Stair Calculator — Riser, Tread & Run

PRACTICAL GUIDE

Stair Calculator guide

Use this calculator to turn a measured finished floor-to-floor rise into a preliminary straight-flight stair layout. It first finds enough equal risers to stay at or below the maximum riser height you enter, then recalculates the exact riser height. For the common layout where the upper floor is the final walking surface, there is one fewer separate tread than risers.

The maximum riser and minimum tread fields are design inputs, not FigureNest recommendations or automatic code limits. Stair rules differ by jurisdiction, building type, occupancy, and stair configuration. Enter values taken from the requirements that apply to your project, then use the result to test whether the flight fits before developing the complete stair design.

FORMULA AND VARIABLES

How the calculation works.

Formula

Risers = ceiling(total rise ÷ maximum riser); actual riser = total rise ÷ risers; treads = risers − 1; total run = treads × tread depth

total rise = vertical distance from finished lower floor to finished upper floor
maximum riser = highest riser height you permit for this calculation
actual riser = equal finished rise after the riser count is rounded up
treads = separate horizontal steps in a straight flight when the upper floor acts as the final surface
tread depth = horizontal run assigned to each tread for this layout estimate

WORKED EXAMPLE

Follow the numbers in context.

108-inch finished floor-to-floor rise

With a maximum riser of 7.5 in, 108 ÷ 7.5 = 14.4, so the count rounds up to 15 risers. Each riser is then 108 ÷ 15 = 7.2 in. A straight flight with the upper floor as the final walking surface has 14 separate treads; at 10 in of run per tread, the modeled horizontal run is 140 in.

Metric layout check

For a 2.70 m finished rise entered as 270 cm and a 19 cm maximum riser, 270 ÷ 19 = 14.21, so use 15 risers. The equal riser height is 18 cm. If the chosen tread run is 25 cm, 14 treads require 350 cm of horizontal run before landings or other geometry are considered.

WHAT THE RESULT TELLS YOU

Use the number for the question this tool answers.

Treat the riser count as the minimum whole count needed to keep equal modeled risers at or below the maximum you entered. The actual riser value is the number to carry into the next layout check.

Treat total run as the horizontal footprint of the repeated treads only. It does not automatically include landings, clearances, framing thickness, or space beyond the first and last nosing.

If changing the maximum riser by a small amount changes the riser count, compare both complete layouts rather than choosing only the shorter flight.

Use the result as transparent geometry you can verify by hand, then reconcile it with the project drawings, product details, and applicable local requirements.

More detail: inputs, assumptions, mistakes and edge cases

HOW TO USE IT

Build a dependable estimate.

Measure vertically from the finished lower-floor surface to the finished upper-floor surface; include the finishes that will actually be present when the stair is complete.
Find the applicable stair requirements for the project and enter the maximum riser height and tread run/depth you intend to test. Do not assume the calculator defaults are legal for your location.
Read the riser count and actual riser height together. Rounding the count upward is what keeps the recalculated riser at or below the entered maximum.
Compare the calculated total run with the real horizontal space available. If it does not fit, changing one dimension can affect the entire layout and should be rechecked as a complete design.
Before construction, verify finished riser uniformity, tread/nosing geometry, headroom, landings, width, guards, handrails, stringers, connections, loads, and all other applicable requirements.

UNITS AND INPUTS

Keep measurements compatible.

Imperial mode uses inches for total rise, riser height, tread depth, and total run. Metric mode uses centimetres for those same quantities.

Measure total rise on the finished-floor basis. Measuring subfloor-to-subfloor and later adding unequal finish thicknesses can change the first or last finished riser.

Total run is horizontal distance, not the sloping stringer length. The two are different dimensions and should not be substituted for each other.

ASSUMPTIONS AND LIMITATIONS

Know what the result includes.

Assumptions

The calculation represents one straight flight with equal risers.
The upper floor serves as the final walking surface, so the model uses one fewer separate tread than risers.
Every tread uses the same entered horizontal run/depth for the total-run calculation.
The entered maximum riser and tread depth are treated as user-supplied design constraints; FigureNest does not infer the governing code.

Limitations

The calculator does not determine whether the entered riser or tread values comply with local building, accessibility, workplace, fire, or other rules.
It does not model landings, winders, curved stairs, alternating treads, spiral stairs, nosing geometry, headroom, stair width, guards, handrails, or door clearances.
It does not size or check stringers, framing, fasteners, connections, loads, materials, or structural capacity.
A geometric fit is not a permit-ready or construction-ready stair design.

COMMON MISTAKES

Avoid preventable errors.

Measuring rough framing instead of finished floor-to-floor height, which can create unequal finished risers at the top or bottom.
Rounding the riser height first and multiplying back. Choose a whole riser count first, then divide the total rise by that count so every riser remains equal in the model.
Using the number of risers as the number of separate treads in a normal floor-to-floor straight flight. The upper floor commonly replaces the last tread.
Treating total run as stringer length. Run is horizontal; a stringer follows the slope and requires separate geometry and structural design.
Assuming a familiar riser/tread rule applies everywhere instead of checking the requirements for the actual jurisdiction and project type.

EDGE CASES

Check unusual inputs carefully.

If total rise divides exactly by the entered maximum riser, no fractional riser count remains, but the finished layout still needs all other checks.

A small change in total rise can add an entire riser when the quotient crosses a whole-number boundary, which also changes tread count and total run.

Different finish thicknesses at the lower floor, upper floor, or treads can change finished riser heights even when the rough framing was laid out evenly.

A stair with a top tread rather than the upper floor as the final walking surface does not match the calculator’s risers-minus-one tread assumption.

FAQ

Questions about this calculation.

Why are there usually fewer treads than risers?

In a typical straight floor-to-floor flight, the upper finished floor acts as the final walking surface. Fifteen vertical rises therefore correspond to fourteen separate treads in this model.

Why does the calculator round the riser count up?

Rounding upward and then dividing the total rise equally prevents the recalculated riser from exceeding the maximum riser value you entered.

Does FigureNest decide the legal maximum riser or minimum tread?

No. Those fields are inputs. Use requirements applicable to your jurisdiction, building type, and stair before relying on the layout.

Is total run the same as stringer length?

No. Total run is horizontal. Stringer length follows the stair slope and is longer; stringer sizing also requires structural checks that this calculator does not perform.

Should I measure from subfloor to subfloor?

For finished stair geometry, use finished floor-to-finished floor height or account explicitly for the finish build-ups so the first and last finished risers remain consistent.

Can I use this for spiral, winder, or curved stairs?

No. Their tread geometry and code checks differ from the straight, equal-riser model used here.

RELATED TOOLS AND GUIDES

Stair Calculator.

Formula & methodology

Worked example

Before you order

Questions people ask