Stair Stringer Calculator

Unit system:

Switch the input labels, table values, chart labels, and exports between inches/feet and millimetres/metres.

Code profile:

The selected profile sets maximum riser height, minimum tread depth, and the displayed code checks.

Total rise:

Use the finished vertical height the stair flight must climb.

Target riser height: {{ formatDisplayLength(target_riser_in, 2) }}

Adjust this to bias the equal-riser count before cutting.

Tread depth / run: {{ formatDisplayLength(tread_depth_in, 2) }}

Use the measured tread run before nosing projection unless your local rule defines it differently.

Stair width:

Measure the finished clear or framing width that the stringers need to support.

Tread support profile:

Pick the support spacing that matches the tread material instructions before estimating stringer count.

Tread thickness:

Use the actual installed tread board thickness, not the nominal label.

Top condition:

Choose whether the top landing supplies the last tread or the stringer carries a final top tread.

Stock stringer board length:

Pick the planned 2x stringer stock length for the purchase warning.

Check stringer inputs

{{ error }}

Custom max stringer spacing:

Stringer board width:

Minimum throat depth:

Riser board thickness:

Board length allowance:

Uniformity tolerance:

Measurement	Value	Use on layout	Copy
{{ row.label }}	{{ row.value }}	{{ row.detail }}

Cut reference	Setting	Field note	Copy
{{ row.label }}	{{ row.value }}	{{ row.detail }}

Check	Status	Reason	Copy
{{ row.label }}	{{ row.value }}	{{ row.detail }}

Embed:

Customize

Include current inputs

Size

Advanced

Width

Height

Aspect ratio

Max height

Collapsible embed

Allow fullscreen

Referrer policy

Sandbox tokens

A stair stringer turns a finished vertical height into a repeatable saw layout. The important measurements are not the rough height of the deck frame or the nominal size of the lumber. They are the finished landing-to-landing rise, the horizontal tread run, the number of equal risers, and the diagonal board length needed to carry the cuts.

Most layout errors start when one of those quantities is treated as flexible after the first cut. If the total rise is divided into too few risers, each step becomes tall. If the tread run is too shallow, the stair feels steep and may miss the selected reference profile. If the bottom cut is not reduced by the installed tread thickness, the first finished step can land higher than the rest.

Stair stringer diagram showing total rise, total run, equal risers, tread run, and diagonal stringer line

The common residential reference is stricter about uniformity than many first-time stair builders expect. The calculated risers can be perfectly equal, but field marking, warped lumber, landing surfacing, tread thickness, and a missed bottom drop can still create a nonuniform flight. A stair-square layout should therefore be checked as arithmetic and as a physical template.

Stringer count is a separate support problem. Wide stairs, composite treads, thin decking, and product instructions can require tighter on-center support than a simple wood-tread layout. Board length and remaining throat depth matter too because a layout that fits the rise and run can still need longer stock or a stronger member than a notched board provides.

How to Use This Tool:

Enter the finished stair geometry first, then use the checks as a layout review before cutting a template stringer.

Choose Unit system and Code profile. The residential profile uses a 7.75 in maximum riser and 10 in minimum tread reference; the public profile uses a 7 in maximum riser and 11 in minimum tread reference. Use Custom local limits when the adopted rule differs.
Enter Total rise from finished lower landing to finished upper landing. Set Target riser height to bias the equal riser count without exceeding the selected maximum.
Set Tread depth / run, Stair width, Tread support profile, Tread thickness, and Top condition. These fields determine tread count, total run, stringer count, and bottom drop.
Pick Stock stringer board length. Use Advanced for custom stringer spacing, board width, minimum throat depth, riser-board thickness, board length allowance, and uniformity tolerance.
Review Layout Ledger, Cut Schedule, Compliance Checks, Stringer Profile, and JSON before transferring the pattern to lumber.

If the input warning appears, fix the named positive measurement first. Total rise, target riser height, tread depth, stair width, selected stringer spacing, stringer board width, and code-profile limits must all be greater than zero.

Interpreting Results:

Risers and Treads on stringer are the first values to check. With the upper landing as the final tread, tread count is one fewer than riser count. With a top tread included on the stringer, the counts match.

Framing square setting is the repeated rise and run used for each notch.
Bottom tread drop reduces the bottom cut by the installed tread thickness so the first finished step matches the rest.
Remaining throat is the wood left below the notch; a pass here is only a field check, not a span design.
Stock board check compares the diagonal plus allowance with the selected board length.
2r + tread comfort near 24 to 26 in is a comfort signal, not a code approval.

A profile pass does not mean the stair is ready for inspection. Local amendments, handrails, landings, headroom, nosings, guards, product instructions, and the actual field-cut uniformity still need separate review.

Technical Details:

A straight stringer layout is governed by equal division of total rise, multiplication of tread runs, and right-triangle geometry. The riser count is the smallest count that can satisfy the selected maximum riser, then candidate counts are scored against the target riser and the common 2r + tread comfort range.

The diagonal length is not the same as the board to buy. The diagonal describes the stair pitch line from lower bearing to upper bearing. The purchase check adds a trim and fitting allowance, then compares that total with the chosen stock length.

Formula Core:

The core geometry uses inches internally, with metric display converted to and from the same base values.

\begin{array}{lcl} n & = & selected riser count with \frac{R_{total}}{n} \leq R_{max} \\ R_{actual} & = & \frac{R_{total}}{n} \\ {Run}_{total} & = & N_{treads} \times D_{tread} \\ L & = & \sqrt{R_{total}^{2} + {Run}_{total}^{2}} \\ Angle & = & atan2 (R_{total}, {Run}_{total}) \end{array}

Stair stringer rules and checks
Check	Rule used	Meaning
Riser height	Actual riser must be within selected minimum and maximum	Too few risers create tall steps; too many create shallow steps.
Tread depth	Tread run must meet the selected minimum	The horizontal foot placement controls steepness and code-profile review.
Stringer count	ceil(stair width / max spacing) + 1, with at least 2 stringers	Wider flights or tighter tread products need more support lines.
Throat depth	Board width minus notch depth must meet the selected field minimum	Notched lumber still needs separate structural judgment.
Stock length	Stringer diagonal plus allowance must fit selected stock	Trim, bearing, and connection fitting need extra length beyond pure geometry.

For a 108 in total rise with a 10 in tread run and an upper landing as the final tread, a 15-riser layout gives 7.20 in actual risers, 14 treads, 140 in total run, and a diagonal near 176.8 in before allowance. A 12 in allowance pushes the board requirement near 188.8 in, so a 16 ft stock check is close while shorter stock fails.

Limitations:

This is a layout and material-planning calculator. It does not design the stair structure, approve the connection, verify headroom, size guards or handrails, or replace the adopted local code.

Measure finished surfaces, including planned decking, flooring, landings, and tread thickness.
Check the first cut stringer as a template before cutting the rest of the stack.
Use manufacturer instructions for composite or specialty tread support spacing.
Ask the local building department or a qualified designer when the stair serves public access, unusual loads, or a structural-critical location.

Worked Examples:

Deck stair with landing as the top tread

A 108 in Total rise, 7.5 in Target riser height, 10 in Tread depth / run, 36 in Stair width, and standard 16 in support produces 15 Risers, 14 Treads on stringer, and a Stringer count of 4. The Compliance Checks row for riser height should pass the residential profile, but the local-code row still says to verify.

Composite tread support

Changing Tread support profile from 16 in wood support to 12 in composite support increases support demand on the same 36 in width. The Stringer count and spacing badge show the tighter on-center layout before the user buys tread boards.

Troubleshooting a failed stock check

If Stock board check says Use longer stock, compare Stringer diagonal with the board-required value that includes allowance. Reducing allowance may hide a real fitting need; the better correction is usually a longer board or a revised landing layout.

FAQ:

Why is tread count one less than riser count?

When Top condition is set to Upper landing is final tread, the landing supplies the last walking surface, so the stringer carries one fewer tread than riser.

Can the code-profile check approve my stair?

No. Compliance Checks compare selected rise and tread references, throat depth, stock length, comfort, and angle. Local adopted code, amendments, inspection, and structural design still control the build.

Why does bottom tread drop matter?

Bottom tread drop subtracts installed tread thickness from the first riser cut. Without that deduction, the first finished step can be higher than the equalized risers.

Why do I see an input warning?

The warning names the invalid field. Enter positive values for the required rise, target riser, tread run, stair width, stock length, spacing, board width, and code-profile limits.

Glossary:

Total rise: The finished vertical distance the flight must climb.
Tread run: The horizontal depth used for each stair-square notch.
Stringer diagonal: The sloped geometric length of the stair layout before allowance.
Throat depth: The remaining wood below the deepest notch in a cut stringer.
Bottom tread drop: The reduction to the bottom riser cut that accounts for installed tread thickness.

References:

Chapter 3 Building Planning, Section R311.7 Stairways, International Code Council, 2021.
Chapter 10 Means of Egress, Section 1011 Stairways, International Code Council, 2021.
2018 International Residential Code Building Provisions, International Code Council.