Stairs look simple until you try to build one. A staircase that is off by even a few millimetres on each step ends up with a top step that is too tall, a bottom step that trips everyone, or a stringer that does not sit flat on the floor. The good news is that stair layout is mostly arithmetic, not talent. If you can measure a total height, do a bit of division, and mark lines carefully, you can lay out a stringer that fits the first time.
This guide walks through the whole process: measuring the rise, working out how many steps you need, figuring out the run, and marking and cutting a stringer. It assumes a straight run of stairs, like a deck stair or a set of garage steps. Winder stairs and curved stairs are a different job.
Start with the total rise
The total rise is the vertical distance from the finished surface you are standing on to the finished surface you are stepping up to. Finished surface matters. If the deck boards are not down yet, or the flooring at the top is not installed, measure to where those surfaces will be, not where the bare framing is today. A stair built to unfinished heights will be wrong the moment the flooring goes in.
Measure in at least two places, left and right, because floors are rarely perfectly level. If the two measurements differ, use the larger one for your calculations and plan to scribe or shim the low side. Write the number down in millimetres or inches, whichever you are comfortable working in, and stick to one unit for the whole job. Mixing units halfway through is how a stringer ends up a centimetre out.
Work out the number of steps
Building codes generally allow a riser height between about 150 mm and 200 mm (roughly 6 to 8 inches), with most comfortable stairs landing around 170 to 190 mm (about 7 to 7.5 inches). Your job is to divide the total rise into equal steps that fall inside that range.
Take your total rise and divide it by 175 mm (7 inches) as a starting point. Round the result to the nearest whole number — that is your step count. Then divide the total rise by that step count to get the exact riser height. For example, a total rise of 2100 mm divided by 175 gives exactly 12, so each riser is 175 mm. A total rise of 2050 mm divided by 175 gives 11.7, which rounds to 12 steps, and 2050 divided by 12 gives a riser height of about 171 mm. That is a comfortable, even staircase.
If the result falls outside the comfortable range, adjust the step count by one and recalculate. Never fudge a single step to absorb the remainder. One odd-height step is the one everyone trips on, because feet learn the rhythm of a staircase and expect every step to be the same.
Understand the run
The run is the horizontal depth of each tread, usually 250 to 280 mm (about 10 to 11 inches) for a comfortable stair. The tread itself is usually a little deeper than the run because of the nosing, the part that overhangs the riser below. A nosing of 20 to 25 mm is typical.
The total run is the number of treads times the run per step. Note the detail that catches beginners out: a staircase with 12 risers has 11 treads if the top step is the landing or deck itself. You step onto the landing as the final step, so you do not need a tread there. Count your treads carefully before you mark the stringer, or the whole staircase will land one tread too far out.
A handy check is the old carpenter’s rule of thumb: twice the riser plus the run should land somewhere around 600 to 650 mm (about 24 to 25 inches). A 175 mm riser with a 260 mm run gives 610 mm, which feels natural underfoot. If your numbers land far outside that range, the stair will feel either cramped or like climbing a ladder, even if every individual measurement looks legal.
Marking the stringer
You will need a framing square, a pair of stair gauges (the small brass or plastic clamps that lock onto the square), a pencil, and a length of straight, dry lumber for the stringer, typically a 2 by 12. Check the board first. A stringer board with a big crown, a twist, or a knot right where a tread notch will go is a problem waiting to happen. Pick the straightest board you have.
Set the stair gauges on the framing square: one arm at the riser height, the other at the run. Clamp them firmly. A gauge that slips halfway through marking will quietly ruin every step after it.
Lay the square on the stringer with the gauges pressed against the edge of the board, and trace the first step: a vertical line for the riser, a horizontal line for the tread. Slide the square along so the gauges sit against the edge again at the next position, and repeat. Work your way down the board, one step at a time, keeping the gauges tight against the edge every time. Number each step lightly in pencil as you go. It is easy to lose count on a long stringer, and counting is much harder to fix after the saw comes out.
Do not forget the adjustments at the ends. At the top, the stringer usually drops by the thickness of the tread material if the top tread sits on the stringer, or it is cut to sit flush under the landing — check how your stair connects before you cut. At the bottom, the first riser must be shortened by the thickness of the tread that will sit on the floor, because the floor itself acts as the first tread surface. Skip this and your bottom step will be one tread-thickness taller than all the others, which is exactly the kind of uneven step that causes falls.
Cutting the stringer
Cut with a circular saw along the marked lines, but stop just short of the corners. A circular saw cuts past the corner on a notch, and that overcut weakens the stringer right where it carries the most load. Finish each corner with a handsaw or a jigsaw, cutting precisely to the line. It takes a few extra minutes and it is the difference between a stringer that lasts decades and one that cracks at a tread notch.
Cut one stringer first and test-fit it before you cut the rest. Hold it in place, check that it sits flat on the floor and tight against the landing, and walk the steps with a level. Check the riser heights with a tape measure. If the first stringer is right, use it as a template to trace the others. If it is wrong, you have only spoiled one board, and you know exactly what to fix.
Common mistakes that ruin a staircase
Uneven risers are the biggest one. Even a 6 mm difference between steps is noticeable underfoot, and a difference of 10 mm or more is a genuine tripping hazard. This usually comes from rounding errors in the division or from forgetting the bottom-step adjustment.
Measuring to unfinished surfaces is the second classic error. Always measure to finished floor levels at both the top and the bottom, including tile, decking, or any underlay that will be added later.
A third mistake is trusting a single measurement of the total rise. Floors slope. Measure both sides, and if the slope is significant, consider levelling the landing or splitting the difference deliberately rather than discovering the problem when the stringer is already cut.
Finally, do not rush the marking. A stair layout rewards patience and punishes hurry. A square that drifted, a gauge that slipped, or a miscounted step will not reveal itself until the stringer is cut, and by then the fix is a new board.
A quick checklist before you cut
Total rise measured to finished surfaces, in two places. Step count worked out so every riser is equal and inside the comfortable range. Run chosen so the stair feels natural, with the two-times-riser-plus-run check done. Top and bottom adjustments accounted for. First stringer cut, test-fitted, and used as the template for the rest.
Get those five things right and the staircase practically builds itself. The steps will feel even, the stringer will sit flat, and nobody will ever notice the work — which, for a staircase, is the highest compliment there is.