Total rise is floor to floor, finished surface to finished surface. Answers round to the nearest 1/16 inch, and the riser height shows its exact decimal too, so you can mark the stringer with a calculator's precision. The dashed line in the drawing is the straight line estimate for the stringer board.
What this calculator doesn't check
It does the geometry, nothing more. It doesn't size lumber for a load, check fasteners, or know your local building code, and the limits you type are yours, not a code. Codes for stairs vary from place to place, so check yours before you cut.
A worked example
Say the total rise is 9′ 0″, the max riser 7 3/4″ and the treads 10″, the numbers already in the calculator.
- Count the risers.
108″ divided by 7 3/4″ is just under 14, so it takes 14 risers to stay at or under the max.
- Find the riser height.
108″ over 14 risers is 7.714″, which marks as 7 11/16″.
- Count the treads.
There's one tread fewer than risers, because the upper floor is the last step. That's 13 treads.
- Total run.
13 treads of 10″ make 10′ 10″ from the bottom riser to the top one.
- Stringer and angle.
The straight line over 9′ of rise and 10′ 10″ of run is about 14′ 1″. The stair climbs at 37.6°.
- The cuts.
With 1″ treads, the bottom riser on the stringer is cut to 6 11/16″, so the first finished step matches the rest. With a 3/4″ floor finish, the top of the stringer sits 7 15/16″ below the upper subfloor.
Stringer length and the board to buy
The stringer figure is the straight line from the bottom of the stair to the top. The board needs to be longer than that, because the bottom cut and the top connection take extra wood, so buy the next stock length up with some to spare.
Each notch cuts into the board. For this example the notch goes 6.11″ deep, square to the board's edge. That leaves 5.14″ of wood under the notches on a 2×12, but only 3.14″ on a 2×10. The calculator flags anything under 5″, the same rule my Stair Calculator app uses.
Is the stair comfortable?
A long standing rule of thumb says two risers plus one tread should come to about 24 to 25 inches. The example gives 25 7/16″. Steeper stairs come in under that, shallow ones over it. If yours is well off, try a slightly shorter max riser or a deeper tread and watch the drawing change.
Measuring tips
- Measure the total rise in two or three places. Floors are rarely level, and a quarter inch matters over 14 steps.
- Measure to the finished floors. If a floor isn't finished yet, add its finish thickness.
- Mark the riser height from the decimal, not the rounded fraction, if you can. Rounding 14 times adds up.
Questions
How do I calculate the number of stairs?
Divide the total rise by the tallest riser you want and round up. That's the number of risers, and there's one tread fewer.
How long should a stair stringer be?
At least the straight line over the total rise and total run, which the calculator shows, plus extra for the bottom and top cuts. Buy the next standard length up.
Can it do landings or L shaped stairs?
Not on this page. It handles one straight flight. My Stair Calculator app does L and U stairs with landings, stringer layout marks and balusters in its Pro version.
Does it work in metric?
You can type millimeters, like 2794 mm. The answers show in feet, inches and fractions.
Stair Calculator
Risers, treads and stringer from three numbers, drawn to scale, plus stringer layout and landings in Pro.