Stair Stringer Calculator
Turn a total rise and tread run into the numbers you need to cut stringers — riser count, exact riser height, total run, stringer length, and stair angle — with a right-triangle diagram and a building-code check.
Stair dimensions
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Right-triangle stringerAbout the Stair Stringer Calculator
A stair stringer is the sawtooth-cut board that supports the treads and risers of a staircase. Getting its geometry right is the heart of any stair build: every step must be the same height, the run must be deep enough to stand on safely, and the whole flight has to meet building-code limits. This Stair Stringer Calculator does that maths for you.
Enter the total rise (the vertical distance from one finished floor to the next), the number of risers, and the tread run (the horizontal depth of each step, also called the going). The calculator divides the rise evenly into risers, derives the number of treads (always one fewer than the risers, because the top landing is the last step up), and computes the stringer length as the hypotenuse of the right triangle formed by the total rise and total run.
It also reports the exact riser height, the total run, and the stair angle (pitch), then checks them against either the US International Residential Code (IRC) — maximum 7¾″ riser, minimum 10″ tread — or UK Approved Document Part K — maximum 220 mm riser, minimum 220 mm going, and a maximum pitch of 42°. The right-triangle diagram updates live so you can see the profile before you cut.
How to Use the Stair Stringer Calculator
- 1 Measure the total rise. Measure vertically from the lower finished floor to the upper finished floor. Switch between imperial (inches) and metric (millimetres) at the top.
- 2 Set the number of risers. Tap the suggestion button for a comfortable step height, or type your own. The riser height and tread count update instantly.
- 3 Enter the tread run. This is how deep each step is front-to-back (excluding any nosing overhang). 10–11″ (250–280 mm) is typical for main stairs.
- 4 Pick your building code. Choose US (IRC) or UK (Part K) to check riser height, tread depth, angle, and the comfort rule against the right limits for your region.
- 5 Read the results and diagram. Use the stringer length to buy your board (add extra for the cuts and any floor connection), and the riser/tread numbers to mark out the sawtooth with a framing square.
Frequently Asked Questions
How is stringer length calculated? ▾
The stringer is the hypotenuse of a right triangle whose vertical side is the total rise and whose horizontal side is the total run (treads × tread run). So stringer length = √(rise² + run²). This is the minimum board length along the cut line — add extra for the top and bottom connections and saw kerf.
Why is the number of treads one fewer than the risers? ▾
The very top "step" is the upper floor or landing itself, so you climb one more riser than the number of treads you actually walk on. A flight with 14 risers has 13 treads. The calculator handles this automatically.
What is the maximum riser height? ▾
In the US, the IRC caps residential risers at 7¾ inches and requires a tread depth of at least 10 inches. In the UK, Approved Document Part K limits domestic risers to 220 mm, requires a going of at least 220 mm, and caps the pitch at 42°. Always confirm the code adopted by your local authority.
What is the 2R + T comfort rule? ▾
Two times the riser height plus one tread depth should land in a comfortable range — about 24–25 inches (the classic Blondel figure is ≈ 24.6″), or 550–700 mm under UK guidance. Stairs that fall outside this feel either cramped and steep or stretched and shallow.
How many stringers do I need? ▾
At least two on the outside edges. For stairs wider than about 36″ (900 mm), or when using thinner treads, add a centre stringer so treads don't flex. Deck and exterior stairs commonly use three or more. This calculator sizes one stringer — multiply the board length by the number you plan to cut.
Does the calculator account for tread thickness? ▾
It works from the finished rise and run, which is what governs code compliance and comfort. When marking the stringer, remember that the bottom riser is cut shorter by one tread thickness so that every finished step ends up the same height — a step often called "dropping the stringer."
Is my data saved or sent anywhere? ▾
No. The calculator runs entirely in your browser — your measurements never leave your device and nothing is stored on a server.