Why use this calculator
Stairs are unforgiving because every riser in a flight must be the same height, and the tolerance for variation is only a few millimetres. Get the riser count wrong and the last step is either a trip hazard or a stumble, which is exactly the defect building inspectors look for first and the most common cause of falls on domestic stairs.
The arithmetic is simple but the constraints are not: riser height, tread going and the relationship between them are all limited by code, the flight has to physically fit the floor space available, and the comfort rule that makes stairs feel right is a separate condition again.
This calculator tests all of them at once and shows the neighbouring riser counts so you can pick the option that fits.
How to use it
- Measure the total rise from the finished floor below to the finished floor above, remembering to include the thickness of the upper floor structure and any floor covering, because measuring to the joist rather than the finished surface is the classic error.
- Enter a preferred riser height as a target, around 175 mm or 7 inches for a comfortable domestic stair, and the calculator will divide the rise into a whole number of equal risers close to it.
- Enter the tread going, which is the horizontal distance from one riser face to the next and excludes any nosing overhang, then the clear width of the stair and how many stringers you intend to use.
What each result means
- Riser height
- The vertical height of each step, which must be identical throughout the flight. Variation of more than a few millimetres is a trip hazard and a code failure.
- Number of risers
- How many vertical steps the flight contains. Most codes require a landing once a single flight exceeds about 18 risers.
- Number of treads
- Horizontal steps, always one fewer than the risers because the upper floor level forms the last step you walk onto.
- Total going
- The floor space the flight occupies horizontally. Check this against the room before committing, as it is what forces most stair redesigns.
- Stringer length
- The straight-line length of the sloping stringer. Order timber longer to allow for the top and bottom fixings and the cutting waste.
- Stair pitch
- The angle of the flight. Domestic stairs are normally kept below about 42 degrees, with anything steeper feeling like a ladder.
- 2R + G comfort rule
- Twice the riser plus the going. This has matched comfortable human stride since the seventeenth century and is the best single test of whether stairs feel right.
- Tread going check
- Whether the going you entered sits inside the code range. Too shallow and the heel overhangs; too deep and the stride breaks.
- Stair width
- The clear width between walls or handrails. Domestic minimums are commonly around 860 to 900 mm, with more needed for accessible routes.
- Total stringer material
- Combined straight length across all stringers, for ordering timber. Add waste for the cutting of each tread and riser notch.
The formula
Number of risers = total rise divided by the preferred riser height, rounded to the nearest whole number and never fewer than two
Actual riser height = total rise divided by that riser count, which is why every riser ends up identical
Number of treads = risers minus one
Total going = treads multiplied by the going
Stringer length = the square root of total rise squared plus total going squared
Pitch = arctangent of riser height divided by going
The comfort rule evaluates 2R + G, twice the riser plus the going.
How this number is used in practice
The 2R + G rule dates to Francois Blondel in 1675 and has survived because it reflects how people actually walk: a stride shortens as it rises, and doubling the riser captures that trade-off. A flight that satisfies the individual riser and going limits but falls outside the 550 to 700 mm band will still feel wrong underfoot, usually described as either cramped or as needing an awkward stretch.
Where floor space forces a compromise, staying inside the comfort band matters more to how the stairs are experienced than optimising any single dimension.
Measuring to the wrong datum is the most expensive mistake in stair setting-out. The total rise must be from finished floor to finished floor, so if the upper floor will receive 20 mm of timber and the lower floor 10 mm of tile, both allowances belong in the measurement.
Discovering the omission after the stringers are cut means new stringers, because you cannot adjust a single riser to absorb the error without creating exactly the unequal step that codes prohibit and that causes falls.
Code figures differ between jurisdictions and the ones used here are indicative. In Australia the National Construction Code sets risers between 115 and 190 mm with goings from 240 to 355 mm and the 2R + G range at 550 to 700 mm. The US International Residential Code caps risers at 7.75 inches with a minimum 10 inch tread.
The UK Building Regulations Approved Document K sets a maximum 220 mm rise and minimum 220 mm going for private stairs with a pitch limit of 42 degrees. Always confirm against the code in force where you are building, and check handrail and headroom requirements, which this calculator does not cover.
Worked examples
A 2,700 mm floor-to-floor height with a 175 mm target riser and 260 mm going
15 risers at 180 mm, 14 treads, 3,640 mm of going and a stringer 4,532 mm long
A tight 2,400 mm rise where floor space is limited, using a 190 mm target
13 risers at 184.6 mm with a 2R + G of 629 mm, still inside the comfort band
A US basement stair with a 105 inch rise, 7 inch target riser and 10 inch tread
15 risers at 7 inches exactly, 14 treads and a total run of 140 inches
When people use it
- Setting out a new flight of stairs for a renovation or extension
- Checking whether an existing stair meets current code before a sale or inspection
- Testing whether a flight will physically fit the floor space available
- Deciding between riser counts when the floor space is constrained
- Ordering stringer timber and working out tread and riser quantities
Tips
- Measure finished floor to finished floor. Forgetting the floor covering allowance is the most common and most expensive setting-out error.
- Every riser in a flight must be identical. Codes allow only a few millimetres of variation across the whole flight.
- Check the total going fits the room before you cut anything. Floor space, not code, is what forces most stair redesigns.
- Use the 2R + G rule as the comfort test. Inside 550 to 700 mm the stairs will feel right even at the limits of the other ranges.
- There is always one fewer tread than risers, because the upper floor forms the final step.
- Flights over about 18 risers normally need a landing. Check the rule in your jurisdiction before designing a long single flight.
What it assumes, and where it stops being accurate
- Assumes a straight flight with equal risers and no winders, landings or turns, each of which changes the setting-out substantially.
- Code limits shown are indicative for domestic stairs. Commercial, industrial and accessible stairs have different and usually tighter requirements.
- Total rise must be finished floor to finished floor. Measuring to a subfloor or joist produces a flight that does not land correctly.
- Going excludes nosing overhang. Codes limit nosing projection separately, commonly to around 25 to 30 mm.
- Headroom, handrail height, balustrade and gap requirements are not checked and are governed separately by code.
- Stringer length is a straight-line dimension and excludes cutting waste and the allowance needed at the top and bottom fixings.
Terms used on this page
- Riser
- The vertical height of a single step, which must be constant throughout a flight.
- Going
- The horizontal depth from one riser face to the next, excluding nosing overhang.
- Stringer
- The sloping structural member supporting the treads and risers at each side.
- Nosing
- The tread edge projecting beyond the riser below, which adds foot room without adding going.
- 2R + G
- Twice the riser plus the going; a comfort rule of thumb dating from 1675.
Questions people ask
- Why is there one fewer tread than risers?
- Because the upper floor surface itself acts as the final tread. A flight with 15 risers has 14 physical treads, with the fifteenth step landing you on the floor above. Ordering 15 treads is a common and avoidable mistake.
- What is the 2R + G rule?
- Twice the riser height plus the tread going should fall between about 550 and 700 mm, or 22 to 27.5 inches. It reflects how stride length shortens as you climb, and stairs outside it feel cramped or stretched even when every individual dimension complies.
- What is the maximum riser height?
- It varies by jurisdiction. Australia's National Construction Code allows up to 190 mm for domestic stairs, the US International Residential Code caps risers at 7.75 inches, and UK Approved Document K allows up to 220 mm for private stairs. Always check the code in force locally.
- Can risers vary within a flight?
- Essentially no. Codes permit only a few millimetres of variation across an entire flight, because an inconsistent step is the single largest cause of stair falls. If a flight does not divide evenly, change the riser count rather than absorbing the difference in one step.
- How do I fit stairs into a small space?
- Increase the riser height toward the code maximum, which reduces the riser count and shortens the total going. Beyond that you need a landing with a turn, winders, or a change of stair type. Never simply shorten the tread going below the code minimum.
- Does this include the landing?
- No. The calculation covers a single straight flight from floor to floor. A landing that breaks the flight adds its own depth to the total going and is usually required once a flight exceeds around 18 risers.
Use this with
Stair Rise and Run Calculator answers one part of the question. These pick up where it stops: Roof Pitch and Rafter Calculator, Brick and Block Calculator, Pythagorean Theorem Calculator and Area Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- Australian Building Codes Board: National Construction Code. Riser, going and 2R+G limits for stairways in Australia
- International Code Council: International Residential Code. US stair riser and tread requirements for dwellings
- UK Government: Approved Document K, protection from falling. UK private stair rise, going and pitch limits
Stair Rise and Run Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.