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.
| Item | Section | Length, mm | Qty |
|---|---|---|---|
| Stringer | 60x40x2 | 4360 | 2 |
| Tread, front | 60x40x2 | 900 | 13 |
| Tread, side | 60x40x2 | 280 | 26 |
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.
2h + b = 600…640 mm, where h is the riser and b the going.
At 185 and 280 that gives 650: a little steep, acceptable outdoors. Indoors 170
and 290 give 630 and climb noticeably more easily. The calculator shows both
figures.
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.
| 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
Why does the actual riser differ from the one I asked for?
Because the number of steps is a whole number. A 2400 mm rise with a 180 mm target riser gives 13.33 steps: the calculator rounds to 13 and the actual height becomes 184.6 mm. That is how it is done — every step must be equal, and a last step of a different height is the commonest cause of falls.
What ratio of going to riser is right?
The comfortable-pace rule: 2h + b should land between 600 and 640 mm. At 180 and 280 that is 640, the upper limit. At 200 and 250 it becomes 650 and the flight is steep.
What are stringers made from?
For a three-or-four-step entrance 60×40×2 is enough. For a full flight use 100×50×3 or a 100–120 channel: the stringer works in bending along its whole length, and there tube loses to channel at equal mass.
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