How to lay out a stair stringer with a framing square

What do I need before I mark the stringer?

Four numbers and four tools. The numbers come from the stair, not the board:

• Total rise, finished floor to finished floor.
• Riser height: the total rise shared equally between the risers.
• Going: the level depth of one step, nosing to nosing.
• Number of treads: one fewer than the risers, because the upper floor is the last step. Note the tread thickness too, for the boards you will fix on top.

The tools: a framing square (a steel or roofing square), a pair of stair gauges (small clamps that lock onto the square's edges), a sharp pencil and a tape. Pick a straight board for the stringer and sight along it: lay out from the edge with the crown, so the slight bow faces up once the stringer is in.

How do I calculate the stringers for my stairs?

Divide the total rise into equal risers, then count one tread fewer. The stair stringer calculator on this site does it in millimetres or feet and inches. By hand, with a 2700 mm floor-to-floor rise and a 240 mm going:

1. Target riser: (600 − 240) ÷ 2 = 180 mm, from the 2R + G rule.
2. Risers: 2700 ÷ 180 = 15, so 15 risers of 180 mm. If the division doesn't come out whole, round to the nearest whole number of risers and divide again: the riser is always total rise ÷ risers, decimals and all.
3. Treads: 15 − 1 = 14 treads, each a 240 mm going.
4. Each step on the board: √(180² + 240²) = 300 mm along the line of the stair, at 36.87°.

Check the riser and going against your country's stair rules before you mark anything. For an Australian house the NCC allows risers of 115 to 190 mm and goings of 240 to 355 mm; 180 and 240 pass, with the going on the minimum. The stair maths guide's table quotes the limits for Australia, New Zealand, England and the US, with sources.

How do I mark each step with a framing square?

Set the square once, then step it up the board, one tread at a time:

1. Set the gauges. Clamp one stair gauge on the outside edge of the tongue (the short, narrow arm) at the riser, 180 mm, and the other on the outside edge of the blade (the long, wide arm) at the going, 240 mm.
2. Mark the first step. At the bottom end of the board, lay the square flat with both gauges hard against the top edge. Pencil along the outside of the tongue (the first riser) and of the blade (the first tread).
3. Step it up. Slide the square along until the tongue's edge sits exactly on the end of the tread line you just drew, gauges against the edge again, and mark the next riser and tread.
4. Repeat for every tread. For this stair that's 14 tread lines. Count them as you go.
5. Mark the top riser. After the 14th tread, draw one more riser line, the 15th, from the end of that tread to the top edge. That's where the stringer meets the trimmer or rim joist; how far the board runs past it depends on how the top is fixed.
6. Mark the seat. At the bottom, draw a line square to the first riser through its foot. That's the level cut that sits on the floor. Then mark the drop, below.

Why is the first riser shorter?

Because the treads sit on top of the cuts. Every step you walk on ends up one tread thickness above its cut line, except the floor at the bottom, which has no tread on it. Left as marked, with 25 mm treads, the first step would be 180 + 25 = 205 mm high and the top step 180 − 25 = 155 mm.

The fix is to drop the stringer: cut the tread thickness off the bottom, parallel to the seat line. Every cut comes down by 25 mm, the first riser on the stringer measures 180 − 25 = 155 mm, and once the treads are on, every step is 180 mm again, top and bottom. If the floor at the top or bottom gets a finish later (tiles or boards), measure the total rise to the finished surfaces and adjust the bottom cut for any difference.

How do I check the layout before cutting?

Measure the inside corners, where each tread line meets the next riser. They lie on one straight line, one step apart: 300 mm for 180 × 240. Hook the tape on the first inside corner and the 14th should read 13 × 300 = 3900 mm. If it reads more or less, a step crept as you moved the square: find it and re-mark before you cut. Measured from the foot of the stringer (the bottom of the first riser), after the 25 mm drop, the corners sit at:

Along the floor is corner × going; up from the seat is corner × riser − the drop. The top of the 15th riser is 15 × 180 − 25 = 2675 mm up and 3360 mm along. Cut to just short of each inside corner with a circular saw and finish with a handsaw, so you don't saw past the corner into the stringer. Try the first stringer in place before you trace the others from it.

Worked example: a 9 ft rise in feet and inches

The same method in inches, with 10 in treads and 1 in thick tread boards. The stair calculator in feet and inches gives:

1. 9 ft = 108 in = 2743.2 mm. Target riser: (600 − 254) ÷ 2 = 173 mm, and 2743.2 ÷ 173 = 15.86, so 16 risers of 108 ÷ 16 = 6 3/4".
2. Treads: 15, each 10" deep.
3. Set the gauges at 6 3/4" on the tongue and 10" on the blade, and step the square 15 times, then draw the 16th riser.
4. Drop: cut 1" off the bottom, so the first riser on the stringer is 5 3/4".
5. Check: each step is √(6.75² + 10²) = 12.06", or 12 1/16". Corner 1 to corner 15 is 14 × 12.06 = 168.9", or 14' 0 15/16".

That stair passes the IRC limits quoted on the stair calculator (riser at most 7 3/4", tread at least 10"), with the tread on the minimum. Your state, city or county adopts the code and decides what applies.

How long a board do I need?

Longer than the stepped part, with room for the fixings. For the 2700 mm stair with the 25 mm drop, the cut line from the foot of the first riser to the top of the last runs 3360 mm along and 2675 mm up, so √(3360² + 2675²) = 4294.8 mm. The stair calculator's stringer length, 4500 mm in Rise only, runs one going past the top riser, so a board that long covers the steps with about 200 mm over for the top fixing. For the 9 ft stair the cut line is √(150² + 107²) = 184.3", or 15' 4 1/4", against the calculator's 16' 1 1/16". More on those line lengths in what the stair totals measure.

How deep the board must be, how much timber has to be left under the cuts, and how many stringers the stair needs and how far apart depend on the timber, the treads and your local rules or deck guide, so check those before you buy.

Common mistakes

• Forgetting the drop. Without it, the first step is a tread thickness too tall and the last one that much too short.
• One tread too many. A flight with 15 risers has 14 treads. The upper floor is the 15th step.
• Rounding the riser. 2700 ÷ 14 is 192.86 mm. Mark 192.9, not 193: 14 risers of 193 mm come 2 mm over.
• Letting the square creep. A small slip at each step adds up over 14 steps. Use gauges, not a pencil tick, and check the corners against the 300 mm spacing.
• Measuring to the bare floor. The total rise is between finished floors. A floor finish added later changes the top or bottom step.
• Laying out from the wrong edge. Mark from the crowned edge, so the bow faces up when the stringer is fixed.

What this guide doesn't cover

• Straight flights only: no landings, winders or spiral stairs. Work each straight flight on its own.
• Housed (closed) stringers, where the treads sit in routed grooves, start from the same riser and going but are marked and cut differently.
• It doesn't size the stringer timber, the stringer spacing or the fixings, and it doesn't decide what complies. Your drawings, the local code and your council, certifier or building department do.

Stairs in MeisterCalc

MeisterCalc's Stairs calculator (Home ▸ Stairs) gives the numbers this layout starts from: risers, riser height, treads, going, total run, stringer length, incline angle and stairwell opening, with Rise Only, Rise + Run and Run Only modes and a live side view of the flight. For feet, inches and sixteenths, go to Settings ▸ Units ▸ Default units ▸ Imperial (ft/in). Stairs is one of three MeisterCalc calculators that are free to use, with Concrete and Check Square, and it works offline on site, on Android 7.0 or later. The other 13 calculators unlock with a weekly subscription that starts with a 3-day free trial, or with a one-time lifetime unlock; Google Play shows the prices in your currency.

Related guides

• Stairs in the MeisterCalc app: every input and result
• Stair maths: risers, treads, stringer length and stairwell opening
• How many steps do I need for a 2.7 m or 3 m floor height?
• What is the 2R + G stair formula, and how do I use it?
• What is the going of a stair, and how is it different from the tread?
• How big does a stairwell opening need to be?
• More guides on the blog