Fence Post Spacing & Wind Load Calculator
Enter fence height, panel type, and wind exposure to get the maximum safe post spacing, footing depth, and how much concrete you need. Nothing uploaded.
Fence elevation
Learn more: fence post spacing and wind load
How the spacing is worked out
The calculator treats each post as a cantilever fixed at the ground. Wind pressure is 0.5 x 1.2 x speed squared, with the speed set by the wind zone (18 m/s sheltered up to 36 m/s severe), and it is multiplied by the panel's drag coefficient: 1.3 for close-board, 1.2 for solid panel, 0.7 for trellis and 0.4 for open rail.
One bay of width L and height h catches a force of pressure x h x L at mid-height. That gives a moment at the base of pressure x L x h squared / 2. The calculator sets this equal to the post's bending strength divided by a safety factor of 1.5, then solves for L, capped at 3 m.
A worked example
Take the defaults: a 1.8 m close-board fence in a normal (24 m/s) zone on 100 x 100 mm softwood posts. The pressure is 345.6 x 1.3 = 449 Pa, and the posts can be 1.22 m apart. The same fence in an exposed zone drops to 0.78 m, and open rail at the same height reaches the 3 m cap.
Spacing falls with the square of the height, so a taller fence needs posts much closer together. Hollow steel is treated using its listed section properties, not as a solid bar, and concrete uses its bending (tensile) strength of about 3 MPa, not its compressive strength.
Footings and post count
Footing depth is a third of the fence height plus 3 cm per metre of spacing, with a minimum of 450 mm. At the defaults that is 631 mm, and a 300 mm hole takes about 44.6 L of concrete.
Post count is the run divided by the spacing, rounded up, plus one, so no bay is wider than the maximum. A 20 m run at 1.22 m needs 18 posts and about 32 bags of 25 L concrete. ScoutMyTool's fence post spacing guide uses a different rule, frost depth plus 6 inches, so check your local frost depth and building rules too.
FAQ
Why is the spacing capped at 3 metres?
The calculator applies a practical ceiling whatever the load calculation says. ScoutMyTool notes that 8-foot spacing can risk sagging within 5 to 7 years compared with 6 feet, which is one reason wide bays are avoided.
Does a steel post allow wider spacing than timber?
Often, yes, because steel has a much higher bending strength. In the defaults, a 50 x 50 x 3 mm hollow steel post gives 1.42 m against 1.22 m for 100 x 100 mm softwood. The model is simplified, so check your local building rules.
Which wind zone should I pick if my fence is only sometimes windy?
Choose the zone that matches your worst realistic conditions, not the average day. Posts fail in storms and gusts, and a zone that is too mild gives wider spacing than is safe.