Introduction
Building stairs starts with getting the stringer right. The stringer is the angled board that holds up your treads and risers, and cutting it wrong means wobbly steps, wasted lumber, or a failed building inspection. This stair stringer calculator takes the guesswork out of the process by figuring out the exact riser height, tread depth, stringer length, and stair angle based on your measurements.
Just enter your total rise — the distance from the finished floor to the finished floor above — along with your tread depth, and the calculator does the rest. It tells you how many steps you need, what each riser height will be, the total run of your staircase, and the length of lumber you'll need for the stringer. It also checks your results against IRC residential building code requirements, so you'll know right away if your riser height exceeds the 7¾-inch maximum or if your tread depth falls below the 10-inch minimum.
Use Basic Mode for quick calculations when you just need the key numbers. Switch to Comprehensive Mode for full control, including automatic or manual step counts, tread thickness adjustments, a step-by-step breakdown table, a stair diagram, and an angle visualization. The calculator works in both imperial and metric units, supports fractional inches down to 1/16", and applies comfort rules like the rise-plus-run check (ideally 17 to 18 inches) to help you build stairs that are safe and easy to walk on.
How to Use Our Stair Stringer Calculator
Enter your staircase measurements below, and this calculator will give you the number of risers, step rise height, total run, stringer length, stair angle, and building code compliance checks. You can choose Basic Mode for quick results or Comprehensive Mode for full control over every detail.
Total Rise (Floor to Floor): Measure the vertical distance from the finished surface of the lower floor to the finished surface of the upper floor. This is the full height your stairs need to climb. You can enter this value in inches, feet, meters, or centimeters.
One Step Run (Tread Depth): Enter the horizontal depth of a single tread, measured from the nose of one step to the nose of the next. The standard range is 10 to 11 inches. In Comprehensive Mode, you can switch between entering one step run or the total run instead.
Calculation Mode (Comprehensive Only): Choose between Automatic and Manual. Automatic mode finds the best number of steps for you based on a target step height. Manual mode lets you set every dimension yourself.
Target Step Height (Comprehensive — Automatic): Pick your preferred riser height from the dropdown. The ideal range is 7 to 7¾ inches. The calculator will figure out how many risers fit your total rise based on this target.
Fixed Number of Steps (Comprehensive — Automatic): If you already know how many risers you want, switch to "Fixed Number of Steps" and type in your count. The calculator will then divide the total rise evenly across that number of risers.
Total Run (Comprehensive — Manual): Enter the full horizontal distance your staircase will cover from the bottom riser to the top. The calculator divides this by the number of treads to find each step's run depth.
Number of Steps / Risers (Comprehensive — Manual): Type in the exact number of risers you plan to use. The calculator will split both the total rise and total run evenly across your steps.
Tread Thickness (Comprehensive Only): Enter the thickness of your tread material, typically 1 to 1½ inches. This adjusts the stringer length calculation so the bottom of your stringer is cut correctly to account for the added tread height.
What Is a Stair Stringer?
A stair stringer is the diagonal board that supports the treads (the flat parts you step on) and risers (the vertical parts between treads) of a staircase. Most stairs use two or three stringers — one on each side and sometimes one in the middle for extra strength. Stringers are usually cut from 2×12 lumber, with notches shaped like a zigzag pattern to hold each step in place. Getting the stringer measurements right is the most important part of building safe, comfortable stairs. If you're working on the overall stair layout first, our Stair Calculator can help you plan the full design before you start cutting stringers.
Key Stair Terms You Should Know
Total rise is the full vertical distance from the finished floor at the bottom to the finished floor at the top. Total run is the full horizontal distance the staircase covers. Step rise (or riser height) is how tall each individual step is, while step run (or tread depth) is how deep each step is from front to back. The stringer length is the actual length of the diagonal board you need to cut — calculated using the total rise and total run with the Pythagorean theorem. If you need help with the underlying geometry, our Right Triangle Calculator walks through the same formula.
Building Code Requirements
The International Residential Code (IRC) sets clear rules for residential stairs. The maximum riser height is 7¾ inches, and the minimum tread depth is 10 inches. All risers in a staircase must be within ⅜ inch of each other in height — uneven steps are a major tripping hazard. The stair angle should stay between about 30° and 42° for safe, comfortable use. You also need at least 6 feet 8 inches of headroom measured vertically from the nose of any tread to the ceiling above.
The Rise + Run Comfort Rules
Experienced builders use two simple formulas to check if a staircase will feel comfortable to walk on. The first is rise + run = 17 to 18 inches. For example, a 7-inch riser paired with a 10½-inch tread gives you 17½ inches — right in the sweet spot. The second formula is 2 × rise + run = 24 to 25 inches. If your numbers fall outside these ranges, the stairs may feel too steep or too shallow.
How to Measure and Calculate Stair Stringers
Start by measuring the total rise from finished floor to finished floor. This must be exact — even a quarter inch off will compound across every step. Divide the total rise by your target riser height (7 inches is a good starting point) and round to the nearest whole number. That gives you the number of risers. Then divide the total rise by the number of risers to get your actual step rise. The number of treads is always one less than the number of risers because the top floor acts as the final step.
Multiply the tread depth by the number of treads to get the total run. Use the formula stringer length = √(total rise² + total run²) to find how long your stringer board needs to be. Remember to subtract one tread thickness from the bottom of the stringer — this is called "dropping the stringer" and keeps the first step the same height as all the others once the treads are installed. When calculating how much lumber to purchase, a Board Foot Calculator can help you estimate the material you'll need for your 2×12 stringer stock.
Tips for Cutting and Installing Stringers
Always use straight, knot-free lumber for stringers. After cutting the notches, the narrowest part of the board (called the throat) should be at least 3½ inches deep for adequate strength. Use a framing square with stair gauges clamped on to mark each step consistently. Secure stringers to the header at the top and a kickboard or concrete pad at the bottom. For stairs wider than 36 inches, add a third stringer in the center to prevent treads from bouncing or sagging. If your staircase includes a landing that connects to a ramp for accessibility, our Ramp Slope Calculator can help you plan compliant slopes. And if the stairs connect to a deck, consider using the Decking Calculator and Framing Calculator to estimate materials for the surrounding structure. For projects that require a concrete pad at the base, our Concrete Calculator will help you determine the right volume to pour.