Why the last fence bay should not be short
The instinct is to set posts every 8 feet and let the last gap fall where it may — which is how you end up with a 3 ft bay at the end of the run, a panel that has to be cut, and a fence that looks wrong from the street. The professional approach is the opposite: decide the number of bays first, then divide the run by that number so every bay is identical.
The two-line method
- Bays = run ÷ maximum spacing, rounded up. A 100 ft run with an 8 ft maximum gives 12.5, so 13 bays.
- Spacing = run ÷ bays. 100 ÷ 13 = 7.69 ft, which is 7 ft 8 in on centre. Fourteen posts, thirteen identical bays.
That spacing is always a little tighter than the maximum you chose, never looser — which is exactly what you want, because post spacing limits are about wind load and rail stiffness, not about round numbers.
Choosing the maximum spacing
Wood and composite fences are normally set at 8 ft; some builders go to 10 ft with thicker rails. Chain-link runs at 10 ft, aluminium panel systems at 6 ft, and vinyl panels must be spaced at the panel length (usually 8 ft) because the rails are built into the panel. If you are building on a slope, remember that the spacing is measured along the fence line, so a steep run needs more posts than the map distance suggests.
Hole depth and concrete
Bury roughly one third of the post's above-ground height, with a 24 in (60 cm) minimum, and keep going deeper if the frost line in your area is lower — frost heave is what makes fence posts lean after two winters. Hole diameter is usually about three times the post width: 8–10 in (20–25 cm) for a 4×4 in post. Volume is π r² × depth, and a 50 lb bag of fast-setting mix fills roughly 0.375 ft³ (0.0106 m³), so an 8 in × 30 in hole takes about three bags. Pour the dry mix, add water, and crown the concrete slightly above grade so rain runs off.
Slopes, corners and gates
Every corner and every gate post is a fixed point that the spacing has to respect: work out the bays between fixed points rather than across the whole run, and remember that a gate opening is normally one bay wide. On a slope, step the fence panels down rather than racking them, which keeps the posts vertical and the rails level.
Frequently asked questions
What is the best fence post spacing?
Eight feet on centre for wood, composite and vinyl panels, ten feet for chain-link, and six feet for aluminium panel systems. Then divide the run by that number of bays, so the spacing comes out slightly tighter and every bay is identical — 100 ft at 8 ft maximum becomes 13 bays at 7 ft 8 in.
How do I keep the last fence bay from being short?
Decide the number of bays first: bays = run ÷ maximum spacing, rounded up. Then spacing = run ÷ bays, and the last bay is the same as the first. Thirteen bays at 7.69 ft beat twelve bays at 8 ft plus one leftover bay of 4 ft.
How deep should fence posts be set?
One third of the above-ground height is the usual rule, with a 24 in (60 cm) minimum — so a 6 ft fence needs about 2 ft of post in the ground. In cold climates set them below the local frost line, which may be 3 ft or deeper, or the posts will heave and lean.
How many bags of concrete per fence post?
An 8 in × 30 in hole holds about 0.87 ft³, and a 50 lb bag of fast-setting mix fills roughly 0.375 ft³, so three bags per post. A shallower 24 in hole takes two. Use fast-setting mix: you can pour it dry, add water and walk away.
How far apart should posts be for a 6 ft fence?
Eight feet for wood with two rails, or 7 ft 8 in if you want 13 identical bays in a 100 ft run. Six-foot fences catch more wind than 4 ft fences, so do not stretch the spacing beyond 8 ft unless you add a third rail.
Does the spacing change on a slope?
The measurement does not, but the amount of post does: spacing is measured along the fence line, which is longer than the flat map distance on a hill. Step panels down the slope rather than racking them, and measure each bay individually if the grade changes.