MEASURE ONCE. PLAN EVERY STEP.
Stair calculator.
Work out your rise, run, and stringer length.
See your staircase take shape as you calculate.
From floor to floor.
Down to the details.
A clearer view of your next step.
View all dimensions & layout points
- First stringer rise cut
- Top tread support below upper floor
- Comfort comparison: 2 × rise + going
- Pitch-line vertical span
The first cut subtracts tread thickness, assuming finished landing levels and no additional packing. Stringer pitch length is the span of repeated rise/run modules, not the board length to order. Connections, end cuts, stringer depth, and bearing need separate allowances.
| Tread | Horizontal position | Finished height |
|---|
Positions are layout references, not saw-cut coordinates. Imperial fractions in exports are rounded to 1/16 in; use unrounded calculations to maintain equal risers.
For straight flights, including indoor and deck stairs. Geometry estimates only; verify local requirements, headroom, supports, and connections before building.
START WITH THE RIGHT MEASUREMENTS
How to calculate stairs.
A good stair plan starts with finished floor levels.
Three steps take you from a measurement to a layout.
Measure the total rise
Measure vertically from the finished lower floor to the finished upper floor or deck. Include the final flooring on both levels. Floor-to-ceiling height is a different measurement.
Set the rise and run
Choose your units and a target riser height, or enter a fixed riser count. Set the tread going, or use an available total run to find the going that fits your space.
Check the whole flight
Read the equal riser height, tread count, footprint, and angle. Open construction details for thickness and top tread position. Check the reference notes, then print your plan.
THE MATH BEHIND THE MEASUREMENTS
No mystery.
Just rise and run.
Risers count the vertical changes in height. Treads count the horizontal walking surfaces. With a standard top position, the upper floor completes the last rise, so there is one fewer tread.
Put your measurements to work ↗N = round up (total rise ÷ target rise)R = total rise ÷ N
Run = tread count × goingAngle = arctan (R ÷ going)
Length = tread count × √(R² + going²)Standard: tread count = N − 1. Flush: tread count = N. The stringer length above follows the tread pitch; the diagonal from lower floor to upper landing uses different endpoints.
CHECK IT WITH A REAL EXAMPLE
Different units. Same approach.
Both examples use the standard top tread position
and round the riser count up.
A 9-foot floor-to-floor rise
108 in total rise ÷ 7.5 in target = 14.4.
Round up to 15 risers at 7.2 in, with 14 treads.
- Tread going
- 10.5 in
- Total run
- 147 in / 12 ft 3 in
- Stair angle
- 34.4°
A 2.8-metre floor-to-floor rise
2,800 mm total rise ÷ 180 mm target = 15.56.
Round up to 16 risers at 175 mm, with 15 treads.
- Tread going
- 280 mm
- Total run
- 4,200 mm / 4.2 m
- Stair angle
- 32.0°
KNOW YOUR STAIRCASE
Every dimension has a job.
Use these definitions when measuring,
comparing results, or speaking to your builder.
Total rise & riser height
Total rise spans the two finished floors. Riser height is one vertical step. Divide the total rise evenly; do not leave a shorter last step to absorb a remainder.
Tread going & nosing
Going is the horizontal spacing between adjacent tread fronts. Nosing is the projecting lip. Adding nosing to every going would overstate your total run.
Tread thickness
The tread sits on the stringer. Subtract its thickness from the first stringer rise cut so the finished first step matches the remaining steps.
Stringer & stair angle
The stringer supports the treads. Stair pitch comes from one riser divided by one going. The pitch span helps plan the layout; it does not size the timber or its supports.
LOOK BEYOND RISE AND RUN
Plan the space around your stairs.
The flight must work with its landings,
floor opening, and edge protection.
Headroom & floor opening
Headroom is the vertical clearance above the sloping line through the tread nosings. Measure to the nearest ceiling, beam, or obstruction. Floor thickness and opening length matter; a staircase can fit its footprint and still have insufficient headroom.
Stair width & landings
Check usable stair width at the relevant handrail height, plus the landing space at each end. Doors and changes in direction affect that space. For a landing between flights, measure and calculate each flight’s rise separately; do not divide the entire storey into one uninterrupted run.
Handrails & guards
A handrail provides a continuous grasping surface. A guard protects an open edge. They serve different functions, even when combined in one assembly. Their height, openings, returns, and attachment require checks beyond the rise-and-run calculation.
Stringer support & connections
The geometry does not determine timber size, allowable span, number of stringers, or connector capacity. For timber decks, use the applicable details in the American Wood Council’s DCA 6 guide (2015 IRC basis) alongside local requirements, or obtain a project-specific design.
SMALL DETAILS. REAL DIFFERENCES.
Common stair calculation mistakes.
Check these before transferring
any dimensions to your material.
| The mistake | Why it changes the result | What to do instead |
|---|---|---|
| Measuring to unfinished floors | Floor finishes change the first or last finished rise. | Use the final walking-surface elevations at both ends. |
| Counting treads as risers | A standard flight uses the upper floor for its final rise. | Count vertical height changes. Standard treads = risers − 1. |
| Adding nosing to every going | It exaggerates the horizontal footprint. | Measure going from one tread front to the next. |
| Rounding each step independently | Small differences accumulate along the flight. | Keep the exact equal rise for layout, and check cumulative heights. |
| Ordering timber from pitch length alone | End cuts and connection details can require extra material. | Confirm the full stringer layout, bearing, and structural design. |
FROM THE FRAMING SQUARE TO YOUR SCREEN
A brief history of stair calculation.
The tools have changed.
The relationship between rise and run remains.
Geometry on the stringer
Traditional stair layout uses a framing square to transfer a chosen rise and run to the board. Stair gauges help repeat the position consistently. This method still connects the calculation to the physical work. See Johnson Level’s framing-square guide.
Less arithmetic by hand
Handheld construction calculators brought stair-specific functions into a portable tool. Johnson’s CALC-1500, for example, includes rise/run calculations, stair settings, and measurement conversion. Read the manufacturer’s manual.
A plan you can compare
Online stair calculators combine those calculations with editable inputs and drawings. This tool adds live unit conversion and a printable plan, making it easier to compare layouts. Measuring the actual site and checking the construction details are still part of the job.
DIMENSIONS ARE ONE PART OF THE PLAN
A reference check.
A better starting point.
The calculator compares selected dimensions with the 2021 IRC, Section R311.7. These are US residential reference values, not a worldwide standard or a building approval.
Your locally adopted edition and amendments govern. Metric selection changes units; it does not change the code reference.
*Headroom is not calculated here. Measure vertically from the sloping nosing line to the obstruction. Also verify stair width, landings, guards, handrails, nosing, and structural support. Exact unit conversions shown; code publications may round metric equivalents.
A FEW THINGS WORTH KNOWING
Stair calculator
questions, answered.
Practical answers before you start
marking out your stairs.
How many steps do I need for a 9-foot rise?+
For a 108-inch rise and a 7.5-inch target, the calculator rounds 14.4 up to 15 risers. Each finished riser is 7.2 inches. Standard mounting gives 14 treads; a flush top tread gives 15.
Can I calculate stairs in metres or centimetres?+
Yes. Select metres, centimetres, millimetres, feet, or inches. Changing units converts your existing measurements rather than reinterpreting the same numbers. For example, 2.8 m equals 280 cm or 2,800 mm. In inch mode you can enter decimals or fractions such as 7 1/2.
How do I calculate stair stringer length?+
For the repeated stair pitch, multiply the number of treads by √(riser height² + tread going²). A standard flight has one fewer tread than risers. This pitch length does not include the extra board needed for end cuts, bearing, connections, or stringer depth; verify the actual cut layout before ordering timber.
Can I use this as a deck stair calculator?+
Yes, for the geometry of a straight deck stair flight. Measure to the finished decking and finished lower landing, and enter the actual tread thickness. Decking gaps, footing design, timber sizing, stringer spacing, and connections require separate design checks.
What is the difference between standard and flush?+
Standard places the highest tread one full riser below the upper floor; the floor completes the climb. Flush adds a tread level with the upper floor, projecting outward. It adds one going to the footprint without adding another riser.
What is the 7–11 rule for stairs?+
It describes a 7-inch riser with an 11-inch going. The related comfort comparison, two rises plus one going, gives 25 inches for that combination. It is a starting point for comparison, not a universal rule or proof of compliance.
Why does my actual riser differ from the target?+
A staircase needs a whole number of equal risers. This calculator rounds the count up and divides the total rise by that count, keeping the finished rise at or below your target. Use fixed riser count to compare a different arrangement.
Does this work for spiral or turning stairs?+
This tool calculates one straight flight. For L-shaped or U-shaped stairs with landings, calculate each straight flight separately and design the landing footprint. Spiral stairs and winders need different geometry and are outside this calculator’s scope.