concrete for posts is estimated here as one 20 kilogram bag per post, which is a planning assumption, not a product specification. Real use depends on the hole size and the post size: a wide hole in loose soil takes more, and a narrow hole in firm soil takes less. Read the bag instructions for the hole size it covers, and buy post concrete rather than general concrete if you want a fast set that lets you finish the fence the same day.
Measure the fence line on the ground, following slopes rather than measuring a straight line on a map, because a slope adds length. Walk the line and mark corners, gates, and places where the ground changes height, since each corner needs its own post and each gate needs a pair of stronger posts. Buy a few spare boards in the same batch: timber shades vary, and a split board next year is a quick fix if you kept spares.
Remember that this is a planning estimate, not a professional quote. It does not allow for gates, gravel boards, post caps, screws, or treatment and paint, so add those separately. Boundary fences may need agreement with neighbours and may be limited by local rules on height and position, so check before you dig. For a long or tall fence, show your numbers to the timber yard and ask them to check the board and post counts. Check the ground for buried cables and pipes before you dig any post hole.