The number of stringers needed for stairs depends on several factors including the width of the stairs, the material and strength of the stringers, and local building codes. Typically, for residential stairs, you would use 2×12 lumber for stringers which spaced about 16 inches apart.

Residential stairs must have a minimum width of at least 36 inches and have stringers every 16 to 18 inches. Stairs that are 36 inches wide require at least three stringers. At least four stringers are required for stairs wider than 36 inches.

The spacing between stringers for stairs depends on various factors such as the load capacity requirements, building codes, material properties, and the intended use of the structure. In general, stringers are typically spaced 16 to 18 inches apart for wooden staircases.

To calculate the number of stringers you need for a staircase, divide the total width of the staircase in inches by the distance between stringers of 18, add 1, and round the results to the nearest whole number. This results in the total number of stringers required for a staircase.

For example, to calculate the number of stringers do i need for 10 foot-wide stair. Total number of stringer for 10 foot-wide stair = (width of the staircase in inches/ spacing between the stringer) + 1 = (10 × 12)/18 + 1 = 120/18 + 1 = 7.66, (or rounded to 8 stringers).

**Here’s a general guideline for how many stringers do I need for 10, 8, 6, 5, 4 & 3 foot wide stairs:**

- You would likely need at least 8 stringers for 10-foot (or 120 inch) wide stairs. Total number of stringers for 10 foot-wide stair = (10 × 12)/18 + 1 = 7.66, (or rounded to 8 stringers).
- You would likely need at least 7 stringers for 9-foot (or 108 inch) wide stairs. Total number of stringers for 9 foot-wide stair = (9 × 12)/18 + 1 = 7 stringers.
- You would likely need at least 6 stringers for 8-foot (or 96 inch) wide stairs. Total number of stringers for 8 foot-wide stair = (8 × 12)/18 + 1 = 6.33, (or rounded to 6 stringers).
- You would likely need at least 5 stringers for 6-foot (or 72 inch) wide stairs. Total number of stringers for 6 foot-wide stair = (6 × 12)/18 + 1 = 5 stringers.
- You would likely need at least 4 stringers for 5-foot (or 60 inch) wide stairs. Total number of stringers for 5 foot-wide stair = (5 × 12)/18 + 1 = 4.33, (or rounded to 4 stringers).
- You would likely need at least 3 stringers for 4-foot (or 48 inch) wide stairs. Total number of stringers for 4 foot-wide stair = (4 × 12)/18 + 1 = 3.66, (or rounded to 3 stringers).
- You would likely need at least 3 stringers for 3-foot (or 36 inch) wide stairs. Total number of stringers for 3 foot-wide stair = (3 × 12)/18 + 1 = 3 stringers.

**Conclusions:**

You would need at least 8 stringers for 10-foot wide stairs, 7 stringers for 9-foot wide stairs, 6 stringers for 8-foot wide stairs, 5 stringers for 6-foot wide stairs, 4 stringers for 5-foot wide stairs, and 3 stringers for 3 and 4-foot wide stairs.