Stair Stringer Calculator

Reference profile:

The profile sets the rise and tread checks; it does not certify the stair.

Custom maximum riser:

Changing the unit converts the value without changing the physical limit.

Custom minimum tread run:

Measure tread run horizontally between the governing reference planes.

Finished total rise:

Use finished surfaces; rough framing can create a different first or last riser.

Preferred riser height:

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The result always equalizes total rise across a whole number of risers.

Tread run:

Tread run drives the total footprint, angle, notch geometry, and comfort check.

Stair width:

Confirm the actual tread product can span the selected support spacing in stair use.

Tread support spacing:

The selected maximum spacing determines the estimated stringer count.

Custom maximum spacing:

Changing the unit preserves the physical support limit.

Tread thickness:

Use actual installed thickness, not the nominal board label.

Top condition:

This choice changes tread count, total run, stock length, and profile geometry.

Stock board length:

Verify usable stock, bearing details, defects, and connection cuts before purchase.

Actual stringer board width:

The throat estimate does not prove stringer capacity, span, bearing, or connection adequacy.

Minimum throat target:

The default 3.5 in is a planning screen, not a universal structural rule.

Trim and bearing allowance:

Neutral default: 0. Add the project allowance before the stock check.

Top riser-board deduction:

Neutral default: 0 for open risers or no deduction.

Displayed decimals:

Presentation-only refinement; it does not change the selected layout.

Cut plan

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Layout method:

Whole-riser selection precedes tread count, total run, diagonal, notch, and stock checks.

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The chart renderer is unavailable. The same measurements remain available in the cut plan and field ledger.

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A straight stair flight has one fixed vertical distance to climb, but it can divide that rise into several possible step counts. Each choice changes the riser height, horizontal footprint, angle, stringer length, and the amount of wood left below every notch. A layout that fits the opening may still be uncomfortable, weak at the throat, or outside the rules adopted for the project.

Finished measurements matter more than rough framing dimensions. Flooring, decking, tread boards, and landing finishes can change the first or last riser. Repeating a perfectly consistent mark along the stringer will not correct a total rise measured from the wrong surfaces.

Riser height: The equal vertical rise between adjacent finished treads.
Tread run: The horizontal advance of one step, measured between the governing reference planes.
Stringer throat: The narrowest wood depth left below the rise-and-run notch.
Top condition: Whether the upper landing supplies the final walking surface or the stringer carries a separate top tread.

Building-code profiles screen rise, tread, and uniformity dimensions. They do not settle every requirement for landings, headroom, guards, handrails, nosings, bearing, fastening, lumber, or structural capacity. Local amendments and the actual tread manufacturer's span instructions can also change an otherwise plausible plan.

Measure the finished total rise at the stair location, including the lower and upper finishes.
Confirm the available footprint before accepting a tread run.
Check the remaining throat and stock length before using the first stringer as a pattern.
Test-fit one cut stringer before transferring its marks to the rest.

How to Use This Tool:

Start with the limits that apply to the job, then enter finished geometry and the stock that will actually be cut.

Choose Reference profile. Use Custom local limits only when you have an applicable maximum riser and minimum tread run from the project authority.
Enter Finished total rise, Preferred riser height, and Tread run. Changing a unit converts the physical measurement rather than reinterpreting the digits.
Set Top condition, tread thickness, top riser-board deduction, stock length, stringer width, minimum throat, and any trim or bearing allowance that affects the cut.
Enter Stair width and choose Tread support spacing. Match that spacing to the installed tread product in stair use.
Review Cut plan first. Use Stringer profile for the overall geometry and Field ledger for the individual measurements and checks.

Interpreting Results:

Riser count, Actual riser, and Tread count define the repeated layout. Total run is the required horizontal footprint. Board required includes the diagonal stringer length plus the entered allowance, so compare it with the selected stock length before marking lumber.

Rise and Tread pass only the selected dimensional profile.
Remaining throat compares geometric wood depth with the entered minimum; it is not a structural approval.
Comfort passes from 609.6 to 660.4 mm, reviews from 584.2 to 685.8 mm outside that pass range, and otherwise says adjust.
Angle passes from 30° to 38°, reviews from 25° to 42° outside that pass range, and otherwise says adjust.

A pass does not certify the stair. Verify the finished end conditions, local code, tread span, lumber and connections, then test-fit the first stringer.

Technical Details:

Equal division governs the vertical layout. Candidate riser counts begin with the smallest whole number that keeps every riser at or below the selected maximum. Where a minimum riser applies, that value limits the largest candidate count. The chosen count balances closeness to the preferred riser with closeness of twice the riser plus tread run to 635 mm.

Formula Core:

All physical lengths are converted to millimetres before calculation. The principal geometry is:

\begin{array}{lcl} n_{\min} & = & ⌈ \frac{R}{r_{\max}} ⌉ \\ r & = & \frac{R}{n} \\ G & = & n_{tread} \times g \\ L & = & \sqrt{R^{2} + G^{2}} \\ d & = & \frac{r \times g}{\sqrt{r^{2} + g^{2}}} \end{array}

R is finished total rise, n is riser count, r is actual riser height, g is one tread run, G is total run, L is the stringer diagonal, and d is notch depth measured perpendicular to the stringer edge. Remaining throat equals stringer width minus d. Board required equals L plus the entered allowance.

Reference profile limits used by the stair calculation
Profile	Riser range	Minimum tread run	Uniformity reference
2024 IRC-style residential	Up to 196.85 mm	254 mm	9.525 mm
2024 IBC public / egress	101.6 to 177.8 mm	279.4 mm	9.525 mm
Custom local limits	Up to entered maximum	Entered minimum	9.525 mm

A landing top condition uses one fewer tread than risers. A separate top tread uses the same count. Stringer count is the greater of two or the ceiling of stair width divided by maximum support spacing, plus one. Actual spacing then equals stair width divided by one fewer than the stringer count.

Bottom cut rise subtracts tread thickness from the actual riser without going below zero. Top run cut subtracts the entered riser-board thickness from tread run without going below zero. Display precision changes only presentation from zero to three decimal places.

Limitations:

This is a straight-flight layout estimate. It does not design the stringer as a structural member or check the complete stair assembly.

Winders, curved stairs, landings, headroom, guards, handrails, and accessibility details need separate design.
Throat depth does not account for defects, overcuts, moisture, species, grade, fasteners, bearing, or loading.
Reference profiles may not match the code edition, occupancy, amendments, or inspection rules at the project location.
Tread support spacing must come from the installed product and assembly, not the planning preset alone.

Worked Examples:

Residential landing-top flight

A finished rise of 108 in with a preferred 7.5 in riser, 10 in tread run, 36 in width, 16 in maximum support spacing, and the landing as the top tread produces 14 risers and 13 treads. The actual riser is about 7.71 in, total run is 130 in, four stringers sit at 12 in actual spacing, and the 16 ft stock fits the roughly 169.01 in diagonal. The profile and comfort checks pass, while the 39.72° angle is marked for review.

References:

2024 International Residential Code, stairway provisions, International Code Council, 2024.
2024 International Building Code, Section 1011 Stairways, International Code Council, 2024.
How to Calculate and Cut Stair Stringers, Trex.