How Many Post and Rail Lengths Do I Need?
Quick Answer
Posts: fence length ÷ bay width + 1. Rails: 2–3 per bay.
Based on: standard 3m bays with 2 rails. A 15m fence = 6 posts + 10 rails (3m lengths)
How We Calculated This
Post and rail fencing uses horizontal rails mortised or nailed into upright posts:
- Bay width: typically 2.4m or 3m (matching standard rail lengths)
- Number of posts: fence length ÷ bay width + 1 (for the end post)
- Number of rails: number of bays × rails per bay (2 or 3)
Example — 15m fence with 3m bays and 2 rails:
- Bays: 15 ÷ 3 = 5 bays
- Posts: 5 + 1 = 6 posts
- Rails: 5 × 2 = 10 rails
Standard Sizes
- Posts: 100 × 100mm or 125mm diameter round, 1.5–1.8m long (600mm in ground)
- Half-round rails: 87mm or 100mm, 3m or 3.6m lengths
- Sawn rails: 75 × 38mm or 100 × 38mm
When to Use 3 Rails
Use 3 rails for fences over 1.2m high, for stock-proof fencing, or when you plan to attach mesh or netting. Two rails are sufficient for decorative boundary fencing up to 1.2m.
Tips
Use pressure-treated (UC4 rated) timber for ground-contact posts. Set posts in postcrete or concrete at a minimum depth of 600mm. Rails should be nailed or screwed from the field side (the side you want to look best).
Last updated: April 2026. Buy 5–10% extra rails to allow for cuts at corners and changes of direction.