How to cut stair stringers
A stair stringer is cut by stepping a framing square down a 2x12 once per riser, then sawing to the lines. The calculator gives every number the layout needs:
- Set the square. Clamp stair gauges (or a straight piece of wood) at the riser height on the narrow tongue and the unit run on the wide body. The heel of the square points away from the edge.
- Step it off. Start at one end with both gauges on the top edge of the 2x12. Trace the riser and tread, slide the square so the riser gauge sits on the end of the last tread line, and repeat. The marks in the results are where each step's nosing point falls along the edge.
- Drop the stringer. Cut the tread thickness off the bottom (the level cut). With 1-1/2 in treads on 7-1/2″ risers, the first rise on the stringer is 6″.
- Cut the top. The top plumb cut sits against the rim or header so the top tread is one riser below the floor.
- Use the first stringer as the pattern for the others, after you test-fit it.
The total rise and riser count come from the stair calculator; this page is for the cut itself.
How long is a stair stringer?
A stair stringer is a little longer than the straight diagonal from the bottom of the first riser to the top of the last one. Measured along the top edge of the board, the length is ((n − 1) × T² + n × R²) ÷ √(R² + T²) for n risers of R and treads of T. For 14 risers of 7-1/2″ and 10-1/2 in treads that is 14′ 4-1/8″, so a 14 ft board works and 16 ft leaves room for a mistake.
| Total rise | Risers | Riser | Stringer length | Throat |
|---|---|---|---|---|
| 3′ 0″ | 5 | 7-3/16″ | 4′ 7″ | 5-5/16″ |
| 4′ 0″ | 7 | 6-7/8″ | 6′ 7″ | 5-1/2″ |
| 6′ 0″ | 10 | 7-3/16″ | 9′ 10-5/8″ | 5-5/16″ |
| 8′ 0″ | 13 | 7-3/8″ | 13′ 2-5/16″ | 5-3/16″ |
| 8′ 9″ | 14 | 7-1/2″ | 14′ 4-1/8″ | 5-1/8″ |
| 10′ 0″ | 16 | 7-1/2″ | 16′ 5-15/16″ | 5-1/8″ |
Throat depth and stringer size
The throat is the wood left below the deepest notch, and it carries the load. On a 2x12 (11-1/4 in) with steps of R by T, the notch takes R × T ÷ √(R² + T²) out of the width. Deeper treads and taller risers leave less wood, which is why most stairs use 2x12 rather than 2x10 stringers. Deck guides set a minimum remaining depth for cut stringers; the deck stair calculator uses it for outdoor stairs[3].
How many stringers?
The count is the stair width divided by the maximum stringer spacing, rounded up, plus one. A 36 in stair with stringers no more than 16 in apart needs 36 ÷ 16 = 2.25 → 3 + 1 = 4 stringers if you count both edges, or 3 if the outer two are spaced closer. The spacing limit comes from the tread material: 5/4 deck boards span less than 2x6 treads[4].
Code checks that apply to the stringer
- Riser and tread variation: the largest riser or tread in a flight may exceed the smallest by no more than 3/8 in under the IRC[1][2]. The drop at the bottom and an uneven floor are the usual culprits.
- Maximum riser 7-3/4 in and minimum tread 10 in (IRC R311.7.5; R318.7.5 in 2024)[1][2].
- For the overall plan, including landings and how much floor the stair takes, use the stair rise and run calculator.
Stair stringer layout mistakes to avoid
A good stair stringer layout depends on a steady square and a test fit; these are the mistakes that waste a 2x12:
- Square set backwards. The riser goes on the narrow tongue and the run on the wide body.
- No stair gauges. Without stops on the square, small errors add up over 13 steps.
- Overcutting the notches. Stop the circular saw at the line and finish the corner with a handsaw so the throat keeps its strength.
- Forgetting the drop for the tread thickness at the bottom.
- Thin throat. DCA 6 wants at least 5 in of wood under the notches of a cut deck stringer[3].
- Cutting all stringers first. Test-fit one, check every riser with a level, then use it as the pattern.
Frequently asked questions
What size lumber is used for stair stringers?
Most cut stringers are 2x12, which is 11-1/4 in wide. That leaves about 5 in of throat on a typical 7-1/2 × 10 in step. 2x10 stringers only suit short, shallow steps.
How do I drop a stair stringer?
Cut the thickness of one tread off the bottom level cut. Without the drop the first step is too tall by a tread thickness and the top step too short.
How far apart should stair stringers be?
Space them to suit the tread: thinner treads need closer stringers. Many outdoor stairs use 16 in or less for 5/4 deck boards. Check the tread maker and your local code.
How long is a stringer for 8 steps?
Eight risers of 7-1/2 in with 10-1/2 in treads need a stringer about 7′ 10-11/16″ long, so buy an 8 ft 2x12.
What is the formula for stair stringers?
Riser = total rise ÷ number of risers; the step diagonal is √(riser² + tread²); the stringer length along its top edge is ((n − 1) × tread² + n × riser²) ÷ that diagonal.
How far can a stair stringer span?
For deck stairs, AWC DCA 6 limits a cut 2x12 stringer to a 6 ft horizontal span and a solid 2x12 stringer to 13 ft 3 in; longer stairs need a mid support or a landing.
How do you calculate stair stringer length?
Along the top edge it is ((n − 1) × tread² + n × riser²) ÷ √(riser² + tread²) for n risers; 14 risers of 7-1/2 in with 10 in treads give 13 ft 11 in. Buy the next longer board.
How to calculate stair stringers step by step?
Divide the total rise by the maximum riser and round up for the riser count, divide the rise by that count for the riser height, choose the tread, then lay out one step per riser with a framing square and drop the bottom by the tread thickness.
Sources
- 2021 International Residential Code, Chapter 3: Building Planning (R301.5, R311.7 stairways, R312 guards)
- 2024 International Residential Code, Chapter 3 (R318.7 stairways, R320 handrails, R321 guards)
- DCA 6: Prescriptive Residential Wood Deck Construction Guide (2015 IRC basis)
- 2021 International Residential Code, Chapter 5: Floors (R502 joists, R507 decks)