← All guides
Time: ~4 min·Difficulty: ●●●●
How to Measure a Window for Blinds, Curtains, or Shades
Inside mount vs outside mount, width tolerance, drop length, three sets of measurements for any window covering, captured in 4 minutes with your iPhone.
Window covering retailers handle 20%+ returns due to fit issues. The math is simple, but the choice of inside-mount vs outside-mount changes which dimensions you need. Here's the workflow for both.
Why measure this
- Inside-mount blinds need exact width within 3 mm
- Outside-mount blinds need to project past the window by 5-10 cm each side
- Drop length determines floor clearance or pool
- Window depth matters for inside-mount tilt mechanisms
What you need
- iPhone with Ruler AR
- Length mode
- Decide upfront: inside or outside mount
Step by step
- Decide: inside vs outside mount. Inside mount: covering sits inside the window frame, flush with the wall. Looks clean, blocks less light at the edges. Outside mount: covering hangs in front of the frame on the wall. Better light blockage, wider light gap-free zone.
- Inside mount, exact window opening. Tap the inside-left edge of the frame to the inside-right edge for width. Top of frame to sill for height. Measure at three points (top, middle, bottom for width; left, center, right for height), old houses are not square, use the smallest measurement.
- Outside mount, total coverage area. Mark on the wall how far past the window you want the covering to extend (typically 5-10 cm each side). Tap the leftmost mark to the rightmost mark. For height: above the frame to your desired drop length.
- Drop length / floor clearance. For floor-length curtains: floor to mount point. Subtract 1-2 cm for floor clearance. For sill-length: window top to sill. For below-sill: window top to your desired drop point.
- Window depth (inside mount). Use Touch Mode against the glass. Tap the glass, then the front of the frame. Most inside-mount blinds need 5-10 cm of frame depth for the headrail and tilt mechanism.
Tips
- For sliding doors, always use outside mount, inside mount blocks the track
- Add 1-2 cm to outside-mount width if you want light-blocking overlap with the wall
- Photo each measurement in the app, order forms always ask for dimensions you forgot
Common mistakes
- Measuring at only one point, windows are rarely perfectly rectangular
- Forgetting to subtract for inside-mount mounting tolerance, order the actual window width, the manufacturer subtracts
- Ordering wrong drop length, easier to shorten than to lengthen
Frequently asked
Do I order the exact window width for inside mount?
Yes, the manufacturer subtracts mounting tolerance (~3-5 mm) automatically. Order your measured width.
How long should curtains be?
Three options: just touching floor (formal), 1-3 cm puddle (relaxed), or sill-length (casual rooms). Each looks intentional; in-between looks wrong.
Inside or outside mount for blocking light?
Outside mount blocks more light because the covering extends beyond the window edges. Inside mount always has a small light gap at the edges.