Centre it on the furniture, not on the wall
Take a 120 inch wall with a sofa whose middle sits 54 inches from the left end, because the wall has a doorway at one end and the sofa had to slide. A 4 by 2 block of 16 by 20 frames at 3 inch gaps is 73 inches wide. Centre that on the wall and it runs from 23-1/2 to 96-1/2, and its middle is at 60. Centre it on the sofa and it runs from 17-1/2 to 90-1/2.
Six inches of difference. On the wall-centred version the block sits six inches right of the sofa, and everybody who walks in will register that something is off without being able to name it, because the eye compares the picture block to the nearest large object rather than to the room. The page reports both positions and the offset between them so the choice is explicit rather than accidental.
Where there is nothing underneath — a stairwell, a hallway — the wall centre is the right anchor and the point mode is unnecessary. It is the presence of a large object below that changes the reference.
Hang by the middle of the block, not the top
Setting the top row first is the natural instinct and it is the wrong end to start from. Add a third row later and the whole arrangement grows downward; take one away and it shrinks upward, away from the furniture. Fix the middle instead and the block grows symmetrically, which is how the wall keeps looking balanced as the collection changes.
On the defaults the block is 43 inches tall with its middle at 58, so the top lands at 79-1/2 and the bottom at 36-1/2. With a 32 inch sofa back that is 4-1/2 inches of clear air, which is about as tight as it wants to be, and 16-1/2 inches below a 96 inch ceiling. Both of those numbers are the ones that decide whether the arrangement looks like it belongs on that wall, and neither is visible while you are working out the spacing.
Gaps multiply, and they multiply by the count minus one
Four pieces across have three gaps. Every extra inch of gap adds three inches to the run, so moving from 2 inch gaps to 4 adds six inches — 70 to 76 on the defaults. In a row of eight it would add fourteen, and that is where a plan that fitted the wall on paper stops fitting it.
The practical consequence is to fix the gap against the tightest wall in the plan rather than the most generous. Tight gaps read as one object; wide gaps read as several objects that happen to be near each other. Neither is wrong, but a set that changes its spacing between walls looks like an accident.
Uniform pieces are the easy case
Everything here assumes every frame is the same size, which is what makes the arithmetic exact. Mixed sizes are a genuinely different problem: they are laid out on shared centre lines or shared edges rather than on equal gaps, and the usual method is to trace every frame on paper, tape the paper to the wall, and move it until it looks right.
What survives from this page into that job is the block dimensions and the height. Work out roughly what rectangle the collection occupies, place that rectangle with its centre where you want it, and then arrange within it. The rectangle is the part that has to relate to the room; the internal arrangement only has to relate to itself.
Marking the wall
The centres reported here are frame centres, not hook positions. The distance between them is fixed for a given hanger, so the efficient sequence is to mark all the frame centres first, work the hook offset out once, and then step every hook up from its centre by the same amount. On a wire that offset is not the drop of the D-rings — the wire lifts when you pull it, and the difference is often more than an inch. That is the whole subject of the hanging height page.
Marking a level line matters more than marking accurate centres. A row of pieces whose spacing is a quarter inch out looks fine; a row that climbs a quarter inch from left to right looks broken, because the eye compares the tops of adjacent frames against each other and against the ceiling line.
Questions people ask
How far apart should pictures be on a gallery wall?
There is no correct figure and this page publishes none. What it will tell you is what your choice costs in wall: with four pieces across there are three gaps, so every extra inch of gap adds three inches to the run — 70 inches at a 2 inch gap and 76 at 4 on the default frames. Tight spacing makes the group read as one object, wide spacing makes it read as several. Pick the gap against the tightest wall in the plan so the same spacing works everywhere.
Should the block be centred on the wall or on the sofa?
On whatever large object sits under it, which is usually not the middle of the wall. On a 120 inch wall with the sofa centred at 54 inches, the two answers differ by six inches, and six inches is plainly visible even though nobody will name it. The page reports the offset between the two positions so you can see the size of the compromise. With nothing underneath — a hallway, a stairwell — the wall centre is the right reference.
What height should the middle of the arrangement be?
It is a field rather than an answer. Galleries often work to a centre line somewhere in the high fifties, which is a convention built around average standing eye level, and it is neither a standard nor right for every room. A space with high ceilings, a wall seen mostly from a seated position, or low furniture underneath will all want something else. Set the number you want and the page gives you the top, the bottom, the clearance over the furniture and the gap under the ceiling.
Why hang by the middle of the block rather than the top row?
Because the middle is the only reference that survives the arrangement changing. Fix the top and adding a row pushes the whole thing down towards the furniture, while removing one lifts it away. Fix the middle and it grows and shrinks symmetrically. It also matches how the block relates to the room: the eye reads the centre of the mass against the centre of the furniture, not the top edge against the ceiling.
Does this work for frames of different sizes?
No. The arithmetic here assumes every piece is the same size, which is what makes the gaps and centres exact. A mixed collection is laid out on shared centre lines or shared edges instead, and the reliable method is still to cut paper templates and move them on the wall. Use this page for the outer rectangle — how much wall the group takes and where its middle sits — and do the internal arrangement on paper.