Do you actually need made-to-measure?
Plenty of windows do not. Before making the case for made-to-measure curtains it is worth being straight about that, because ordering curtains made to your own dimensions when a ready-made pair would have done the job is a waste of everyone's time.
This piece sets out where ready-made is the sensible answer, where it stops being the sensible answer, and what made-to-measure actually changes when you do need it.
When ready-made is the right choice
Ready-made curtains are sold in a fixed grid of widths and drops. If your window fits that grid, they work.
They are the right answer when the window is a standard size and shape, and a stock drop lands close enough to the floor or the sill that nobody will look twice. They are the right answer when the budget is tight and the money is better spent elsewhere in the room. They are the right answer in a rented flat or a house you expect to move out of, where curtains sized to somebody else's windows are a poor thing to be carrying around.
They are also the right answer for a room you have not made your mind up about. If you suspect you will redecorate within a couple of years, something that will do for now is a perfectly reasonable decision, not a compromise you need talking out of.
None of that is a criticism of ready-made curtains. It is what they are for.
Where ready-made runs out
The limits show up in a few specific situations, and they are usually about dimension before they are about anything else.
Drop is the first. A stock drop that finishes an inch or two above the floor reads as a mistake, and a stock drop that finishes below it drags. There is no fixed drop that suits a Victorian sash and a modern flat, and a curtain that is nearly the right length is the single most obvious sign of a curtain that was not made for its window.
Width is the second. Ready-made pairs are sold by the width of fabric, not by the width of your pole, and the fullness you end up with is whatever falls out of that arithmetic. On a narrow window you can get too much fabric bunched into a small space. On a wide one you can get a thin, under-gathered curtain that never looks quite right.
Then there are windows that simply are not on the grid. Bay windows, where each section needs its own dimension. Full-height glazed doors. Windows either side of a chimney breast that need to match each other. Anything tall, anything unusually wide, anything at an angle.
Fit is the part you can see from across the room
When curtains are made to the measurements you supply, the drop is the drop you asked for. The curtain meets the floor the way you decided it should, and it does that at both ends, which matters more than people expect in older houses where the floor is not level.
The width is set against the hardware you are actually fitting rather than against a standard size, so the pair closes properly in the middle and covers the run when drawn. On a bay, each drop can be sized to its own section instead of being approximated.
This is the difference that reads without anyone knowing why. A room where the curtains fit looks settled. A room where they nearly fit looks slightly off, and it is hard to point at the reason.
The fabric opens up
A ready-made range is a small matrix: a handful of colours in a handful of sizes. Made-to-measure takes the size constraint out of it entirely, so the fabric can be chosen on its own merits, from furnishing fabrics rather than from what happens to be made up in your drop.
That is a real gain if the curtains are doing anything in the scheme beyond covering the glass. It also means you can judge the fabric properly before committing, which is a decision worth taking slowly. The guide to choosing curtain fabric covers how weight, weave and pattern scale change what you end up with.
Heading and lining become decisions, not defaults
With ready-made curtains the heading comes with the curtain. With made-to-measure it is a choice, and it is the choice that most changes how the curtain looks at the top and how it folds when open. The curtain headings guide sets out what each one does.
Lining is the same. It affects light, privacy, how the curtain handles and how it hangs, and it can be specified for the room rather than accepted as whatever the manufacturer used. The curtain linings guide goes through the options and what each one is for.
These two together are why made-to-measure is not simply "the same curtain, resized". The construction changes with the room.
The hardware and the curtain are specified together
The other thing that changes is the order of operations. Ready-made curtains are usually bought first and the pole chosen to fit them. Made-to-measure lets you decide the pole or track position first, then size the curtain to it.
That matters because the position of the hardware determines the finished drop, how far the curtains stack back off the glass, and whether the curtains clear a radiator or a sill. Deciding it first and measuring second removes most of the ways this goes wrong.
How to tell which you need
Measure the window and see whether a standard size lands where you want it to. If it does, and the fabric you like exists in that size, buy the ready-made pair. If it does not, or the window is a bay, a set of doors, or anything tall or wide, made-to-measure is not an upgrade, it is the only way to get a curtain that fits.
If you are somewhere in between, take the measurements first. The curtain measurement finder walks through what to record. Then look at the made-to-measure curtains service to see what is involved, or book the free design consultation and have someone go through the window with you before anything is ordered.