Measure the line, not the panels
Run a tape along the fence line at ground level and follow the boundary rather than a straight chord between corners: a curved or stepped boundary is longer than it looks on a plan. Write down each straight run separately, because posts are set per run and every change of direction needs a post of its own. Add the gate openings to the same sketch: a 3 ft gate removes 3 ft of fence but still needs a post on each side.
Choose post spacing before anything else
Eight feet (2.44 m) is the North American default for timber panels; 1.8 to 1.83 m is the metric equivalent used in the UK, Germany and Australia. Wider spacing saves posts but lets rails sag under load, so keep the supplier maximum. The calculator divides each run into full bays at your spacing and adds one closing bay. If that closing bay would be shorter than a quarter of the spacing, the difference is shared across the run instead, so no bay looks like a mistake.
Count infill per bay, not per metre
One picket bay holds floor((bay width minus picket width) divided by (picket width plus gap)) plus one board. One panel bay holds exactly one panel. The last bay is usually shorter and needs its own count, which is why a 100 ft run at 8 ft centres gives 12 full bays of 24 pickets plus a 4 ft bay of 12, not 13 bays of 24.
Size the footing for your frost line
A 10 in hole 30 in deep takes about two 80 lb bags of concrete once a 4x4 post is in it; the same hole at 48 in takes three. In Canada and the northern US the post must sit below the frost line or it lifts with the first hard freeze, while 600 mm is normally enough in the UK and Australia. The calculator subtracts the post volume from the hole and never returns less than 35% of the hole volume.
Add waste where it is actually needed
Order 5 to 10% extra pickets and fixings: boards split, knots fall out and a few cuts go wrong. Do not add waste to panels, posts or gates, because these are bought as whole units and a spare panel rarely goes to waste. The waste field applies to pickets and fixings only.
Check the rules before you dig
In the US, call 811 before digging so utilities can mark the line. In the UK, anything over 2 m next to a boundary or highway usually needs planning permission. In New Zealand the Fencing Act 1978 sets who pays for a boundary fence, and in Germany the Nachbarrecht of each state decides height and position. The country notes under the calculator carry the details; they are guidance, not legal advice.