How to Measure Curtains for Tension Rods | Inside Mount, Done Right

One wrong measurement sends you back to the store with curtains that either puddle on the floor or float awkwardly above the sill. Whether your project is a bathroom privacy panel or a kitchen café curtain, getting tension rod curtains right comes down to two measurements: the inside width of the frame and the drop you want the fabric to cover. Measure these correctly once, and the rest is straightforward.

How to Measure the Window Opening for a Tension Rod

For a tension rod, you measure the inside width of the window frame—specifically, from the inner edge of one side of the trim to the inner edge of the other side. This is the distance the rod must span to grip the frame with spring pressure.

Openings are rarely perfectly square, so measure in three places:

• Across the top of the opening, just below the header trim
• Across the middle of the opening
• Across the bottom of the opening, just above the sill

For a tension rod, use the smallest of the three measurements as your working width, since the rod needs to fit through the narrowest part of the opening. The Home Depot curtain measuring guide emphasizes measuring the inner edges of the frame rather than the visible glass.

Once you have that smallest width, shop for a rod whose adjustable range brackets it.

Using the Right Rod Length for Curtain Width

The most common mistake homeowners make is basing curtain panel width on the glass width rather than the full rod span. Your curtains should cover the entire length of the rod, not just the window opening it sits inside.

For a full look, choose curtain panels whose combined width is 2 to 3 times the rod’s full length. This allows the fabric to gather properly rather than hanging flat and stretched.

Two more points worth noting:

• Tension rods are inside-mount hardware. They are not designed for outside-mount installation where the rod sits above the frame on brackets.
• The rod must be able to hold itself in place through spring pressure alone—these rods are built for lightweight or sheer curtains, not heavy drapes or blackout panels.

Curtain Length: Sill or Floor?

Since tension rods mount inside the frame, the curtains almost always end at the windowsill. Measure the drop from the rod position down to the sill—the sash-to-sill distance—and use that as your curtain length. You can choose to hang panels that touch the sill exactly or hover just above it.

If you plan to hang curtains with rings, measure from the floor up to the bottom of the rings themselves, not to the top of the rod. The rings add an inch or more of drop that changes your final length.

If you are ready to pick panels, see our tested roundup of the best curtains for tension rods, matched to real window measurements and fabric weights.

Finally, if the opening width varies significantly between top and bottom, trust the smallest measurement for your rod selection. A rod that fits the widest point may not clear the narrower middle, leaving you with a rod that dents the frame or refuses to grip at all.

Wayfair’s measuring guide also highlights checking the rod by its full span, including any finials or end caps, since those extend past the adjustable shaft. For tension rods specifically, this matters less—there are no finials—but it confirms why the inner frame edges are the reference point, not the visible window pane. John Lewis’s curtain pole guide similarly directs buyers to measure the recess width for inside-mount installations.