The end hinges are the layout
A hanging leaf tries to rotate about the bottom hinge, and the only thing stopping it is the top hinge pulling back. Take moments about the bottom: the weight acting at half the leaf width plus the pivot offset, divided by the distance between the two end hinges. On a 36 inch leaf weighing 130 lb, the arm is 18-1/4 inches and a 62 inch span gives 38.3 lb of horizontal pull at the top hinge and the same push at the bottom.
The middle hinge does not appear in that sum. It takes a share of the vertical weight and it keeps the leaf from bowing, but the couple lands on the pair at the ends whatever you do with it. Which is the honest answer to the argument about where the third hinge goes: for the load, it barely matters; for how the door looks in the opening, it does.
Two inches of span is a real number
Drop the top hinge an inch and bring the bottom up an inch and the span goes from 62 to 60. The pull goes from 38.3 lb to 39.5 lb — 1.2 lb, about 3 percent, for a change nobody would notice looking at the door. Do it on a heavy leaf and the same 3 percent is worth more. It is the mechanism behind a rehung door that starts to sag when the original never did: the new marks gathered the end hinges in, and the jamb has been carrying more ever since.
Transferring to the jamb
Every jamb position is its leaf position plus the head gap, because the leaf hangs that far below the head jamb. An eighth of an inch, and it is the eighth that decides whether the leaf ends up sitting proud at the top or scrubbing the head. From the floor, the same hinge on an 80 inch leaf with a half inch gap under it sits at 73-1/2 inches — but only if the floor it is measured from is the floor that ends up there, which on a job where the flooring goes in afterwards it usually is not.
What the page will not do
It will not tell you whether a hinge is up to the job. If you enter a per-hinge figure from your supplier it divides the weight by it and says so plainly, and that division ignores three of the four things a hinge rating actually depends on — leaf width, frequency of use and bearing type as well as weight. The manufacturer sizes hinges with the door schedule in front of them. And on a rated assembly, the prep question comes before the layout question, because cutting one outside its listing voids it. Related: the door slab weight calculator for the weight that goes in the form, and the door hardware schedule calculator for the counts across a set of openings.
Questions people ask
Where does the third hinge go on a door?
For the load it makes little difference, which is not the answer most people expect. The horizontal couple lands on the top and bottom hinges regardless — it is the weight times the arm to the centre of the leaf, divided by the distance between those two. The middle hinge takes vertical share and stops the leaf bowing. Evenly between the other two is the usual setting out and it looks right; packing it up near the top is a real option on a leaf where the top hinge is working hard.
How much force is on the top hinge?
Weight times the arm to the centre of the leaf, divided by the span between the end hinges. A 130 lb leaf 36 in wide with a quarter inch pivot offset has an arm of 18-1/4 in, and across a 62 in span that is 38.3 lb pulling the top hinge out of the jamb, with the same force pushing the bottom one in. The vertical weight splits between all the hinges; this does not.
Does moving the hinges closer together matter?
It is worth more than it looks. Shortening the span from 62 to 60 inches — an inch at each end — raises the top hinge pull from 38.3 to 39.5 lb on the default leaf, around 3 percent. On a heavy leaf that is a few pounds of extra pull on the same screws forever. It is a common reason a rehung door sags when the original one never did.
How do I transfer the hinge positions to the jamb?
Add the head gap to every leaf position. The leaf hangs below the head jamb by that gap, so a hinge 7 inches down the leaf is 7-1/8 down the jamb at an eighth of an inch of gap. Working from the floor instead is fine as long as the floor you measure from is the one that ends up there — flooring going in after the frame was set moves every mark by its build-up.
How many hinges does a door need?
Not something this page decides. If you enter a per-hinge figure from your hinge supplier it divides the leaf weight by it and prints the result as arithmetic, plainly labelled as such. A real hinge selection weighs leaf width, how often the door gets used and the bearing type alongside the weight, and the manufacturer does it against the door schedule. Heavier leaves, tall leaves and doors with closers on them commonly get more than three, and what yours needs comes from the people who make the hinge and the door.