Woodworking & Carpentry · Calculator
Stair Stringer Calculator
This stair stringer calculator turns your total rise into a step count, exact riser height, tread count, stringer length and framing square settings, checked against the IRC. Enter the rise, pick your stock, and the diagram redraws your stringer to scale.
Your stairs
Measure from the finished lower floor (or landing pad) straight up to the finished upper floor or deck surface.
The IRC allows up to 7-3/4 in. A lower max gives you more, shorter steps and an easier climb.
How the stair stringer calculator works
Start with total rise, measured from the finished lower floor to the finished upper floor. Measure it where the bottom step will actually land, because floors and patios are rarely level. An eighth of an inch error here repeats on every step.
Divide by the max riser height and round up. That gives you the fewest risers that stay under code. Then divide the total rise by that whole number to get the exact riser height, which is always equal to or less than your max.
Treads are one fewer than risers, because the upper floor is the top step. Multiply treads by tread depth to get the total run, which tells you where the stairs land. Finally, check the stringer length and the throat: the wood left below the notches after you cut them.
IRC stair code requirements
These are the International Residential Code limits most US jurisdictions use for one and two family homes. Your local code may amend them, so confirm with your building department before you cut.
| Item | Requirement | IRC section |
|---|---|---|
| Riser height | 7-3/4 in max | R311.7.5.1 |
| Tread depth | 10 in min, nose to nose | R311.7.5.2 |
| Riser or tread variation | 3/8 in max in one flight | R311.7.5.1 / .2 |
| Nosing | 3/4 to 1-1/4 in (not required on 11 in treads) | R311.7.5.3 |
| Open risers | 4 in sphere may not pass (rise over 30 in) | R311.7.5.1 |
| Headroom | 6 ft 8 in min, measured from the nosing line | R311.7.2 |
| Stair width | 36 in min clear above the handrail | R311.7.1 |
| Handrail height | 34 to 38 in above the nosings | R311.7.8.1 |
| Handrail required | Flights with 4 or more risers | R311.7.8 |
| Max rise per flight | 151 in (12 ft 7 in) between landings | R311.7.3 |
| Landings | Top and bottom, at least 36 in deep | R311.7.6 |
Two rules trip people up most. The 3/8 inch variation limit means your top and bottom steps must match the rest, which is why the stringer gets dropped by one tread thickness. And 2R + T (two risers plus one tread) between 24 and 25 inches is not code, but it is the best predictor of whether a stair feels natural to climb.
How to lay out and cut a stringer with a framing square
- Pick a straight 2x12 and sight down it. Lay it crown up, so the notches are cut from the crowned edge.
- Set stair gauges on your framing square: the riser height on the tongue (the short, narrow leg) and the tread depth on the body (the long, wide leg). For the example above that is 7-1/2" and 10 in.
- Mark the first step near the bottom end, with both gauges tight to the top edge. Trace the outside of the tongue and body.
- Slide the square up so the tongue mark lines up with the end of the last body mark, and repeat until you have one riser more than treads.
- Mark the bottom level cut by extending the first tread line through the bottom of the board, then take one tread thickness off the bottom riser. That is the drop.
- Mark the top plumb cut where the stringer meets the rim joist or header, allowing for any hanger or ledger.
- Cut with a circular saw, stopping at the lines, then finish each corner with a jigsaw or handsaw so you do not overcut into the throat. Our guide to the best circular saws covers the models that handle 2x12 stock easily.
- Use the first stringer as a template for the rest, and test fit it before cutting the others.
If you need to rip treads or riser boards to width, see our walkthrough on how to rip cut with a circular saw.
Deck stair notes
Outdoor stairs follow the same IRC rules, and the AWC DCA 6 deck guide adds a few more for cut stringers. Here is what matters:
- Use 2x12 stock with at least 5 in of throat left after notching.
- Space stringers 18 in on center or less. Composite treads usually need 12 in, so check the manufacturer.
- Limit the span. A cut stringer can span 6 ft horizontally (7 ft for southern pine 2x12s). Longer runs need a support post or a landing.
- Land on something solid. A concrete pad keeps the bottom of the stringers out of the dirt. Our concrete calculator sizes the pad and bag count.
Measure rise at the landing spot
Common stair building mistakes
- Measuring to the subfloor. Always measure finished floor to finished floor. If the flooring is not in yet, add its thickness at the top and bottom.
- Forgetting the drop. Skip it and your first step is a tread thickness too tall.
- Buying stock too short. The layout needs more than the straight line length once you add end cuts. Buy the next size up.
- Overcutting the notches. A saw kerf past the corner weakens the throat right where the load is highest.
Framing a roof next? The roof pitch calculator uses the same rise and run math for rafters, and the board foot calculator prices the lumber. Run your numbers through the stair stringer calculator one more time before you cut the first stringer.
Frequently asked questions
How do I calculate how many stairs I need?
Divide the total rise in inches by 7.75 (the IRC maximum riser height) and round up. That is your number of risers. Divide the total rise by that number to get the exact riser height. You need one fewer tread than risers when the upper floor serves as the top step, so a 105 inch rise needs 14 risers at 7-1/2 inches and 13 treads.
What is the maximum stair riser height allowed by code?
The IRC allows a maximum riser height of 7-3/4 inches and a minimum tread depth of 10 inches for homes. Within one flight, the tallest and shortest risers can differ by no more than 3/8 inch, and the same limit applies to tread depths. Some local codes are stricter, so check with your building department.
How long should a stair stringer be?
The stringer is roughly the hypotenuse of the total rise and total run: the square root of rise squared plus run squared. For a 105 inch rise with 13 treads at 10 inches, that is about 167 inches, or 13 feet 11 inches. Buy the next standard board length with room to spare for the end cuts, which means 16 foot stock in that case.
Why do you cut the bottom of a stair stringer shorter?
You cut the bottom riser short by one tread thickness, called dropping the stringer, so every finished step comes out the same height. Without the drop, the first step would be taller by the thickness of the tread and the top step shorter by the same amount, which is a trip hazard and a code violation.
How many stringers do I need for stairs?
For deck stairs, the AWC DCA 6 guide limits cut stringers to 18 inches on center, so a 36 inch wide stair needs at least 3 stringers. Many builders use 4 stringers at 12 inches on center for 36 inch stairs, and most composite tread makers require 12 inch spacing. Check your tread manufacturer before you buy lumber.
Can I use a 2x10 for stair stringers?
Usually not. A 7-1/2 inch riser and 10 inch tread cut a notch 6 inches deep, which leaves only 3-1/4 inches of wood in a 2x10. That is below the 3-1/2 inch rule of thumb and well below the 5 inch minimum DCA 6 sets for deck stairs, so use 2x12 stock for cut stringers.
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