Example data table
| Mount | Opening width | Finished deduction | Fabric deduction | Estimated fabric width |
|---|---|---|---|---|
| Inside | 36 in | 1.5 in | 1.0 in | 33.5 in |
| Inside | 900 mm | 38 mm | 25 mm | 837 mm |
| Outside | 36 in | — | 1.0 in | (36 + 2×2) − 1.0 = 39.0 in |
| Outside | 1200 mm | — | 30 mm | Custom 1300 − 30 = 1270 mm |
Formula used
Inside mount
Finished shade width = Opening width − Finished shade deduction (total)
Fabric width = Finished shade width − Fabric deduction (total) − 2×Extra side gap (each)
Outside mount
Finished shade width = Opening width + 2×Overlap each side (or custom finished shade width)
Fabric width = Finished shade width − Fabric deduction (total) − 2×Extra side gap (each)
How to use this calculator
- Measure opening width at top, middle, and bottom; use the smallest.
- Select inside or outside mount, then choose the outside sizing method if needed.
- Enter your supplier’s deductions for finished shade and fabric width.
- Set optional extra side gap if you want more clearance.
- Press Calculate, review the notes, then export CSV or PDF.
Why fabric width matters
Roller shades use a tube, brackets, and a hem bar. Fabric cannot reach bracket edges, so installers allow clearance. Too wide can rub hardware, wrinkle, or stall. Too narrow increases side light gaps. A fabric-width estimate balances smooth operation and coverage, especially for tight inside mounts. It also helps you communicate requirements clearly when comparing vendors.
Inputs that change the deduction
Mount type drives the math. Inside mounts start with the opening width and subtract clearances. Outside mounts start from the desired shade width and subtract clearances. Hardware style matters: cassette systems and motor drives often need more room than a standard clutch. Control side may influence the hardware package you choose.
Typical industry allowances
Many suppliers build a standard \u201clight gap\u201d into finished shade width. Fabric is usually narrower than the finished shade, and the finished shade is narrower than the opening for inside mounts. Common total allowances range from 1.0 to 2.0 inches, depending on tube size and bracket design. This calculator lets you set clearances and optional side gaps. If you know your vendor\u2019s published deduction, enter it directly for consistent ordering.
Using the results on site
Measure the opening in three places and use the smallest value for inside mounts. If the frame is out of square, increase clearance slightly to prevent rubbing. For outside mounts, pick a target coverage width that overlaps glass or trim by your preferred amount. The calculator reports fabric width and estimated finished shade width so you can check coverage before ordering. Recheck after painting or tiling, because thickness changes measurements.
Quality checks before ordering
Confirm units, rounding, and whether your supplier lists \u201cfabric\u201d or \u201cshade\u201d width on the order form. Compare the output to a similar shade you already own, if available. When in doubt, follow the manufacturer\u2019s minimum deductions. A small side gap is normal and helps the shade roll smoothly. Document your final settings for future replacements and matching rooms over time.
FAQs
1) What is the difference between fabric width and shade width?
Shade width refers to the full hardware-to-hardware finished size. Fabric width is narrower so it clears brackets and rolls smoothly. Order forms may ask for either, so confirm your supplier’s terminology.
2) Why does an inside mount need a deduction?
Inside mounts must fit within the smallest part of the opening. A deduction prevents the shade from binding against uneven jambs, paint buildup, or slight out-of-square conditions during operation.
3) How much overlap should I use for outside mounts?
Many installers overlap 1–3 inches per side for privacy and light control. Use more if trim is narrow or the window gets strong side light. The best value depends on your room and aesthetics.
4) Do motorized shades require more clearance?
Often, yes. Motor heads and larger idlers can increase the hardware envelope. Use your supplier’s published deductions, and consider adding a small extra allowance if your measurements are tight.
5) Should I round my measurements?
Only if your supplier requests rounding. Many manufacturers cut to exact decimals or fractions. If you do round, keep it consistent across all shades in the same room for a uniform appearance.
6) What if my window opening varies top to bottom?
Use the smallest width for inside mounts. If the variation is large, increase clearance or consider an outside mount to avoid rubbing and to improve coverage along the narrower section.