Calculate hull plank widths for lapstrake or carvel boat building.
6 planks on a small hull
8 planks on a mid-size hull
10 planks on a larger hull
e.g., inches, mm
e.g., inches, mm
per side
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Download NowClinker and carvel planking lives or dies on a fair, predictable run of widths from the keel up to the sheer, and the plank width calculator sizes that run. Divide the girth, the developed distance around a station from keel edge to sheer, by the number of planks allotted to one side and you get the width each strake should carry at that frame.
Because a hull narrows toward bow and stern, the same plank is wide amidships and pinches fine at the ends, so the tool compares two stations and reports the width at each plus their average, giving you a working target that balances a full, efficient midsection against a graceful, narrow run aft.
Enter the girth at two stations and the number of planks per side, and the calculator returns the plank width at each station and the average, the raw numbers for marking out strides and planning the taper of every strake.
Width at A = Girth at A ÷ Number of Planks; Width at B = Girth at B ÷ Number of Planks; Average Width = (Width A + Width B) ÷ 2| Hull Size | Planks per Side | Plank Width Feel |
|---|---|---|
| Small dinghy | 3 - 5 | Wide, few strakes |
| Daysailer | 5 - 8 | Moderate strides |
| Cruising launch | 8 - 12 | Narrower, more strakes |
| Large classic | 12 - 20 | Fine, many strakes |
Plank counts and typical girth behaviour.
Formula verified against standard naval-architecture references · Updated October 2026
Divide the developed girth at a station, the distance around the section from keel to sheer, by the number of planks you have allotted to one side. A 60 inch girth with eight planks gives seven and a half inch strakes there. Because the girth shrinks toward the ends, do the sum at two stations to see the taper and mill to the average.
It is the length measured around the outside of a hull section from the keel edge up to the sheer, following the curve rather than straight across. Measured with a flexible batten or read from the offsets on the lines plan, the girth is the total width of planking to be shared out among the strakes at that station, so it is the number you divide by plank count.
It depends on hull size and style, from three or four on a small dinghy to twelve or twenty on a large classic. More, narrower planks bend easier across a double-curved hull and show finer work; fewer, wider planks are quicker and suit flatter sections. Pick the count from the lines and the plank stock you can realistically fair, then compute widths.
Because the girth does. A hull narrows fore and aft, so the same number of strakes must fill a shorter distance, and each plank tapers as it runs into the deadwood or stem. Dividing the girth at two stations reveals that taper so you can fair the widths into a smooth, graceful diminution rather than abrupt steps.
At a given station, equal division gives a fair starting point, but in practice you fair the widths along the length and may let the sheer strake stay a little wider for strength and fastening. The goal is a smooth, believable taper with no plank suddenly pinching, so treat the calculated width as a target line, not a rigid rule.
A little over the calculated width, because each strake loses edge material to its bevel or round, to trimming to the marked stride and to the grain at the taper. Milling slightly proud and shaping down keeps every plank full where it matters, whereas cutting to the bare number risks thin, breakable edges at the sheer.