FIELD CALCULATIONS / WORKED EXAMPLE
Equal spacing between posts: gaps versus centres
By Eutopix Labs · 9 September 2026
Equal clear gaps are the empty spaces between objects. Centre-to-centre spacing includes the width of an object. Decide which measurement the drawing specifies before dividing a run.
Example: six posts across a 120-inch outside span
Assume six posts, each 3½ inches wide, with the first and last outside faces defining the 120-inch span. There are five gaps between the posts and no extra end gaps.
- Total post width: 6 × 3½ = 21 inches.
- Space left for gaps: 120 − 21 = 99 inches.
- Divide by five gaps: 99 ÷ 5 = 19.8 inches per clear gap.
- Centre spacing: 19.8 + 3.5 = 23.3 inches.
Check the layout
Five centre intervals cover 116.5 inches. Add half a post at each end, 1.75 + 1.75 inches, to recover the original 120-inch outside span. This is a useful independent check of the endpoints.
What changes if the endpoints are centres?
If 120 inches is the distance between the first and last post centres, divide 120 by five intervals instead. The result is 24 inches centre to centre, with a 20½-inch clear gap for 3½-inch posts.
Round at the layout stage
A decimal-inch result may fall between marks on a tape. Choose a suitable marking precision and check cumulative positions from the same reference, rather than repeatedly adding a rounded gap.
Use the equal-spacing tool
Onsite Math includes equal-spacing calculations for field work. Check object count, actual width and endpoint assumptions before applying the answer. These are layout examples, not guardrail, structural or code requirements.