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Stair Calculator

Lay out a straight stair: equal risers, treads, total run, angle and pitch line from the floor height.

By Nitin Raturi · Last reviewed

Your stair

How to use the Stair Calculator

Lay out a straight stair with equal risers from the floor-to-floor height.

  1. Choose millimetres or inches.
  2. Enter the total rise between finished floors.
  3. Enter the tallest riser you allow and the tread depth.
  4. Read the number of risers, their height, the run and the angle.

Good to know

  • A straight flight has one tread fewer than risers; the upper floor is the last step.
  • Leave room at the bottom and top landings beyond the run.
Unit

Unit

Whether you measure in millimetres or inches.

How to fill it in

  • Switching converts the three measurements you entered.

Example

108 in with risers up to 7¾ in needs 14 risers of 7.71 in.

Total rise

The height from finished floor to finished floor that the stair climbs.

How to fill it in

  • Measure from the lower finished floor to the upper finished floor, not the slab.
  • Enter 100 to 10,000 mm.

Example

A 2,800 mm rise with risers up to 190 mm needs 15 risers of 186.7 mm.

Maximum riser

The tallest step you will allow.

How to fill it in

  • The calculator uses the fewest equal risers that stay at or below this height.
  • Enter 100 to 300 mm.

Example

Lowering the limit to 175 mm gives 16 risers and a 3,750 mm run instead of 3,500 mm.

Keep in mind: Check the riser and tread limits in your local building bye-laws before building.

Tread depth

The horizontal depth of each step, from one nosing to the next.

How to fill it in

  • Enter 150 to 450 mm.
  • Deeper treads make a longer, gentler stair.

Straight stair

15 risers of 186.7 mm

15 equal risers of 186.7 mm and 14 treads of 250 mm: the stair runs 3,500 mm at 38.7°.

Total run
3,500 mm
Pitch line (stringer estimate)
4,482 mm
Angle
38.7°
2 risers + tread
623 mm

Nosing heights above the lower floor:

  1. 187 mm
  2. 373 mm
  3. 560 mm
  4. 747 mm
  5. 933 mm
  6. 1,120 mm
  7. 1,307 mm
  8. 1,493 mm
  9. 1,680 mm
  10. 1,867 mm
  11. 2,053 mm
  12. 2,240 mm
  13. 2,427 mm
  14. 2,613 mm
  15. 2,800 mm

Straight flight; the upper floor is the last step, so there is one tread fewer than risers. Landings, nosing overhang and floor finishes are not included.

ChangelogHow we test this calculatorEmbed this calculator

How this calculator works

  1. Step 1Risers⌈rise ÷ max⌉The fewest equal steps within your limit.
  2. Step 2Run(risers − 1) × treadOne tread fewer than risers.
  3. Step 3Slopeatan(rise ÷ run)The angle and the pitch line.

Every riser in a flight should be the same height, so a missed step does not trip anyone. The calculator divides the total rise into the fewest equal risers that stay within your maximum.

A straight flight has one tread fewer than risers, because the upper floor is the last step. The run is the treads times their depth.

Worked example: 2,800 mm floor to floor

Risers up to 190 mm, 250 mm treads
Risers
⌈2,800 ÷ 190⌉
Run
(risers − 1) × 250
  1. Risers2,800 ÷ 190 = 14.7, rounded up15
  2. Riser height2,800 ÷ 15186.7 mm
  3. Treads15 − 114
  4. Total run14 × 2503,500 mm
  5. Angleatan(2,800 ÷ 3,500)38.7°

The pitch line from the bottom nosing to the top floor edge is 4,482 mm, a first estimate of the stringer length.

Read the result

  • Up to 190 mm15 risers186.7 mm each, 3,500 mm run, 38.7°.
  • Up to 175 mm16 risers175.0 mm each, 3,750 mm run, 36.7°.

A lower riser limit adds a step and lengthens the stair: here by 1 riser and 250 mm of run.

In inches, a 108 in rise with risers up to 7¾ in needs 14 risers of 7.71 in.

Assumptions and limits

  • Straight flightNo winders, turns or mid landings.
  • Finished floorsMeasure the rise between finished surfaces.
  • Not a code checkLocal bye-laws set riser, tread and headroom limits.
  • Pitch line onlyThe cut stringer is longer than the pitch line.

Other building estimates

A stair is one part of a build. Concrete, tiles and slopes have their own tools.

For a concrete stair or landing, use the concrete calculator. The tile calculator covers treads and floors, and the slope calculator gives the angle of any rise over run.

Common mistakes

  • Mistake: Measuring slab to slab before flooring.

    Instead: Allow for both floor finishes, or the top and bottom risers come out uneven.

  • Mistake: Counting as many treads as risers.

    Instead: A straight flight has one tread fewer.

  • Mistake: Forgetting headroom and landings.

    Instead: Check clear height above the pitch line and space at both ends.

Checking the method

A separate script re-works every golden value in exact fractions, counting risers up from one and finding the angle with an inverse sine.

Read how we test our calculators, or browse more everyday tools.

Frequently asked questions

How many steps for a 2,800 mm floor height?

With risers up to 190 mm, 15 risers of 186.7 mm and 14 treads; with 250 mm treads the stair runs 3,500 mm.

Why is there one tread fewer than risers?

In a straight flight the upper floor is the last step, so it needs no tread of its own.

How does a lower riser limit change the stair?

At 175 mm the same rise needs 16 risers and runs 3,750 mm, at 36.7° instead of 38.7°.

Does it work in inches?

Yes. A 108 in rise with risers up to 7¾ in needs 14 risers of 7.71 in.

Does it check building rules?

No. Enter the riser limit from your local bye-laws, and check tread, headroom and handrail rules separately.

Sources and methodology

The formulas are standard time-value-of-money results. We check every figure against golden values worked out independently, by closed form and by month-by-month simulation; see the methodology.

Written and maintained by Nitin Raturi, a software engineer (about the author). Last reviewed 4 Nov 2026; next review due by . Found an error? Tell us.

Changelog

  • : Initial release.

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