Measuring a stair for its skirt board
A skirt board is laid out from the same numbers that built the stringer. Pull them off the actual stair, not the original plan — field-cut stringers rarely match the drawing exactly.
Measure the built riser and tread
With the stringer already cut (or the finished stair already framed), measure one riser height and one tread depth directly off the stringer's notches. Use the actual cut numbers, not the numbers from a planning calculator.
Count the steps
Count total risers in the flight, including the top step into the upper floor. This, with riser height, gives the calculator the total rise and total run it needs for the rake length.
Decide wall-side or open
A closed stringer against drywall gets a flat skirt with a square-cut bottom that meets the baseboard. An open, exposed stringer usually gets a mitered return at the bottom so no end grain shows.
Check the top condition on site
Walk up to where the skirt terminates — a wall corner, a newel post, or a landing header. Measure how far past the top riser the board actually needs to run; this varies by house and can't be assumed.
The five formulas behind every result
Nothing on this page is a black box. Here's exactly what runs when you change a number.
Rake Length
Rise = riser × steps, Run = tread × (steps − 1). Same hypotenuse math as the stringer itself — the skirt board runs parallel to it.
Rake Angle
The angle the board is cut at on both ends, and the miter-saw bevel setting is 90° − θ from square.
Minimum Rip Width
The perpendicular distance from the nosing line down to each notch's inside corner is constant and equals riser·cos(θ). Add your top reveal and bottom margin on top of that floor.
Layout Cut Length
The rake length plus whatever the board needs to extend at the bottom (baseboard butt or mitered return) and at the top (into the wall or landing).
Board Length to Buy
Add the cut-and-mistake allowance before rounding up to a length actually sold — 6, 8, 10, 12, 14, 16, 18 or 20 feet.
Walking through the default 7.5" riser, 10.5" tread, 14-step stair
Same five formulas, filled in step by step, for the values this calculator loads with — a closed, wall-side stringer.
Rake length
Rise = 7.5" × 14 = 105". Run = 10.5" × 13 = 136.5". Rake = √(105² + 136.5²) ≈ 172.2" — this is the board's diagonal, before either end extension.
Rake angle
θ = atan(7.5 ÷ 10.5) ≈ 35.5°. Both end cuts on the board are made at this angle, and a miter saw is set to 90 − 35.5 = 54.5° from square to make them.
Minimum rip width
7.5 × cos(35.5°) ≈ 6.10". Add the default 0.375" top reveal and 0.5" bottom margin: 6.10 + 0.375 + 0.5 ≈ 6.98". A 1x8 (7.25" actual) technically clears this by a quarter inch — tight enough that most trim carpenters would size up to a 1x10 for a safer margin, which is why this calculator defaults to 1x10 stock.
End extensions and cut length
Square bottom (0.75" baseboard butt) plus a 6" top extension into the upstairs wall: 172.2 + 0.75 + 6 = 178.95" layout length.
Board to buy
Add the 10% cut-and-mistake allowance: 178.95 × 1.10 ≈ 196.8". A 16-footer (192") looks close enough at a glance, but it's actually 4.8" short once the allowance is included — the next standard length up is an 18-footer (216"). This is exactly the kind of gap that shows up only after the waste allowance is added, not from the rake length alone.
This is why "just measure the stringer and round up" undersizes lumber more often than it should — the rake length alone almost always fits inside the next board size down from what you actually need once extensions and waste are added.
Common skirt board stock sizes
Actual dimensions run 3/4" under nominal on both thickness and width for surfaced (S4S) lumber and most primed finger-joint or MDF skirt stock.
| Nominal | Actual Width | Actual Thickness | Typical Use |
|---|---|---|---|
| 1x6 | 5.5" | 0.75" | Very shallow-angle stairs or narrow skirt reveals only. |
| 1x8 | 7.25" | 0.75" | Shallow-angle stairs (under ~33°) with tight reveals. |
| 1x10 | 9.25" | 0.75" | Most residential stairs in the 30–37° comfort range — this calculator's default. |
| 1x12 | 11.25" | 0.75" | Steeper stairs, taller risers, or extra margin for scribing to an uneven wall. |
| 5/4x10 or 5/4x12 | 9.25" / 11.25" | 1.0" | Thicker stock for open-stringer skirts taking a mitered return at the bottom. |
Sheet-stock MDF and primed finger-joint skirt board are typically milled to the same actual widths as S4S pine — confirm with your specific supplier before ordering.
Joinery & wall prep
A skirt board can be cut to the exact right length and angle and still look wrong on the wall. These three things trip up an otherwise-correct board.
Housed (notched) vs. flat skirt
What this calculator sizes is a flat board that runs parallel to the stringer. A "housed stringer" instead has the tread and riser ends routed directly into pockets in the skirt itself — a different, more involved joinery method. Use the Stair Stringer Calculator for that layout.
Scribing to an uneven wall
Drywall is rarely a perfectly straight plane along the full rake. Dry-fit the board, scribe the back edge to the wall's actual surface with a compass, and trim to that line rather than trusting a single straight rip.
Fastening for wood movement
A long skirt board expands and contracts across its width more than its length. Nailing it rigidly along both edges over a long rake can split it seasonally — many installers glue and nail the top edge only, letting the bottom float slightly against the baseboard.
Sequencing with treads and risers
On most builds the skirt goes up before the treads and risers, which are then scribed and butted to it — the reverse order leaves visible gaps at every step where the skirt can't be perfectly registered afterward.
These are planning-stage estimates for material and cut length, not a finish-carpentry install guide. Wall conditions vary enough that final trimming is always done on site.
Common mistakes when cutting a skirt board
Most skirt board problems trace back to one of these five planning errors, not to a bad cut.
Ripping to a fixed width without checking the math
A board that looks plenty wide by eye can still land short of the minimum rip width on a steep stair, leaving a visible gap right at the tightest notch corner.
Cutting to the exact rake length only
Skipping the bottom and top extensions leaves the board floating short of the baseboard or the upstairs wall — both ends need their own allowance, not just the diagonal.
Ordering with zero waste allowance
A compound end cut on an 14–17 foot board is unforgiving — one miscut angle and the whole board is scrap. Buy the next size up, not the exact calculated length.
Assuming the wall is straight
Drywall bows and stud lines drift over a long rake run. Skipping the scribe step is the single most common reason a "correctly cut" skirt still shows daylight along the wall.
Installing before confirming tread/riser stock
If the skirt is set first, its reveal has to match the tread and riser thickness that will butt against it later — confirm that stock before the skirt goes up, not after.
When to use a stair skirt board calculator
Skirt board sizing comes up once the stringer is already built and it's time to finish the sides of the stair.
Finishing an open stringer
Covering a cut, exposed stringer with a clean finish board and mitered returns.
Wall-side trim in new construction
Sizing the board that hides the drywall gap along a closed stringer before painting.
Basement stair finishing
Trimming out a basement stair that was framed with a bare stringer against block or drywall.
Paint-grade vs. stain-grade planning
Deciding between MDF and stain-grade hardwood stock before ordering, based on the required rip width.
Matching skirt to baseboard height
Coordinating the skirt's bottom reveal with the adjoining room's baseboard for a level, continuous line.
Trim scope & material takeoff
Estimating board count and linear footage for a bid or a material order before a job starts.