A stair calculator finds the number of risers, actual riser height and total run for a straight staircase. Enter the floor-to-floor rise, desired riser and tread depth, and the tool checks IRC R311.7 code limits and the 2R+T comfort formula. A 9-foot rise with a 7.5-inch riser gives 15 risers at 7.20 inches.
Lumber, Framing and Carpentry
The vertical distance from the top of the lower finished floor to the top of the upper finished floor, in inches.
Your target riser height. IRC R311.7.5.1 allows a maximum of 7-3/4 inches (7.75 in). Common range is 7 to 7.5 inches.
The horizontal depth of each tread from nosing to nosing. IRC R311.7.5.2 requires a minimum of 10 inches.
Stair summary
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How to Use the Stair Calculator
- Enter the total rise, the floor-to-floor height in inches.
- Enter the desired riser height. IRC allows a maximum of 7.75 inches; 7 to 7.5 inches is common.
- Enter the tread depth. IRC requires a minimum of 10 inches; 10.5 to 11 inches is typical.
- Read the number of risers, actual riser height, total run, stair angle and comfort check.
Here is what each result line means:
| Result | What it means |
|---|---|
| Number of risers | How many vertical steps from bottom to top. |
| Actual riser height | The exact riser height after dividing the total rise evenly. |
| Number of treads | One fewer than risers; the top floor counts as a landing, not a tread. |
| Total run | Horizontal distance the stair covers from first riser to last. |
| Stair angle | The incline of the staircase in degrees. Comfortable stairs are 30 to 37 degrees. |
| 2R+T comfort check | Twice the riser plus the tread. The ideal range is 24 to 25 inches (Blondel formula). |
| IRC code check | Whether the riser and tread meet IRC R311.7 limits. |
For stringer layout and material, see the stair stringer calculator.
What Is a Stair Calculator?
A stair calculator computes the number of risers, actual riser height, total run and comfort score for a straight residential staircase. It divides the floor-to-floor height into equal risers and checks the result against the IRC building code and the classic 2R+T comfort formula.
Builders, remodelers and homeowners use it to lay out stairs that are both code-compliant and comfortable before cutting stringers or submitting plans for permit review.
How the Stair Calculator Works
The calculator rounds the number of risers up so that the actual riser height stays at or below your target. It then checks whether the actual riser exceeds the IRC maximum of 7.75 inches and whether the tread depth meets the 10-inch minimum.
- Divide the total rise by the desired riser and round up to get the number of risers.
- Divide the total rise by the number of risers to get the actual riser height.
- Subtract one from the risers to get the number of treads.
- Multiply treads by tread depth to get the total run.
- Compute 2R+T and compare to the 24-25 inch ideal range.
Stair Calculator Worked Example
You have a floor-to-floor height of 108 inches (9 feet) and want risers around 7.5 inches with 10.5-inch treads.
Result: The calculator gives 15 risers at 7.20 inches each, 14 treads, and a total run of 147 inches (12 ft 3 in). The 2R+T score is 24.9 inches, which falls inside the 24-25 inch comfort zone. The actual riser of 7.20 inches is below the 7.75-inch IRC maximum, and the 10.5-inch tread exceeds the 10-inch minimum, so the stair passes code.
Factors That Affect Stair Layout
Floor-to-floor Height
A taller floor-to-floor rise needs more risers. A 10-foot rise needs at least 16 risers at 7.5 inches each, compared to 13 risers for an 8-foot rise.
Riser Height
Shorter risers are more comfortable but increase the total run and the number of treads. IRC allows up to 7.75 inches; 7 to 7.5 inches is the comfort sweet spot.
Tread Depth
Deeper treads are safer and feel more spacious. The IRC minimum is 10 inches. Standard residential treads run 10 to 11.25 inches (a 2×12 tread measures 11.25 inches).
Available Floor Space
The total run determines how much horizontal space the stair occupies. Shorter treads or more risers reduce the run, but can push the comfort score outside the ideal range.
Nosing Projection
IRC R311.7.5.3 requires a nosing projection of 3/4 to 1-1/4 inches on stairs with solid risers. Open-riser stairs do not require a nosing if the tread depth is at least 10 inches.
Stair Dimension Comparison
How different riser heights affect the layout for a 108-inch (9-ft) floor-to-floor rise with 10.5-inch treads.
| Desired riser | Risers | Actual riser | Treads | Total run | 2R+T |
|---|---|---|---|---|---|
| 7.00 in | 16 | 6.75 in | 15 | 157.5 in (13 ft 1.5 in) | 24.0 in |
| 7.25 in | 15 | 7.20 in | 14 | 147.0 in (12 ft 3.0 in) | 24.9 in |
| 7.50 in | 15 | 7.20 in | 14 | 147.0 in (12 ft 3.0 in) | 24.9 in |
| 7.75 in | 14 | 7.71 in | 13 | 136.5 in (11 ft 4.5 in) | 25.9 in |
A 7.75-inch riser saves one riser and almost a foot of run, but the 2R+T score of 25.9 is above the ideal range.
When to Use a Stair Calculator
Laying Out Stairs for New Construction
Calculate riser count and total run before cutting stringers or framing the stairwell opening.
Remodeling with a New Floor Height
When adding a floor or changing ceiling height, recalculate risers so the stair still meets code.
Checking Comfort Before Building
Run the 2R+T check to confirm the stair will feel comfortable. An uncomfortable stair is expensive to rebuild.
Estimating Stairwell Size
The total run tells you how long the stairwell opening needs to be for adequate headroom.
Common Stair Calculation Mistakes
1. Measuring Total Rise to the Subfloor
Total rise is from finished floor to finished floor, not subfloor. Add the thickness of the upper floor finish.
2. Forgetting the Top Riser
The last riser goes from the top tread to the upper floor surface. Omitting it throws off the riser count.
3. Using the Code Maximum as the Target
A 7.75-inch riser is the maximum, not the recommendation. Aim for 7 to 7.5 inches for comfort.
4. Ignoring the 2R+T Check
A stair can meet code but still feel awkward. A 2R+T score outside 24-25 inches signals a comfort problem.
5. Not Accounting for Floor Finish
If the upper floor finish is not installed yet, the actual riser at the top step will change when the finish goes down.
Accuracy and Limitations
The calculator divides the total rise into equal risers and checks against IRC R311.7 code limits. It assumes a straight single-flight stair with uniform risers and treads.
What it covers
- Riser count and actual riser height
- Total run and stair angle
- 2R+T comfort formula (Blondel)
- IRC R311.7 riser and tread limits
What it does not cover
- L-shaped, U-shaped or winder stairs
- Spiral staircase geometry
- Stringer sizing or material quantity
- Headroom clearance from framing
How We Calculate Stair Dimensions
Frequently Asked Questions About Stair Calculations
How many stairs do I need for a 9-foot ceiling?
A 9-foot floor-to-floor rise (108 inches) typically needs 14 to 15 risers. At 7.5 inches per riser you get 15 risers with an actual height of 7.20 inches each.
What is the ideal riser height for residential stairs?
The IRC allows up to 7.75 inches, but 7 to 7.5 inches is the comfort range. A riser in this range paired with a 10.5-inch tread scores 24 to 25 on the 2R+T formula.
What does 2R+T mean?
It is the Blondel comfort formula: twice the riser height plus the tread depth. A score of 24 to 25 inches signals a comfortable stride on the stair.
What is the minimum tread depth for residential stairs?
IRC R311.7.5.2 requires a minimum tread depth of 10 inches, measured from nosing to nosing. Deeper treads of 10.5 to 11 inches are more comfortable.
How do I measure total rise?
Measure the vertical distance from the top of the lower finished floor to the top of the upper finished floor. Include any floor finish not yet installed.
What is the maximum riser height allowed by code?
IRC R311.7.5.1 sets the maximum riser height at 7-3/4 inches (7.75 inches). The maximum variation between the tallest and shortest riser in one flight is 3/8 inch.
How much floor space does a stair need?
The total run depends on the tread depth and number of treads. A 14-tread stair with 10.5-inch treads has a total run of 147 inches (about 12 feet 3 inches), plus landing space at top and bottom.
What stair angle is comfortable?
Residential stairs are typically 30 to 37 degrees. Below 30 degrees feels like a ramp; above 40 degrees feels steep. The 2R+T formula helps keep the angle in the comfortable range.
Do I need a landing?
IRC R311.7.6 requires a landing at the top and bottom of every stair flight. The landing must be at least as deep as the stair is wide (minimum 36 inches).
Is my data saved?
No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.
Sources
- IRC 2021 Section R311.7, Stairways (residential stair riser, tread, width, headroom and uniformity requirements).
- Fine Homebuilding, 2 Rules for Comfortable Stairs (2R+T comfort formula, 24-25 inch target range, worked examples).
- Viewrail, Residential Stair Code Guide (IRC R311.7 riser, tread, headroom and nosing requirements explained).
- Architectural Graphic Standards, Stair Design (R+T = 17.5 inch rule of thumb for stair proportions).
Related Calculators
Laying out stairs?
This calculator checks stair dimensions against IRC R311.7 residential stairway requirements and the 2R+T comfort formula. Local code amendments may differ. Confirm stair dimensions with your local building department before construction. Spotted an error? Let us know.