Stair Stringer Calculator

What a stringer actually is

The stringer is the angled board that carries the stairs: every riser and tread is cut into it or fastened to it. Two stringers hold a narrow flight; a full-width stair usually gets a third down the middle so the treads can't spring.

Total rise is the vertical drop from finished deck surface to finished grade or landing, plumb. Total run is horizontal: tread run × tread count. The stringer is the hypotenuse of that triangle, so it is always longer than the run.

Risers, treads, and why risers = treads + 1

You derive the riser height, you don't pick it: divide the total rise by a riser near the legal maximum, round up to a whole number of risers, then divide the rise by that count.

There is always one more riser than there are treads. The first steps from the ground onto the bottom tread; the last steps from the top tread onto the deck, where no tread is needed. Count risers when you divide the rise, treads when you work out the run and order material.

Notched stringers and dropped stringers

• Notched (cut) stringer. Mark the riser on one leg of the framing square and the tread run on the other, walk the square down the board, and cut the sawtooth so the treads drop into the notches. Standard, and the cheapest to build, but keep notch depth to no more than a third of the board depth.
• Dropped stringer. The board isn't notched into the rim; it is dropped so its top sits below the deck surface and hung on the face of the rim with an approved hanger, leaving the deck edge as the top riser. Full depth is kept at the top.

Cut every stringer from the same marks.

How the 3/8" variation rule works in practice

R311.7.5.1 caps the riser at 7¾″ and adds a second limit: the greatest riser may not be more than 3/8" taller than the smallest. R311.7.5.2 does the same for treads — 10″ minimum nosing to nosing, no tread more than 3/8" off another. A stair can sit under the maximum and still fail on the spread.

In practice you cannot shim a tread to fix a sag, leave the bottom step short, or lay the decking after the stringers are set — the deck board shifts the top riser by its full thickness. One riser off, usually the bottom one, is the most common stair failure.

Nosing — and when you don't need it

R311.7.5.3 requires the nosing to project ¾″ to 1¼″ beyond the riser below, with a radius no larger than 9/16" or a bevel no larger than ½″. The projection counts toward tread depth, so the code measures nosing to nosing. Nosing isn't required where the tread is at least 11″ deep.

Handrails and the 34″–38″ band

R311.7.8 requires a handrail on at least one side of any flight with four or more risers, set 34″ to 38″ above the nosing line, measured vertically and not along the rake. A deck stair that just clears grade often comes out at four risers. Also checked: 36″ clear width (R311.7.1) and 6′8″ headroom (R311.7.2).

Worked example — a 30″ deck rise

The calculator opens on a 30″ rise and a 10″ tread run. Work it by hand:

• Risers: 30 ÷ 7.75 = 3.87 → 4
• Riser height: 30 ÷ 4 = 7.5″, under the 7¾″ maximum
• Treads: 4 − 1 = 3
• Total run: 3 × 10 = 30″ of floor space
• Stringer length: √(30² + 30²) = 42.4″ — the hypotenuse, not the run
• Angle: about 45°. Cut line 10″ horizontal and 6″ vertical — riser minus the 1.5″ tread thickness
• Bottom cut: reduced by the tread thickness so the bottom riser still measures 7.5″
• Handrail: four risers triggers one at 34″–38″ (R311.7.8)

R311.7.6 wants a 36″ landing top and bottom, so the stair eats 30″ of run plus that landing; R311.7.3 caps a flight at 12′7″ of rise.

Common mistakes

• Measuring total rise from the wrong floor level. The rise is between finished surfaces; measure to the frame and every riser shifts by the deck board added later.
• Ignoring the 3/8" variation rule. It fails stairs that are under 7¾″ on every step.
• Too few stringers for the width. Two carry a narrow flight; on a full-width stair the tread springs in the middle.
• Forgetting the bottom of the stringer is cut short. It is reduced by the tread thickness so the first tread sits level.
• Assuming the base IRC numbers apply. Some states allow 8¼″ risers with 9″ treads (CT, VA, MA) or 8″ with 9″ (OR).
• Cutting stock to the hypotenuse. The notches, bottom cut and top connection all eat length.

Stair stringer FAQ

Related pages

• Stair math: riser and tread rules with a worked example
• Deck stairs: how many steps, landings and the stair footing rule
• Deck board spacing calculator
• Deck footing calculator
• Deck beam and joist span calculator