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The largest riser you measured anywhere in the flight.
Measure nosing to nosing on the finished surfaces, not off the bare stringer, and take every riser rather than the ones that look suspect. The first and last are where the outliers almost always are, because they are the ones affected by floor finishes going on after the stair was set.
The smallest riser you measured anywhere in the flight.
Use the same measuring method for both extremes. Mixing a tape reading at one end of the flight with a laser reading at the other introduces a difference that belongs to the instruments rather than to the stair.
The variation your adopted code allows across one flight.
The North American model codes both allow three eighths of an inch between the largest and smallest riser in a flight. Other jurisdictions are tighter and some express the requirement as uniformity with a construction tolerance instead, so take the figure from the code that governs the work rather than from habit.
How many risers the flight contains from floor to floor.
Count risers, not treads — a flight has one more riser than it has treads, because the last riser lands on the floor above. Getting this wrong by one is what makes the uniform height below disagree with what you measured.
Finished floor to finished floor, measured over the whole flight.
This gives the height every riser in the flight should have been, which is the useful comparison once you know a flight has drifted: it tells you whether the error is one bad step or a gradual accumulation, and those have different remedies.
Spread between tallest and shortest riser
0.25 in
The riser spread measured is 0.12 in inside the variation entered. Re-measure after the floor finishes go down at both ends, because that is when a flight that was inside the limit most often stops being. Matching the figures quoted is not compliance. The rest of the requirement, and the installed work, are outside what this page can see.
- Permitted variation for this flight
- 0.38 in
- Margin left before the limit
- 0.12 in
- Uniform riser height for this flight
- 7.5 in
- Tallest riser above the uniform height
- 0 in
- Shortest riser below the uniform height
- 0.25 in
They open the calculator with your figures already in it
Stair Riser Spread Tolerance Checker: 0.25 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- IRC R311.7.5.1 and IBC 1011.5.4: the greatest riser height within a flight shall not exceed the smallest by more than 3/8 in (9.5 mm) — a flight-wide rule, measured between the extremes rather than between neighbours
- Other jurisdictions set the figure differently, which is why the permitted variation is entered rather than fixed: Approved Document K and the National Construction Code both express riser uniformity in their own terms
- Arithmetic only: spread = tallest measured riser − shortest measured riser, compared against the variation you enter
Inputs used
- Tallest Measured Riser
- 7.5 in
- Shortest Measured Riser
- 7.25 in
- Permitted Variation
- 0.38 in
- Risers in the Flight
- 16
- Total Rise of the Flight
- 10 ft
Intermediate steps
- Permitted variation for this flight
- 0.38 in
- Margin left before the limit
- 0.12 in
- Uniform riser height for this flight
- 7.5 in
- Tallest riser above the uniform height
- 0 in
- Shortest riser below the uniform height
- 0.25 in
Confidence note: The riser spread measured is 0.12 in inside the variation entered. Re-measure after the floor finishes go down at both ends, because that is when a flight that was inside the limit most often stops being. Matching the figures quoted is not compliance. The rest of the requirement, and the installed work, are outside what this page can see.
What this calculation does not cover
- Works from the two extremes you supply. It cannot find an outlier you did not measure, and the rule it applies is flight-wide rather than pairwise.
- Riser height is one of several stair checks. Tread depth uniformity, nosing projection, headroom and handrail geometry are all separately enforced and a flight can pass here and fail those.
Add the equipment this sizes
This result is a specification — 0.25 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-08-30 · in the site-wide review of 2026-09-06 · v1.0.0
Regulatory standards & verification citations3
- IRC R311.7.5.1 and IBC 1011.5.4: the greatest riser height within a flight shall not exceed the smallest by more than 3/8 in (9.5 mm) — a flight-wide rule, measured between the extremes rather than between neighbours
- Other jurisdictions set the figure differently, which is why the permitted variation is entered rather than fixed: Approved Document K and the National Construction Code both express riser uniformity in their own terms
- Arithmetic only: spread = tallest measured riser − shortest measured riser, compared against the variation you enter
Which documents these citations point at
- International Residential Code — R311.7.5.1 (United States)One- and two-family dwellings and townhouses up to three storeys — structure, fire safety, energy, plumbing, mechanical and electrical in a single document.
- International Building Code — 1011.5.4 (United States)Buildings other than the dwellings the IRC covers — occupancy, egress, fire resistance and structural provisions.
- Approved Documents to the Building Regulations (England) — K (United Kingdom)Practical guidance on meeting the Building Regulations, one lettered part per subject — Part A structure, Part B fire, Part K protection from falling, Part L conservation of fuel and power, and the rest.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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