The ratio is the first question, not the waste
Every article about herringbone starts with the waste percentage. That is the second question. The first is whether the plank can make the pattern at all, and the test is arithmetic: in a herringbone the end of each plank butts against the long side of its neighbour, so the length has to be a whole number of widths. Four to one, three to one, two to one all work. A ratio of 4.0002 does not, and neither does 3.94.
Products actually sold as herringbone are milled to an exact ratio for exactly this reason. Ordinary plank that happens to be roughly four times as long as it is wide is the trap, because the pattern looks fine for the first metre and then opens up.
Why the error never averages out
In a running-bond floor, a plank that is a sixteenth short just moves the next joint a sixteenth and nothing else notices. In a herringbone each plank is located by the one before it, in a chain that runs diagonally across the room. A sixteenth of error per unit is a sixteenth added every unit, in the same direction, for as long as the run goes on. The page turns that into the only number that matters on site: how many units you get before the gap reaches whatever you decided you could live with.
The fix, when the ratio is out, is to trim every plank down to the nearest exact multiple of the width. It works, and it costs a cut on every board plus the offcut. The page prices that as a percentage of the material so it can be compared against buying the right product.
Where the border pieces come from
A floor has a fixed amount of joint per square foot — one over the width plus one over the length. How many of those joints meet a given wall depends on the angle between them. Planks running parallel to a wall present only their end joints to it; turn the whole field 45 degrees and both families of joints meet every wall, at an angle that puts them closer together along it. That is the whole reason a herringbone border has more pieces in it, and it is why the extra material is at the edges of the room rather than spread through it.
The counts here are worked from that density rather than by drawing the floor, so they are good to within a piece or two per wall and shift with where the pattern starts. The comparison against the same plank laid straight uses the better of the two straight directions, so it is not flattering the pattern.
Labour is the part this cannot price
The material gap between a herringbone and a straight floor is real but modest. The labour gap is not. Every board is placed against two neighbours instead of one, the field has to be set out from working lines rather than a wall, and every border piece is a 45 degree cut that has to be scribed rather than measured. If the floor is being paid for by the hour, that is where the money is, and no arithmetic on this page reaches it.
Questions people ask
What plank ratio do I need for herringbone?
A whole number: the length has to divide by the covered width exactly, so 2 to 1, 3 to 1 and 4 to 1 all work and 3.94 to 1 does not. Boards sold as herringbone are milled to that ratio deliberately. Ordinary plank that happens to be about four times as long as it is wide is where the trouble starts, because the pattern opens up a fraction at every unit and keeps going.
Can I use a 23-5/8 in plank that is 5-29/32 wide?
That is exactly 4 to 1, because four times 5-29/32 is 23-5/8 to the sixteenth, and it closes forever. The pair that catches people is a 23-5/8 in board against a 6 in width, which is 3.94 to 1 and out by three eighths every unit. Put your own measured figures in the fields rather than the carton sizes, because that is exactly the difference between those two cases.
How much more material does herringbone need?
On plank count, less than most people expect, because the extra is all in the border and the border pieces are triangles that often pair up off one plank. The page prints the herringbone order against the same plank laid straight so the gap is visible for your room. What it deliberately does not do is price the labour, which is where the real difference between the two floors sits.
Where do I start a herringbone floor?
Off two working lines at 45 degrees crossing near the middle of the room, not off a wall. A herringbone laid from a wall inherits every bit of that wall being out of straight, and a 45 degree field shows a racked start at both ends of the room where the border triangles change size along the run. Setting out from the centre puts any error in the border, which is where it can be absorbed.
Is chevron the same thing?
No, and none of the ratio arithmetic applies. In a chevron the plank ends are mitred so two boards meet in a point and form a continuous zigzag, which means the boards come left-handed and right-handed and cannot be swapped. In a herringbone the ends are square and butt into the side of the neighbour, which is why the length has to divide by the width in the first place.