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.

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.

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.

Materials and weight, part by part

For 20 m of fence, 2 m high with posts every 2.5 m, the calculator gives:

• 60×60×3 posts — nine of 2900 mm (2000 above ground plus 900 in concrete), 26.1 m at 5.37 kg per metre.
• 40×20×2 rails — sixteen of 2440 mm: the rail sits between the posts, so it is shorter than the spacing by the width of a post. 39 m at 1.76 kg per metre.
• 0.5 mm profiled sheet — 40 m², about 180 kg.

The frame is about 209 kg, about 389 kg with the sheet. Piece by piece: a post weighs about 16 kg and a rail about 4.3 kg, so a fence can be built single-handed once the posts are in. A third, middle rail adds eight pieces, about 34 kg and 16 joints — tick the box in the calculator and the list updates. The 32 joints and 3.8 m of weld take about 0.07 kg of wire or 0.11 kg of electrodes: a fence has the least welding per metre of any project here.

Tools you need

• A welder: 90–110 A on MIG or 70–90 A with a 2.5 rod is enough for the rails; post caps on a 3 mm wall want up to 140 A or a 3.2 rod.
• An extension lead — a fence line is always a long way from a socket. On a long, thin cable the welder loses voltage and current; the voltage drop calculator picks the cross-section for the length.
• Setting out: string line, 20–30 m tape, spirit level, a storey rod marked with the rail heights, and a laser level if you have one.
• For the posts: an auger, gravel for the base, concrete, temporary braces.
• An angle grinder with cutting and flap discs, clamps, a magnetic square.

Setting out and tacking

A fence starts with the end posts. Set them first, plumb in both directions, and stretch two lines between them — one near the top, one near the ground. Line every intermediate post up to the strings rather than plumbing each one on its own: that way the run comes out straight even if a single level reading is a little off.

• Post positions are measured from the first post with one tape: 2500, 5000, 7500… — never from the previous post. Otherwise the error builds up and the last bay ends up a hand's width shorter or longer than the sheet.
• Rail heights are marked with the storey rod from one line — the string or the laser beam — not with a tape from the ground: the ground is uneven and the rails will wave. The top rail sits 200–300 mm below the top of the sheet.
• Each rail goes between the posts, is clamped and tacked top and bottom at both ends. Then a level on the rail, and only then the full welds.

Step-by-step order with settings

1. Posts in concrete. No welding. Weld the rails once the concrete has set and the posts no longer move: a rail welded to a post in fresh concrete drags it along.
2. Post tops cut to one line; on a slope, step them bay by bay.
3. Rails. Set up for the thin rail wall: 0.8 mm wire, 90–110 A, 17–19 V, or a 2.5 mm rod at 70–90 A, arc aimed at the post. The side faces of the joint are vertical welds; the rail is thin and not load-bearing, so it can be run vertical down with a short arc. The underside is an overhead weld — do it last.
4. Post caps — 3 mm wall: 0.8–1.0 mm wire, 120–140 A, 19–21 V, or a 3.2 mm rod at 90–120 A. Plastic caps are an alternative.
5. Clean, prime, paint — before the sheet goes on.
6. Profiled sheet — fixed to the rails with roofing screws.

How to keep posts and rails straight

On a fence it is less the frame than the posts that move: weld every rail on the same side in sequence and each weld pulls its post over, so by the end of the run the posts lean like a herringbone. The order that prevents it:

• Tack the whole run first, check the posts against the string, then weld.
• Every other bay: first, third, fifth… then second, fourth.
• Joint faces in turn: top, bottom, then the sides — not all the way round in one pass — so the thin rail does not bow.

Painting and rust protection

The usual order: knock off spatter, dress the welds, degrease, anti-corrosion primer, two coats of enamel, welds brushed separately. Two things are particular to a fence. First, paint before the sheet goes on: the face of the rails behind the sheet cannot be reached later. Second, the ground line: a post rusts first where it leaves the concrete, because that is where moisture sits. Give that zone an extra coat, and coat the buried part with bitumen before concreting. Weld or cap the post tops — an open tube fills with water.

How long it takes

20 m of fence is 32 joints and 3.8 m of weld. The welding is short; the time goes into the ground:

• Setting out and digging nine post holes — the heaviest stage physically.
• Setting and concreting the posts, then a break while the concrete sets.
• Rails — a quick stage if the pieces are cut beforehand and the storey rod is marked.
• Painting and sheeting — with drying times as per the product sheet.

A gate in the same fence is worked out separately — the gate calculator adds its posts to the list too. Typical fencing designs are reviewed on the metal fences page.

What usually spoils the job

Beginner mistakes on a fence

• Posts plumbed, no string. Each one is vertical and the line wanders. Intermediate posts go to the string between the end posts.
• Measuring post to post. The error adds up and the last bay does not match the sheet. Take every mark from the first post.
• Rails measured up from the ground. They follow the ground and the sheet waves. Mark from the string or the laser.
• Rails welded to posts in fresh concrete. The post follows the weld.
• Sheet before paint. Bare rails stay hidden behind the sheet, and they rust first.

Author: Dmitry Frolov, welder and metal fabricator Updated: 27 September 2026