The wire is not the drop
Here is the standard mistake, in numbers. A 24 by 30 frame with D-rings 6 inches down the back, 4 inches in from each side. You want the centre at 58, so the top of the frame lands at 73 and the ring line at 67. Put the hook at 67 and stand back: the picture is four inches low.
It is low because the wire does not stay on the ring line. Pull it up and it forms a triangle — 16 inches of span between the rings along the bottom, and half of the 18 inch wire, 9 inches, running up each side. The apex sits above the ring line by the square root of 9 squared less 8 squared, which is the square root of 17, or 4.12 inches. So the hook belongs at 71.12, and the number you measure down from the top of the frame is 6 less 4.12, which is 1.88 inches rather than 6.
This is why hanging by the ring measurement puts pictures high — the frame rides up to meet a hook that was set as if the wire were flat against the back. Four inches on a 30 inch frame is 14 percent of its height, which is more than enough to look wrong next to anything else on the wall.
Two ways to get the lift, and one of them takes no arithmetic
If you know the wire length between the rings, the lift is the square root of half the wire squared minus half the span squared. If you would rather not, hook a finger under the middle of the wire, pull it up until it is tight, and measure from the ring line to your finger. That is the lift, directly, and the page will work the wire length back out of it if you want to check.
The finger method is better on a frame that already exists, because a wire measured slack on a bench is not quite the wire under load — it stretches, the knots pull down, and the rings rotate. The calculation is better when you are setting up a batch of frames the same way and want one number to work to.
Why a tight wire is hard to hang accurately
The sensitivity figure the page reports is how far the picture moves for every inch of wire length, and it is not a constant. It is the wire length divided by four times the lift, which means it grows without limit as the wire approaches the span.
On the defaults it is 1.09, so half an inch more wire raises the picture by about half an inch — manageable. Shorten that wire to 17 inches and the lift drops to 2.87 and the sensitivity climbs to 1.48. At 16.5 inches of wire on a 16 inch span the lift is 2.02 and the sensitivity is 2.05, so a quarter inch of wire is half an inch of picture. Near the limit, tying the knot a fraction differently moves the picture visibly.
The general shape of it: slack wire hangs predictably in height and lets the frame slide sideways on the hook; tight wire holds its sideways position and becomes unpredictable in height. Neither is correct. Knowing which failure you are choosing is the useful part.
Two hooks instead of one
Hanging straight off the D-rings on two hooks removes the wire and with it the whole triangle — the hook height is the ring line, full stop. What it also removes is the self-levelling. A wire lets the frame find its own horizontal on a single hook; two hooks fix it at whatever angle your two pencil marks describe.
So the two-hook method is more accurate about height and less forgiving about level. Measure both marks up from the same reference rather than measuring the second across from the first, because an error transferred sideways is exactly the error you cannot see until the frame is up.
The offset holds wherever you hang it
Once the hook offset is known for a frame it does not change with height. On the defaults the hook sits 13-1/8 inches above the centre of the picture, and that is true at 52 inches, at 58 and at 64 — the whole assembly slides up and down together. This is what makes a gallery wall tractable: work the offset out once per hanger type, mark all the frame centres, and step every hook up from its centre by the same amount.
It also means that if a picture ends up wrong, the error is in one of three places and they are easy to separate. Wrong centre line means everything is wrong together. Wrong lift means one frame is high or low against the others. Wrong ring drop means the frame is not where its own hanger says it should be, which usually shows up as the rings being uneven side to side.
Questions people ask
Why do my pictures always hang too high?
Almost always because the hook was set at the D-ring drop instead of at the wire lift. The wire rises when it takes weight, and the hook meets it at the apex of a triangle rather than on the ring line. On a 24 inch wide frame with rings 4 inches in and 18 inches of wire between them, that apex sits 4.12 inches above the rings — so a hook placed at the ring drop puts the picture 4.12 inches high. The taller the frame, the less it shows; on a small frame it is glaring.
How do I measure the wire lift without doing the arithmetic?
Hook a finger under the middle of the wire, pull it up until the wire is tight, and measure from the ring line to your finger. That distance is the lift. It is more reliable than a calculation on an existing frame, because a wire under load stretches slightly and the knots settle. Switch the page to the measured-lift option and it will work the wire length back out of your measurement, which is a useful check that the rings are where you think they are.
What height should the centre of a picture be?
The page takes it as an input and states no standard. A centre line somewhere in the high fifties is a common gallery convention built around average standing eye level, but a room seen from a sofa, a wall with a high ceiling, or a piece above a console all argue for something else. What matters more than the exact figure is using the same figure across a wall, so that everything relates to one line rather than to whatever furniture happens to be under each piece.
Is it better to hang on a wire or on two hooks?
They fail differently. A wire on one hook lets the frame find its own level and slide sideways to correct a mark that was slightly off, at the cost of a height that depends on the wire length. Two hooks in the D-rings give an exact height with no triangle to work out, but the frame is then exactly as level as your two pencil marks and cannot self-correct. On heavy or wide pieces the two-hook method also spreads the load across two points instead of one, though whether any given hook or wall takes it is a question for the hardware maker and for whoever knows what is in that wall.
Why does a short wire make the height so unpredictable?
Because the triangle flattens. The lift is the square root of half the wire squared less half the span squared, and as the wire approaches the span that difference shrinks fast. On the defaults an extra inch of wire raises the picture 1.09 inches. Shorten the wire from 18 to 16.5 on the same 16 inch span and an extra inch of wire moves it 2.05 inches, so a knot tied a quarter inch differently is half an inch of picture. A wire pulled nearly straight is also the one that snaps.