Curtains hung at the wrong height make a room look worse than bare windows. Most of the time it is not the curtains that are wrong, it is the rod. Move the rod up and the same curtains change the proportions of the whole room.

Here is the measurement, the reason behind it, and what to do when the window or the ceiling fights you.

The rule: six inches above the frame, then to the floor

Mount the rod six inches above the top of the window frame, and hang the curtains so they either touch the floor or stop an inch above it. That is the whole rule and it applies to almost every ceiling height.

The reason is visual rather than practical. A window becomes a tall opening in the wall rather than a hole in it. The eye reads the gap between the rod and the ceiling as ceiling, and the strip of wall beside the window as a small margin rather than a large expanse. The room gains height without anything being added to it.

Hanging curtains level with the frame, or worse level with the bottom of the window, does the opposite. It cuts the wall into two small bands and makes the room feel shorter, which is why a curtain that looks fine in a shop looks wrong on a normal ceiling.

Measuring the drop

Measure from where the rod will sit, not from the ceiling. If the rod is six inches above the frame, add six inches to the drop measurement from that point down to where you want the curtains to finish.

• Standard ceilings, eight feet: rod at about six inches above the frame, curtains landing at or just above the floor.
• High ceilings, nine feet or more: the same rule, but consider a drop of a foot or more above the rod for a taller room.
• Bay windows: measure the widest part, since the rod has to clear the whole recess.

Curtains that stop mid-sill are the single most common expensive mistake. They read as an unfinished room even when everything else is good, because they leave a strip of bare wall under the window and interrupt the vertical line.

Width, which matters as much as height

A curtain rod should extend three to six inches beyond the window frame on each side. That is what makes the window look wider and the wall around it look smaller. A rod fitted exactly to the frame width leaves the curtains bunched in the middle and the full width of wall visible either side.

For fullness, the curtain fabric should be roughly twice the width of the rod when gathered. A hundred inch rod wants curtains that measure two hundred inches across before they are hung. This is the number people get wrong, because it looks like far too much fabric until it is on the rod and flat.

When the window sits awkwardly

Windows that stop at a low level, or sit in a deep reveal, are the two cases that need a decision rather than a rule.

For a low window, hang the curtains from just above the frame all the way to the floor. The curtains become the tall element in the room rather than the window, which suits a low window much better than trying to emphasise the glass.

For a deep recess, mount the rod inside the reveal if there is depth to spare, not across the glass. Curtains mounted in the recess open flat and let the full width of the window show. Mounted across the glass, they cut the window in half and lose most of the light.

Making this work in a rental

All of this is achievable without touching a wall, but it depends on the mounting. A tension rod inside the recess is the cleanest option and takes proper curtains as well as a cafe pole. If the recess is too shallow, brackets that clamp to the window frame need no screws and hold a rod at the right height.

Measure the recess depth before you buy anything. Most tension rods need about half an inch of clearance each side, and a deep sill swallows that in one side. Getting the rod high is the single change that does the most for a room, and it is the one most often abandoned because the rod does not reach.

Check the recess fits your rod, fit the rod six inches above the frame, and leave the curtains long. Those three numbers decide whether a room looks finished. The rug size guide covers the other measurement that changes a room as much as this one.