Steel stairs: tread and stringer calculator

Give the rise to climb and the going, and the calculator returns the number of steps, the actual riser, the going in plan, the stringer length and the whole cutting list.

A worked example: 2400 rise, 900 wide

A flight to the next floor: 2400 mm of rise, 900 mm wide, a target riser of 180 mm, a going of 280 mm, stringers and tread frames in 60×40×2.

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.

Thirteen steps over 2400 mm give an actual riser of 184.6 mm, not the 180 asked for. That is how it should be: the height is fitted to a whole number of steps, not the other way round.

Equal steps are safety, not neatness

People climb stairs without looking: the foot memorises the height by the second or third step. A step that differs by a centimetre breaks that rhythm, and that is exactly the one people trip on. So:

• Every riser equal. A spread of more than 5 mm in a flight is a defect, not a tolerance.
• The last step does not take up the remainder. If the height does not divide evenly, change the number of steps, not the height of the last one.
• The top landing is a step too: its level has to fall in the same rhythm.

Stringers

• A three-to-five-step entrance — 60×40×2 is enough.
• A full flight — 100×50×3 or a 100–120 channel. For the same mass a channel is stiffer in bending, which is why it is used.
• Treads — a frame of 40×40 angle carrying grating, plate or timber. Solid 4 mm plate over the whole flight adds around a hundred kilograms and is almost never needed.

Welding settings

There are more joints here than in any other project in this section: about eighty in a thirteen-step flight. They are all identical, and this is the case where a jig and production-line welding pay for themselves. 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

The example above — 2400 mm rise, 900 mm width, 60×40×2 tube — breaks down like this:

• Two stringers of 4360 mm. That is the hypotenuse of a 3640 mm going (13 treads of 280) and a 2400 mm rise. The flight pitch is about 33°.
• Tread frames: 13 front bars of 900 mm and 26 side pieces of 280 mm.
• 27.7 m of tube in total at 3.01 kg per metre — about 84 kg without treads.

Piece by piece: a stringer weighs about 13 kg, one tread frame (front bar plus two sides) about 4.4 kg. Without treads the whole flight is a two-person carry, which is a good reason to weld it in the workshop and bring it to site finished. A 4 mm plate deck over the 3.28 m² of the flight would add about 103 kg. The 80 joints and 16 m of weld take about 0.3 kg of wire or 0.44 kg of electrodes. A 4360 stringer cut from a 6 m bar leaves a 1640 offcut, which gives five side pieces; the full layout is easiest in the cutting optimiser.

Tools you need

• A welder good for 90–110 A on MIG or 70–90 A with a 2.5 rod — plenty for a 2 mm wall.
• An angle grinder or chop saw: a staircase has only two angled cuts — the top and bottom of each stringer; everything else is square.
• A step template — a plywood triangle with legs of 184.6 and 280 mm. You mark the stringer with it, not with a tape from one step to the next.
• A flat base longer than the stringer: two trestles and a beam or the workshop floor, plus clamps, a square and a level.
• Anchors for the base plate and the top fixing.

Marking the stringers and tacking

The main trap in marking out is cumulative error. Measure each step from the previous one and a millimetre per step becomes a centimetre by the top of the flight. So:

1. Work out the step along the stringer: 4360 mm over 13 steps is about 335 mm.
2. Take every mark from one zero — the bottom of the stringer: 335, 671, 1006… — never from the last mark.
3. Mark both stringers together, side by side with the ends aligned: a step that is skewed between the stringers shows up at once.

Assembly starts with the two stringers on a flat base: set them parallel at the flight width, check the diagonals of the rectangle across the end steps and clamp. Then tack the bottom and top tread frames — they fix the geometry — and after them the rest. Tack each frame at two points on each side and check it with a level across straight away: a tread that tips back or forward cannot be corrected once it is welded out.

Step-by-step order with settings

1. Cut to the list. Stringers, front bars, sides; deburr and clean the weld areas to bright metal.
2. Angled stringer cuts. The bottom is cut level so the stringer stands flat on the floor, the top to bear on the landing or slab.
3. Tacks to the marks as above, levelling every tread.
4. Welding out. 2 mm wall throughout, one setting: 0.8 mm wire, 90–110 A, 17–19 V, or a 2.5 mm rod at 70–90 A. If the stringers are heavier — 100×50×3, as recommended for a full storey — the setting still follows the thin wall of the frame, with the arc aimed at the stringer. The frame-to-stringer joint is a fillet weld, so fillet weld technique is what matters here.
5. Supports. At the bottom, a plate for anchors welded all round to the stringer ends; at the top, a bearing angle or cleat to the slab.
6. Clean, prime, paint, then fit the treads or boards.

How to keep the stringers straight

Eighty joints, all on one face of the stringer — the inside. Weld the treads in order from the bottom up and the inside welds pull the stringer into a curve, so the flight closes inwards. What works against it:

• Alternate stringers: a tread on the left, the same tread on the right.
• Start in the middle of the flight and work towards the ends rather than from the bottom.
• Keep the stringers clamped to the base until they are fully cold; check the spacing with a tape at the top, bottom and middle.
• Joint faces in turn — top of the bar, bottom, then the ends — not one continuous weld round the joint.

Painting and finishing

A staircase is touched by hands and feet, so welds in reach of a palm or a foot are ground flush with a flap disc. After that the usual order: degrease, anti-corrosion primer, two coats of enamel, welds and edges brushed separately first. On outdoor steps cap the open tube ends — water inside a stringer rusts it from within. Chequer plate or grating is slip-resistant on its own; smooth plate or a painted tread outdoors is slippery, especially in winter.

How long it takes

Of all the projects in this section, this one has the most welding: 80 joints and 16 m of weld against 32 joints on the carport. That is why a jig pays for itself — a stop that each tread frame drops into without trial fitting. Where the time goes:

• Cutting 41 pieces and deburring — a real share of the job, even though nearly every cut is square.
• Marking and tacking — the stage that matters most; rushing here costs more than rushing the welding.
• Welding out, alternating stringers, with pauses to cool.
• Grinding welds where hands and feet go, primer, paint — with drying times as per the product sheet.

The different designs — stringer, central spine, spiral — are reviewed on the steel stairs page.

What usually spoils the job

Beginner mistakes on a staircase

• Rise taken from unfinished floors. The rise is measured from finished floor below to finished floor above. A forgotten screed or tiles makes the first or last step a different height.
• Treads left out of the sums. If a board goes on every frame, cut the bottom of the stringer down by its thickness. Otherwise the first step comes out higher than the rest by that thickness, and the last one lower.
• Marking with a tape from step to step. The error adds up; take every mark from one zero.
• One stringer finished first. It curves, and the second no longer matches. Weld the stringers alternately.
• A "make-up" top step. The leftover height does not go into the last step — change the number of steps instead.

Author: Dmitry Frolov, welder and metal fabricator Updated: 28 September 2026