Steel fence: post and rail calculator
Give the length, the height and the post spacing, and the calculator counts posts, rails, bays, the mass of steel and the area of sheet.
A worked example: 20 metres, 2 m high
A typical run: 20 m of fence, 2 m high, posts every 2.5 m of 60×60×3 set 900 mm into the ground, rails of 40×20×2 and 0.5 mm profiled sheet.
| Item | Section | Length, mm | Qty |
|---|---|---|---|
| Post | 60x60x3 | 2900 | 9 |
| Rail | 40x20x2 | 2440 | 16 |
Mass is worked out without the corner radii of the section, so it comes out two to three per cent below the standard tables. No allowance is added for saw kerf or cutting errors: when buying, add one commercial length.
Note the two sections in the list: posts and rails are different, and that is not a detail. Rails made from the same 60×60 tube as the posts add over a hundred kilograms of steel to twenty metres of fence and buy nothing in strength.
Post spacing and the number of rails
2.5 m is not tradition, it is the size of the sheet: profiled sheet comes in strips that are cut per bay, and at that spacing there is almost no offcut.
- 2.5 m — the standard, two rails, no argument.
- 3 m — two posts fewer every thirty metres, but a 40×20 rail visibly sags over that span: you need a third rail or 40×40.
- From 2 m of height a third rail is needed at any spacing: the sheet starts to drum in the wind, and what holds it is not the stiffness of the post but the middle rail.
Wind load. A solid fence 20 × 2 m is forty square metres of sail, and in open country that is a different conversation from a street between houses. On an exposed plot the spacing comes down and the section goes up — a decision made on site, not by a calculator.
Posts
- Concrete all of them, not every other one: wind acts on the whole plane evenly.
- Embedment of 0.9–1.2 m, below the frost line.
- Cap the tops — an open tube fills with water and splits along the seam in winter.
- Weld the rails only once every post is lined to a string, plumb, and the concrete has set.
Welding settings
Two different sections are welded here: a 3 mm post and a 2 mm rail. Set the current for the thin part — the rail — or it will burn through before the post is even warm. The values are the same as on the welding square tube page; the calculator above shows the row for the wall thickness you chose.
| Wall, mm | Section | MIG/MAG | MMA |
|---|---|---|---|
| 1.2 | 20×20, 20×40 | 0.8 mm · 50–70 A · 16–17 V | 2.0 mm · 50–65 A, stitch welding |
| 1.5 | 40×20, 40×40 | 0.8 mm · 70–90 A · 17–18 V | 2.0 mm · 55–70 A, stitch welding |
| 2.0 | 40×40, 60×40 | 0.8 mm · 90–110 A · 17–19 V | 2.5 mm · 70–90 A |
| 3.0 | 60×40, 80×40 | 0.8–1.0 mm · 120–140 A · 19–21 V | 3.2 mm · 90–120 A |
| 4.0 | 80×40, 100×50 | 1.0 mm · 140–170 A · 19–21 V | 3.2 mm · 110–140 A |
What usually spoils the job
| What went wrong | Cause | How to avoid it |
|---|---|---|
| The frame twisted | Welds run one after another on the same side | Alternate corners, weld in short runs |
| The diagonals came out unequal | Nobody checked them before tacking | Measure both diagonals twice |
| Burnt through the wall | Too much current for 1.2–1.5 mm | Turn it down, stitch it with pauses |
| The corner welded on one side only | Saving time | Weld both sides wherever the joint takes load from two directions |
| Rust from the inside within a year | Open ends | Caps, or weld them shut |
Frequently asked questions
What post spacing should I choose?
2.5 m is the standard that sheet and rails are cut to: the bay comes out a multiple of the panel and there is almost no offcut. 3 m saves posts, but a 40×20 rail sags over that span and a third one is needed.
Two rails or three?
Up to 1.8 m of height two are enough. From 2 m the sheet starts to drum in the wind and a third rail in the middle removes it completely. Tick the box in the calculator to see the difference in metres.
How much steel goes into 20 metres?
Twenty metres of 2 m fence at 2.5 m spacing: nine posts of 60×60×3 at 2.9 m and sixteen rails of 40×20×2 — about 65 m of section and 190 kg of steel, plus 40 m² of sheet.
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