Measure each flat section of the bay separately, total the widths with extra stack-back room at both outer ends, and take the drop from where the pole or track will actually sit.
Skipping the section-by-section step produces curtains that look right on the hanger and wrong on the window — too narrow when closed, or swallowing glass when open. Here’s the order that works for two-, three-, and four-angle bays, plus the allowance most people forget.
Why You Measure Each Section Separately
A bay is several flat panes joined at an angle, so it has no single width. Each straight section gets its own measurement, and those numbers are added afterward. Treating the whole bay as one span is the most common mistake, and it fails worst on bays with unequal side walls — which is most of them.
- Sketch the bay from above and label each straight section left to right (A, B, C, D on a four-angle bay).
- Measure each section’s width on its own, then add them up.
- Check the angles rather than assuming they match — rarely do.
- Note any return walls or side clearance needed to stack curtains clear of the glass.
B&Q’s bay window guidance walks through the same section-by-section approach with separate diagrams for two-, three-, and four-angle layouts, so you can match your sketch to the right one. Bow windows — the curved cousins — use identical logic.
How Much Extra Width Do You Add?
Add about 8 to 12 inches (20–30 cm) of extra allowance at each outer end so curtains can stack back off the glass when open. Without that room, the fabric has nowhere to go and ends up covering the window even in daytime.
The measurement you’re after is the pole or track length, not the window opening. If hardware is mounted, run the tape along the pole or track. If starting fresh, measure along the line where the pole will be fitted and include stack-back at both ends.
Stack-back matters most for an outside-the-bay hang, common wherever a returning wall or enough side clearance exists — curtains pull clear of the bay instead of sitting inside it. Inside-the-bay designs need less total width but tighter tolerances, so check handles, deep sills, and trim for anything that would catch the fabric. If you’d rather start from a shortlist, this roundup of the best bay window curtains for awkward angles is a practical companion to your measurements.
Measuring the Drop Correctly
Measure the drop from the top of the planned installation point down to where you want curtains to finish — floor, sill, or just below. The window frame is the wrong starting line; the pole or track position is the right one.
Dunelm’s buying guide makes the same point: measure the hardware location, not the glass. The margin above the window decides mounting. In tight headers, a ceiling-mounted track is often the only option.
- For full-length curtains, take the drop in several spots and use the longest if the floor is uneven.
- Decide your finish point before measuring — sill-length and floor-length need very different numbers.
| Measurement | Where to Measure | Extra Allowance |
|---|---|---|
| Width per section | Each flat pane, individually | None — record exact |
| Total width | Sum of all sections | 8–12 in (20–30 cm) per outer end |
| Drop | Pole or track position to finish point | None — use the longest reading |
| Mounting height | Space above the window | Under 6 in (15 cm) leans ceiling mount |
| Side clearance | Return walls and outer edges | Enough to stack clear of glass |
| Obstructions | Handles, deep sills, trim | Note anything catching fabric |
The Mistakes That Cost You the Whole Project
- Measuring as one span. Splitting the bay into sections is the entire method.
- Forgetting stack-back. Skip it and open curtains block the light you bought them to let in.
- Starting at the frame. Measure where the pole or track sits, not the window edge.
- Ignoring protrusions. Handles, deep sills, and thick trim interrupt curtain travel.
- Assuming the angles. Verify every corner before ordering made-to-measure.
Write the numbers next to your sketch, in inches or centimeters — one unit throughout. US retailers typically order in inches, so converting once at the end is simpler than mid-measurement.
FAQs
Do I add extra width to each section of the bay?
No. Each flat section is measured exactly, then totaled. The extra allowance goes on the two outer ends only, to give curtains somewhere to stack when open. Adding slack to every section throws the total off and produces a pole too long for the bay.
Can I measure a bay window without a pole or track fitted yet?
Yes, and it’s often easier. Pick the spot where the pole or track will be installed, then measure along that line rather than the window. This keeps your drop and width figures consistent with the hardware you’ll actually fit, which is the number curtain makers work from.
What if there’s almost no wall above my bay window?
Ceiling mounting is usually the answer. Measure the clearance first, then choose hardware that matches it.
References & Sources
- B&Q. “How to Measure for Bay Windows.” Section-by-section measuring method and diagrams for 2, 3, and 4-angle bays.
- Dunelm. “How to Measure for Tracks and Poles.” Measuring from the pole or track position rather than the window frame.
- Marks & Spencer. “Curtains Buying Guide.” Stack-back allowance and mounting clearance above the window.
