Mark the geometry of a chosen flight
A stair stringer supports repeated riser and tread positions. This calculator helps inspect their geometry after you choose a whole riser count and horizontal going. It does not automatically choose an allowed step size. Measure the total rise between finished walking surfaces, including the intended floor coverings. A change in that measurement affects every riser, so unfinished subfloor levels can produce a misleading layout.
The landing replaces the last separate tread
Riser height = total rise ÷ riser count; run = (riser count − 1) × goingThe model assumes the upper landing is the final walking surface. There is therefore one fewer separate tread position than vertical risers. The overall diagonal connects the lower starting level to the upper landing across the full horizontal run. It is a bounding geometry measurement, not a finished cut length. The repeated step pitch is calculated from one riser and one going; its angle differs slightly from that overall diagonal because the flight has one fewer going than risers.
Sixteen risers across a 280 centimetre rise
Dividing 280 centimetres by 16 produces a 17.5 centimetre riser. Fifteen tread positions with a 28 centimetre going produce a horizontal run of 420 centimetres. The bounding diagonal is the square root of 280 squared plus 420 squared, approximately 504.78 centimetres. The repeated step pitch is about 32.01 degrees. These numbers describe the layout convention; they do not establish that the flight meets a local building rule.
Leave the end details to the actual construction
Top attachment, lower bearing, tread thickness, riser boards and any nosing projection change the end cuts. Going is the horizontal distance between corresponding tread edges; it is not necessarily the physical depth of a tread board with an overhang. Do not order a stringer solely from the displayed diagonal. Prepare its actual end details and confirm the available stock length. Notching also removes material from the stringer, so timber size, remaining throat, number of supports and connection capacity require their own checks. Headroom, landing dimensions, handrails and uniformity are outside this worksheet. Enter at least two risers and a positive going to obtain a meaningful repeated-flight layout.
A reproducible calculation with complete inputs
For a reproducible stair stringer calculator example, use the metric input system and enter Finished total rise: 280 cm; Number of equal risers: 16 risers; Horizontal going per tread: 28 cm. These settings describe one specific scenario, not a new default for every job. The result follows the method and allowances already explained above. Keep the entire set of values when comparing it with a hand calculation; omitting a selected material or an allowance can change the result even when the dimensions match. The following rows show the unrounded calculation values alongside the precision used by the interface.
| Result | Calculation value | Displayed value | Unit |
|---|---|---|---|
| Overall layout diagonal | 504.777178565 | 504.78 | cm |
| Equal riser height | 17.5 | 17.50 | cm |
| Tread positions below landing | 15 | 15 | treads |
| Total horizontal run | 420 | 420.00 | cm |
| Step pitch angle | 32.0053832081 | 32.01 | ° |
How should I prepare these entries?
Finished total rise: enter the value in cm. Number of equal risers: enter the value in risers. Horizontal going per tread: enter the value in cm. Choose the unit system before transferring your measurements. A depth, length, area and coverage can have different field units even within one system. Read each label individually rather than entering the same unit convention in every box. Optional openings or allowances should describe your actual scenario instead of being silently left at a sample value. The worked inputs above identify exactly which selections produced the table.
How should I compare another measurement?
Change one measurement or option at a time and keep the other inputs fixed. The new result reflects the whole stated formula, including its conversions, allowances and purchase rounding. A quantity that depends on several dimensions does not necessarily respond linearly to every field. Compare intermediate area or volume outputs when present before interpreting the final rounded purchase quantity. This makes a misplaced unit easier to detect than judging the final total alone. Record the selected unit system beside each scenario so that a copied number is not silently reused under different field units.
How do I turn an estimate into an order?
Keep an unrounded physical quantity separate from a rounded count of packages or pieces. Rounding upward to a whole container addresses the purchase unit, whereas a waste or coverage margin addresses an assumption in the formula. They should not be added twice merely because both increase an order. Compare the actual product sizes, coverage and any stated allowances with the assumptions above before buying. A calculated estimate cannot account for an unmeasured feature or a different product simply by displaying more digits.
Frequently asked questions
Is the diagonal a finished stringer length?
No. It is the bounding rise-and-run diagonal. End bearings, attachment cuts and tread thickness must be accounted for in the actual stock layout.
Why one fewer tread than risers?
The model treats the upper landing as the final walking surface. A different top-step arrangement needs a different layout convention.
Is going the same as tread-board depth?
Not always. Going measures horizontal spacing between successive tread edges; a nosing can make the physical board deeper.
Does this choose a legal riser height?
No. You select the riser count and going. Check the resulting dimensions, headroom and landings against the applicable requirements.
Can it size the supporting timber?
No. The geometry does not check strength, notch depth, connections or loads. Those are separate structural decisions.