A stair can look right on a plan and still cause trouble on site if the rise, going or landing detail has been read incorrectly. Knowing how to calculate stair stringers means working from the finished floor levels first, then checking that every step, cut and fixing point suits the stair design before timber is marked.
This is not a calculation to rush between other tasks. A small error repeated across 14 or 15 risers can leave the bottom step out, create an awkward top transition or put the stair outside the approved details.
Start with the total rise
The total rise is the vertical distance from the finished floor level at the bottom of the stair to the finished floor level at the top. Measure finished surface to finished surface, not slab to slab or frame to frame, unless those are the specified finished levels.
Include all floor finishes in the calculation. Tile, timber flooring, carpet underlay, deck boards and screed can all affect the final rise. On a renovation, confirm whether the existing floor will remain, be built up or be removed. This is where many stair set-outs go wrong.
Record the total rise in millimetres. For example, if the finished floor level difference is 2700 mm, your total rise is 2700 mm.
Choose the riser height and number of risers
Next, select a target riser height that suits the approved stair design and applicable project requirements. Divide the total rise by that target height to find a workable number of risers.
The basic calculation is:
`Number of risers = total rise ÷ target riser height`
Using a 2700 mm total rise and a target riser of 180 mm:
`2700 ÷ 180 = 15 risers`
Then calculate the actual riser height:
`Actual riser height = total rise ÷ number of risers`
`2700 ÷ 15 = 180 mm`
That result is clean. In practice, the first division will often produce a decimal. Round to a whole number of risers, then recalculate the actual riser height. Never leave one step slightly different to make the numbers work. Consistent risers matter for safety, comfort and compliance.
Check the relevant National Construction Code requirements, approved drawings and engineering details before locking in the dimensions. Permitted riser and going ranges vary with the building class and stair application. A stair to a dwelling is not automatically detailed the same way as access stairs in a commercial build, external stair or service area.
Count treads separately
For a conventional stair running from one floor or landing to the next, the number of treads is usually one less than the number of risers. The upper floor or landing acts as the final tread.
With 15 risers, that commonly gives 14 treads. But confirm the construction detail. If the design includes a physical top tread, a winder, a landing or a stair box arrangement, the count may change. Do not rely on the rule without checking where the user actually places their final foot.
Set the going and total run
The going is the horizontal distance from the face of one riser to the face of the next. It is not necessarily the full depth of the tread board, because tread nosing may project past the riser below.
Once the going is selected, multiply it by the number of treads to find the total horizontal run:
`Total run = number of treads × going`
For a stair with 14 treads and a 250 mm going:
`14 × 250 = 3500 mm total run`
This is the horizontal space required from the bottom riser line to the upper landing line. Check it against the stairwell opening, headroom zone, bottom clearance and surrounding framing. A stair can meet the preferred riser and going figures but still not fit the available opening.
A tighter site may force a different design solution, such as a landing, turn, winder or changed floor layout. Reducing the going simply to make a straight flight fit can produce a stair that is uncomfortable or non-compliant. Resolve the layout before cutting stringers.
Calculate the stringer length and pitch
With the total rise and total run confirmed, calculate the diagonal length of the stringer. Treat the stair as a right-angle triangle:
`Stringer length = √(total rise² + total run²)`
For the example stair:
`√(2700² + 3500²)`
`√(7,290,000 + 12,250,000)`
`√19,540,000 = approximately 4420 mm`
Allow extra material at each end for the top and bottom connection details. The calculated 4420 mm is the diagonal line between floor levels, not automatically the length to order or cut. Top hanger arrangements, birdsmouth-style bearing details, bottom landing returns and support blocks all affect the required stock length.
The stair angle can also be checked with:
`Angle = arctan(total rise ÷ total run)`
In this example, the pitch is approximately 37.6 degrees. The angle is useful when checking the stair against drawings, fabricating steel supports or setting out handrail components, but the rise and going dimensions remain the primary layout controls.
Mark out the stringer from the same reference point
Once the dimensions are approved, use a framing square or stair gauges to mark the repeated rise and going pattern on the stringer stock. Set one leg to the actual riser height and the other to the actual going - not the target figures if they differ.
Mark each step consistently from the same datum. It is good practice to lay out one stringer, check the count and overall dimensions, then use it as the template for the remaining stringers. Do not assume two separately marked stringers will match closely enough just because the calculations are right.
Before cutting, check these points on the full layout:
- The number of riser spaces matches the calculated total rise.
- The number of tread spaces matches the actual stair and landing detail.
- The top cut suits the fixing method and finished floor level.
- The bottom cut allows for the specified tread, flooring and bearing condition.
- The remaining timber depth below each notch is adequate for the span and structural design.
The last point deserves attention. Cutting deep notches into a timber stringer reduces its effective section. A long flight, heavy-use stair or wider stair may need larger stock, a centre stringer, engineered timber, steel support or a design specified by an engineer. The geometry alone does not prove the stringer is structurally adequate.
Allow for tread and finish details
The finished stair is governed by finished dimensions, not just carcass dimensions. If tread boards sit on top of notched stringers, their thickness changes the top and bottom treatment. If flooring is added after the stair is installed, the first or last riser may need adjustment to remain equal.
For example, a 19 mm timber tread can affect the bottom riser detail if the lower floor finish is not yet installed. Likewise, a top floor finish may alter the final transition at the landing. Work through the build-up in section and use the drawing details where provided.
Open-riser stairs, housed stringers and steel stair systems need different set-out methods. The total rise, tread count and total run still apply, but the way they transfer to the supporting members changes. Do not apply notched timber stringer cuts to a stair system designed for housed or fabricated supports.
A practical site check before installation
Dry-fit the first stringer or hold it in position before committing to final fixing. Check the top and bottom levels, the landing line, the opening and the headroom. Measure several planned riser positions rather than checking only the first and last point.
Keep the calculation with the job record. On a busy site, that gives the team a clear reference for the total rise, riser count, going, run and any finish allowances used. A field tool such as Builders Calculator can help retain the figures alongside diary notes, so the set-out can be checked later without relying on a marked-off scrap of timber.
Accurate stringers begin with finished levels and end with a physical check. Take the extra few minutes to verify both - it is far quicker than remaking a stair after the treads are on.