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

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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 }}

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A straight stair flight has to divide one fixed height into steps that feel even, fit the available footprint, and leave enough wood in every notched stringer. The finished rise is the height people will actually climb after decking, flooring, tread boards, and landings are in place. The tread run is the horizontal distance each step advances. Those two dimensions set the rhythm of the stair before any board is cut.

Stringers turn that rhythm into a physical pattern. A framing square marks the same riser and tread over and over on the board, then the diagonal stringer line carries the load between the landings. When the rise is measured from rough framing instead of finished surfaces, or when the bottom cut ignores tread thickness, the first or last step can end up different from the rest even though the repeated marks looked consistent on the lumber.

Several terms matter before the layout makes sense. A riser is the vertical step height, and an actual riser is the equalized height after the total rise is divided by a whole number of steps. A tread is the walking surface, while tread run usually means the horizontal cut distance before any nosing projection is considered. Total run is the footprint of all tread runs in the flight. Stringer throat is the depth of board left below the notch, not the full nominal board width.

Code limits and walking comfort are related, but they are not the same check. Residential references commonly allow a taller riser and shorter tread than public means-of-egress stairs. A layout can pass a simple rise-and-tread profile and still feel awkward if the pitch is very steep, the run is cramped, or the first finished step differs from the rest. The common comfort check of twice the riser plus tread run near 24 to 26 inches is a useful clue, not a permit rule.

Use finished landing heights, not only framing heights, when the finished surfaces are known.
Decide whether the upper landing supplies the final tread before counting tread cuts.
Match stringer spacing to the tread material, especially for composite or thinner boards.
Check landings, handrails, guards, headroom, nosings, fasteners, bearing, and local amendments separately.

The first cut stringer is usually the proof piece. It should sit on the lower landing, meet the upper landing, and keep the finished first and last risers within the same tolerance as the repeated middle risers. After that field check, the same pattern can be copied to the remaining stringers with much less risk.

How to Use This Tool:

Enter the finished stair dimensions first, then use the result tabs to check geometry, material fit, and profile warnings before marking the board.

Choose Unit system and Code profile. The residential profile checks a 7.75 in maximum riser and 10 in minimum tread, the public profile checks a 7 in maximum riser and 11 in minimum tread, and Custom local limits lets you enter another maximum riser and minimum tread.
Measure Total rise from finished lower landing to finished upper landing. Set Target riser height near the step height you prefer; the calculated actual riser still has to fit the selected profile.
Set Tread depth / run, Tread thickness, and Top condition. These values decide tread count, total run, bottom tread drop, and whether the upper landing or a final top tread supplies the last walking surface.
Enter Stair width and select the Tread support profile. Use the profile that matches the tread manufacturer instructions, not only the spacing you used on nearby framing.
Open Advanced for custom stringer spacing, stringer board width, minimum throat depth, riser-board thickness, board length allowance, and field uniformity tolerance.
Read Layout Ledger for risers, treads, run, angle, and stringer count. Then check Cut Schedule and Compliance Checks; a layout is ready to test-fit only when the warning box is clear and the stock, throat, rise, and tread checks make sense for the job.

If an alert asks for valid inputs, correct the named field before using the numbers. Total rise, target riser, tread run, stair width, stock length, selected stringer spacing, stringer board width, and profile limits must be greater than zero.

Interpreting Results:

Start with Risers, Actual riser, and Treads on stringer. Those three values tell you how many repeated riser marks and tread cuts belong on the board. Total run shows the footprint, while Stringer diagonal and the board-required value show whether the selected stock length has enough allowance for trimming and bearing.

Framing square setting is the repeated rise-by-run mark for every notch.
Bottom tread drop is the tread thickness deducted from the bottom cut so the first finished riser matches the others.
Top run deduction accounts for a riser board at the top cut reference when closed risers are modeled.
Remaining throat is the wood depth left below the deepest notch. It is a field screening value, not a structural span approval.
2r + tread comfort near 24 to 26 in suggests a more familiar walking rhythm, but local code and site constraints still control the build.

A passing profile badge should not be treated as final approval. Recheck finished rise, top condition, stock length, throat depth, tread support spacing, and the local adopted code before copying the pattern to every stringer.

Technical Details:

Straight stringer layout is equal-division geometry. The total rise is divided into a whole number of risers, the selected tread run turns that count into a horizontal footprint, and the diagonal stringer length comes from the right triangle formed by total rise and total run.

The riser count is not chosen by rounding the target riser alone. Candidate counts begin at the smallest count that does not exceed the selected maximum riser. They continue up to the count allowed by the minimum riser, capped at 80 candidates. The chosen count balances closeness to the target riser against a comfort value centered on 25 inches for twice the riser plus tread run.

Formula Core:

The calculations use inches as the base length unit, with metric display values converted to and from millimetres or metres.

\begin{array}{lcl} n_{min} & = & ⌈ \frac{R_{total}}{R_{max}} ⌉ \\ R_{actual} & = & \frac{R_{total}}{n} \\ score & = & | R_{actual} - R_{target} | + 0.08 \times | (2 R_{actual} + D_{tread}) - 25 | \\ {Run}_{total} & = & N_{treads} \times D_{tread} \\ L & = & \sqrt{R_{total}^{2} + {Run}_{total}^{2}} \\ θ & = & atan2 (R_{total}, {Run}_{total}) \end{array}

Variables and derived fields used in the stair stringer formulas
Symbol or field	Meaning	Result effect
Rtotal	Finished vertical height between the lower and upper walking surfaces	Sets the required riser count and diagonal length.
Ractual	Equal riser height after a whole-number count is selected	Appears in the framing square setting and rise profile checks.
Dtread	Horizontal tread run marked for each notch	Sets total run, comfort value, stair angle, and tread profile checks.
Ntreads	Treads carried by the stringer	Equals riser count when a top tread is included, or one fewer when the landing is the final tread.
L	Pure diagonal stringer length before allowance	Board-required length adds the selected board allowance to this value.
θ	Stair angle measured from horizontal	Feeds the pass, review, or adjust angle status.

Remaining throat is estimated from the notch geometry. The step diagonal is the hypotenuse of one actual riser and one tread run. The notch depth along the board is actual riser x tread run / step diagonal, and remaining throat is stringer board width minus that notch depth.

Stair stringer checks and status boundaries
Check	Pass or review rule	Practical meaning
Residential profile	Actual riser from 4 in to 7.75 in, tread run at least 10 in	Matches the residential reference values exposed by the calculator.
Public profile	Actual riser from 4 in to 7 in, tread run at least 11 in	Uses a stricter public-stair reference for rise and tread depth.
Comfort	Pass when 24 in ≤ 2r + tread ≤ 26 in; Review from 23 to 27 in; Adjust outside that wider range	Flags unusually steep or shallow walking geometry.
Stair angle	Pass from 30° to 38°; Review from 25° to 30° and 38° to 42°; Adjust below 25° or above 42°	Highlights layouts likely to raise comfort, headroom, or landing concerns.
Stringer count	At least two stringers, otherwise ceil(stair width / maximum spacing) + 1	Controls on-center spacing based on the selected tread support profile.
Stock length	Stringer diagonal plus board allowance must be no longer than the selected stock length	Allows extra length for trimming, bearing, and fitting.

For a 108 in total rise, 7.5 in target riser, 10 in tread run, and an upper landing as the final tread, the score selects 14 risers. The actual riser is about 7.71 in, the stringer carries 13 treads, total run is 130 in, and the pure diagonal is about 169.01 in before the 12 in board allowance is added.

Limitations:

The results are for straight stair stringer layout and first-pass material planning. They do not design the stair structure, prove connections, or approve a stair for a specific jurisdiction.

Use the local adopted code, permit notes, and inspection guidance for final rise, tread, handrail, guard, nosing, landing, headroom, and opening rules.
Follow the tread manufacturer instructions before relying on 16 in, 12 in, or 10 in support spacing.
Check actual lumber size, splits, knots, overcuts, fastener locations, bearing, and span separately from the remaining-throat value.
Cut and test-fit one pattern stringer before copying the layout to the rest of the boards.

Advanced Tips:

Use Custom local limits when the permit set, inspector, or jurisdiction uses a different maximum riser or minimum tread than the built-in residential and public profiles.
Set Board length allowance to cover the trim, bearing, and layout margin you actually need. A longer allowance can turn a pass into a Use longer stock warning.
Use the Stringer Profile tab to check the off-by-one tread decision before cutting. If the top landing is the final tread, the tread count should be one less than the riser count.
When composite or thin tread boards are used, choose the closer support profile first and then compare Stringer count with the product guide.
Copy the Compliance Checks or JSON only after the field measurements are final. Draft measurements can make a report look more certain than the site is.

Worked Examples:

Deck stair with the 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, 1 in Tread thickness, and 16 in wood support produces 14 Risers and 13 Treads on stringer. Actual riser is about 7.71 in, Stringer diagonal is about 14 ft 1.01 in, and the board-required value with a 12 in allowance is about 15 ft 1.01 in, so 16 ft stock passes.

Public profile with a wider flight

An 84 in rise with the public profile, 7 in target riser, 11 in tread run, 48 in width, and an upper landing as the final tread produces 12 Risers, 11 Treads on stringer, and a 7.00 in Actual riser. The 48 in width at 16 in maximum support spacing yields 4 Stringer count with 16 in on-center spacing, and Stair angle is about 34.8°.

Composite support and a stock-length warning

A 120 in rise, 7.5 in target riser, 10 in tread run, 42 in width, and a 12 in composite support profile gives 16 Risers, 15 Treads on stringer, and 5 Stringer count at about 10.5 in on-center. If Stock stringer board length is set to 14 ft, Stock board check should report Use longer stock because the diagonal plus allowance is about 17 ft 0.09 in.

FAQ:

Why is the riser count not exactly total rise divided by my target?

The target riser is a preference. The calculator first keeps the actual riser within the selected maximum and minimum profile range, then chooses the candidate that best balances the target riser and 2r + tread comfort.

Why is tread count sometimes one less than riser count?

When Top condition is Upper landing is final tread, the landing supplies the last walking surface. Choose Top tread included on stringer when the stringer carries that final tread instead.

Does a passing code profile mean the stair is approved?

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

Why did changing tread support profile add stringers?

Stringer count comes from stair width and maximum on-center spacing. A 12 in composite profile or 10 in tight-support profile can require more stringers than the 16 in wood profile for the same stair width.

What should I fix when the warning box appears?

Read the warning text and correct the named positive measurement first. Zero or negative rise, target riser, tread run, width, stock length, stringer spacing, board width, or profile limits prevent a usable layout.

Glossary:

Total rise: The finished vertical height from the lower walking surface to the upper walking surface.
Actual riser: The equal riser height produced after total rise is divided by the selected riser count.
Tread run: The horizontal depth marked for each tread cut on the stringer.
Top condition: The choice that decides whether the upper landing or a final top tread supplies the last walking surface.
Stringer throat: The remaining board depth below the deepest notch in a cut stringer.
Board length allowance: Extra length added to the diagonal stringer length for trimming, bearing, and fitting.

References:

Chapter 3 Building Planning, Section R311.7 Stairways, International Code Council, 2021.
Chapter 10 Means of Egress, Section 1011 Stairways, International Code Council, 2024.
TimberTech Decking Installation & Maintenance Guide, TimberTech.
How to Calculate and Cut Stair Stringers, Trex.