Window Blind & Curtain Size Calculator

Window Blinds and Curtains Guide

What's the difference between Inside and Outside Mount?

Inside mount (within recess): the blind fits inside the window frame, giving a clean, built-in look. Measure the exact width of the recess at the top, middle, and bottom — use the narrowest measurement, then deduct 10mm for operation clearance. Measure the drop from the top of the recess to the sill. The blind manufacturer will usually make a small deduction from width automatically. Outside mount (on wall or above frame): provides better light blocking (covers the frame edges), can make small windows appear larger by covering the surrounding wall or frame, and is the only practical option where the recess is too shallow for an inside-mounted blind's brackets and headrail. The trade-off is that outside-mounted blinds sit slightly proud of the wall and let a small amount of light leak in at the edges.

What's the key thing to understand about Curtain Measurements?

Track or pole position: typically 150mm above the frame (to give the illusion of height) and 200-300mm beyond the frame on each side (so curtains stack off the glass when open). Pole width = window width + 400-600mm (200-300mm each side). Curtain width: total fabric needed = pole/track width × fullness factor (1.5-3, depending on heading type). Each curtain panel = total fabric width ÷ 2. Drop: sill length = from eyelet/hook to sill minus 5-10mm. Floor length = from eyelet/hook to floor minus 15-20mm, to leave a small gap that stops the hem dragging and gathering dust. For a "puddle" look, add extra length instead of subtracting it — typically 50-150mm depending on how much fabric pooling is wanted.

What do I need to know about Blackout and Thermal Properties?

True blackout blinds require the fabric to be bonded with a blackout coating and the blind to be fitted with side channels or close to the wall to eliminate light gaps around the edges. A standard 'blackout' roller blind without side channels will allow light in around the sides — most effective for darkening a room at night, less effective for shift workers or babies in summer. Thermal properties: cellular (honeycomb) blinds trap air and provide genuine insulation improvement — measured by the

What should I know about Child Safety Requirements?

Since 2014, UK regulations require all new blinds to be supplied with cord safety devices or be cordless to prevent strangulation risk to children. The Child Safety Regulations apply to all blinds with accessible cords — including Roman, Venetian, and vertical blinds. Looped cords must be fitted with a safety device that breaks the loop under tension. When buying second-hand or non-UK blinds, check cord safety as a priority. Cordless roller and cellular blinds are the safest option and now common in new-build and rental properties for exactly this reason. UK regulations (BS EN 13120) require blinds sold since 2014 to be supplied with safety devices as standard — if you have older blinds without one, a retrofit cleat or tensioner kit is a cheap, quick fix.