How it is worked out
The fence length ÷ post spacing gives the bays, rounded up. There is always one more post than there are bays.
Rail is the fence length × the number of rails. Suppliers sell rails in set lengths, so match those to your post spacing when you order.
Each paling covers its own width less the overlap. The fence length ÷ that figure is the paling count, rounded up. With no overlap it is simply the fence length ÷ the paling width.
Corners, gates and steps down a slope are not included. Add a post for each corner and each side of a gate.
Worked example
The numbers this page opens with: fence length 20 m, post spacing 2.4 m, rails per bay 3 rails, paling width 100 mm, overlap between palings 0 mm, extra for splits and cuts 5 %.
The answer: bays 9 bays; posts 10 posts; rail 60.0 lineal m; palings, exact 200 palings; palings with 5% extra 210 palings.
Questions
What post spacing and height should I use?
That comes from your fence design, your supplier's rail lengths and your council's rules. This page counts materials once you have those numbers.
Does it include plinths, capping or nails?
No. Plinth and capping each run the full fence length, so order the fence length in lineal metres for each one you are using.