Shiplap Calculator

Shiplap is sold by the board and imagined by the wall, and the conversion runs through one number most buyers forget: the board’s exposed width once its rabbeted edge disappears into its neighbour. This calculator does the conversion honestly — wall dimensions, board length, exposed face width — and returns rows, boards, linear feet and the waste-included order, for walls and ceilings alike.

The Exposed Width Is the Whole Trick

A nominal 1×6 shiplap board is not six inches of wall. The board's actual width is about 5.5–5.75 inches to begin with (nominal lumber dimensions are historical, not tape measurements), and its rabbeted lap then hides a further slice inside the joint above — what remains visible, the exposed face, is typically 5 to 5.5 inches depending on the profile and the gap style (a nickel-gap installation deliberately opens the reveal a fraction wider, shrinking the exposure further). Every board-count error in the wild is this number ignored: coverage computed at a full 6 inches underbuys by about 10%, discovered on the last wall, on a Sunday. The calculator makes exposed width a first-class input — measure your actual boards, from the face to where the next board's face will sit, and enter that; the product literature's "coverage width" figure, where given, is exactly this number wearing its trade name.

Worked Example: A 12 × 8 Accent Wall

The defaults: a 12 ft wide, 8 ft high wall, 1×6 boards exposing 5.5 inches, 12 ft board lengths. Rows: the wall's 96 inches of height ÷ 5.5 = 17.45, round up to 18 rows (the 18th is ripped narrower at the top — plan its cut so the partial row lands where the eye forgives, usually the ceiling). Boards per row: the 12 ft wall takes exactly one 12 ft board — 18 boards, 216 linear feet, and 20 boards with the standard 10% cutting-and-defect allowance. Sanity-check by coverage: each board covers 12 ft × 5.5 in = 5.5 sq ft of wall; 18 × 5.5 = 99 sq ft against the wall's 96 — the surplus is the ripped row's offcut, which is precisely correct and precisely why per-area division alone (96 ÷ 5.5 = 17.45 → "18 boards") accidentally works here while failing on any wall whose length forces joints: rows and per-row counts, the calculator's structure, are the geometry; area division is a coincidence that breaks at scale.

Joints, Staggering and the Long Wall

When the wall outruns the board (a 16 ft wall with 12 ft boards), each row becomes one full board plus a cut piece, and the professional pattern staggers those joints — alternating which side takes the full board, using the previous row's offcut to start the next where lengths allow. The calculator's per-row count (ceil of wall ÷ board length) prices the material honestly under a conservative assumption — every row bought fresh — and the waste allowance then funds the offcut economy: on long runs, staggered joints with offcut reuse commonly beat the estimate by a board or two, which is the correct direction for an estimate to err. Two craft notes belong here: joints land best on studs, so row pieces are cut to the stud layout, not to abstract halves; and end joints in shiplap are butt joints, visibly cleaner with a slight bevel (the "butt-joint shadow" is a finish-carpentry signature). Vertical installations transpose everything — swap width and height, as the calculator's note invites: rows become columns, and the ripped piece hides along a corner instead of the ceiling.

Ceilings, Wainscot and Partial Walls

The same arithmetic serves any rectangular field of boards: a ceiling is a wall with its height renamed (mind the board direction and the joist spacing for nailing, and add a row's allowance for the out-of-square every real ceiling owns); a wainscot is a wall at chair-rail height (enter 3–3.5 ft) topped by trim the board count needn't include. Openings — windows, doors — are subtracted by the honest method: compute the full rectangle, then deduct the opening's rows only where the opening truly removes full boards (a door removes most of its rows but the header strip above it still needs boards; window walls in practice save less than their glass area suggests, because sills and headers consume the offcuts). Beginners deduct optimistically and re-buy; the calculator's advice is to run the whole wall and let the 10% allowance absorb modest openings, deducting only for genuinely large ones (garage doors, picture windows) — with the deducted case run as its own second calculation.

Acclimation and Movement: Buy for the Wall It Becomes

Wood boards arrive at the moisture of the yard and retire at the moisture of the room, shrinking across their width as they settle — which is why shiplap is acclimated in the room (stacked, stickered, for several days) before the rows are counted against it, and why the installation leaves a small expansion allowance at the wall's ends, hidden by trim. The count on this page is not changed by acclimation — rows are rows — but the pitch is: boards installed green-tight can open visible gaps as they dry (each board surrendering a fraction of a millimetre across its face, summed over 18 rows into a wandering last-row width), while the nickel-gap style masks exactly this movement by designing the shadow in from the start. Buy the count dry-eyed, install it acclimated, and the ripped final row stays the planned one.

The Cost Column

The count converts to a budget in one multiplication, worth doing at the kitchen table rather than the lumber desk: the worked example's 20 boards at a representative $9 per 12 ft board is $180 of material for a 96 sq ft wall — under $2 per square foot of finished surface before fasteners and finish — which is the figure that belongs in the accent-wall comparison against tile, board-and-batten, or paint. Prices wander by region and species (cedar and primed MDF live at different altitudes from knotty pine), but the structure never changes: boards × price, with the waste boards already inside the count this page returns.

Related Tools

More from the Construction Calculators hub: the Lumber Weight Calculator — what this many boards will weigh on the truck — and the Square Feet to Cubic Yards Calculator for the slab beneath the wall.

Frequently Asked Questions

How much shiplap do I need for 100 square feet?

At the common 5.5-inch exposure, each linear foot of board covers 5.5/12 ≈ 0.458 sq ft, so 100 sq ft needs about 218 linear feet before waste — roughly 20 boards at 12 ft with the allowance, the worked example's neighbourhood. But let the calculator do it by rows for your actual wall: the row round-up, not the area ratio, decides the true count on short walls, where the ripped last row is a whole board bought for a third of its face.

What is the exposed width of a 1×8 or 1×4?

Nominal 1×8 boards (7.25 in actual) typically expose about 6.75–7 inches after the lap; 1×4s (3.5 in actual) expose about 3–3.25. Profiles vary by mill and by gap style — MDF and PVC shiplap products publish a coverage width, pine boards vary by supplier — so measure a dry-laid pair on the floor: two boards lapped as installed, face edge to face edge across both, divided by two, is the number to enter.

Should shiplap gaps be included in the face width?

They are part of it: the exposed face per board includes the reveal (the shadow gap) the profile creates — measure from one board's face edge to the identical point on the next installed board, gap included, because that pitch is what advances up the wall per row. Nickel-gap installations simply have a larger pitch fraction spent on shadow; the calculator is indifferent, since pitch is pitch.

How much waste should I really order?

Ten percent funds cutting errors, the occasional split or warped board, and joint staggering on single-room projects; fifteen is wiser for mixed lengths, diagonal installation (a different, hungrier geometry — herringbone and chevron layouts are not this calculator's rectangular maths), or lumberyard stock you cannot inspect. Straight runs of full-length boards on a clean rectangle can run at 5% — if you enjoy second trips, which are the tuition this page exists to discount.

Can I run shiplap in a bathroom or kitchen?

Yes, with material judgement: real pine wants sealing on all faces (the back included) in humid rooms, and PVC or MDF shiplap products exist precisely for wet zones. The counting arithmetic is unchanged — only the board's exposure figure, which the synthetic products publish reliably. Behind showers and inside wet walls proper, tile and code take over; shiplap's territory is the splash-adjacent wall, where it performs indefinitely when sealed.

Do I need furring strips or can I nail to studs?

Horizontal shiplap nails directly to studs (16 or 24 in on centre), the standard case the board lengths assume; vertical installation needs horizontal furring or blocking to nail to, which adds material this calculator doesn't count but also doesn't need to — its concern is boards. Mark the stud layout before the first row: the layout discipline (level first row, checked every few rows against cumulative pitch error) is worth more than any count on this page, since a quarter-inch of pitch drift over 18 rows is a visible 4-inch lean at the ceiling.