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 U-value reduction they add to the window as a whole.

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.

What's the key thing to understand about Inside Mount vs Outside Mount Measuring?

Correctly measuring a window for blinds depends critically on deciding between an inside mount (where the blind fits within the window recess, sitting flush against the window frame) and an outside mount (where the blind is mounted on the wall or frame surrounding the window, covering the recess and some of the surrounding wall), since the measuring approach and the margin of error allowed differ significantly between the two. For an inside mount, the blind needs to be measured to fit within the exact recess width and depth, typically with a small deduction (often around 10-15mm depending on the blind type and bracket design) from the exact recess measurement to allow the blind to be fitted and operated without rubbing against the sides of the recess — getting this wrong by even a small margin can mean a blind that doesn't fit the mechanism correctly or binds when raised and lowered. For an outside mount, the blind is generally measured to overlap the window opening by a reasonable margin on each side (commonly 5-10cm total, depending on the desired look and how much light-blocking coverage is wanted) and extends above the window frame rather than being constrained by the recess depth at all, giving more flexibility but requiring different wall-fixing hardware than an inside mount. Depth of the recess also matters specifically for inside mounts with obstructions like window handles, a radiator positioned close under the window, or deep window sills, which can restrict how far a blind can sit back into the recess regardless of the recess's overall width and height, and should be checked before finalising a purchase, not discovered during installation.

What should I know about Choosing the Right Blind Type for Light Control and Privacy?

Different blind types offer quite different combinations of light control, privacy, and insulation value, which matters more for some rooms than others. Blackout blinds, using a fully opaque fabric or lining, block essentially all light when closed, making them particularly suited to bedrooms (especially for shift workers or households wanting to sleep later without early morning light) and media or home cinema rooms, though they offer an all-or-nothing choice between full darkness and full exposure rather than the more graduated light control some other blind types allow. Day and night (or 'vision') blinds use alternating opaque and sheer fabric bands that can be adjusted to allow varying degrees of light and visibility while maintaining some privacy, offering more flexibility between full privacy and full light than a simple roller or blackout blind, though generally at a higher cost and with a more contemporary look that doesn't suit every room style. Venetian or vertical slatted blinds allow precise, continuously adjustable control over both light level and the viewing angle from outside, letting light in while still blocking direct sightlines at certain slat angles, which makes them popular for rooms needing flexible daytime privacy without sacrificing all natural light, such as home offices or ground-floor rooms facing a street. Thermal or insulated blinds, often using a cellular or honeycomb fabric structure that traps a layer of air, can provide a genuine, measurable improvement in a window's heat retention in winter and heat exclusion in summer compared with a simple single-layer blind, which is worth considering specifically for older properties with single glazing or particularly large, heat-losing window areas.

What do I need to know about Fabric Openness and Light Control for Roller Blinds?

Roller blind fabrics are often rated by 'openness factor' — the percentage of the weave that's literally open space between threads — which determines how much light, glare and outward visibility the fabric allows even before colour or coating is considered.

Worked example: a bedroom window has an inside-mount recess 950 mm wide and 1,400 mm deep. The standard trade allowance for a roller blind inside a recess is to deduct about 3 mm per side from the recess width (for bracket clearance) and about 10 mm from the recess height. Cut width = 950 − (3 × 2) = 944 mm Finished drop = 1,400 − 10 = 1,390 mm Those small deductions matter: ordering a blind cut to the exact recess width risks a fabric roll that binds against the brackets or simply won't fit, while over-deducting wastes light-blocking coverage at the edges. These are typical trade allowances rather than universal rules — some manufacturers specify their own tolerances, so check the fitting instructions for the specific product before cutting or ordering a bespoke size.

Worked Example: Measuring and Costing a Bay Window Blind

Bay windows are one of the trickiest measuring jobs because each pane usually needs its own blind rather than one continuous piece. Take a three-pane bay with a flat centre pane 90cm wide and two angled side panes 70cm wide each, all 140cm drop, fitted as inside mount on each individual recess. Each blind is measured and ordered to its own recess: 90cm x 140cm for the centre, and 70cm x 140cm for each side pane — not the total bay width of roughly 230cm as a single order, which would never fit the angled sections correctly. At a typical mid-range roller blind price of around £35-45 per square metre made-to-measure, the centre pane (1.26m²) costs roughly £45-57, and each side pane (0.98m²) around £34-44, bringing the full three-blind set to approximately £115-145 before fitting — noticeably more than a single large window of the same total width, because bay windows always price per-pane, and angled corner brackets or a specialist bay pole often add a further fitting cost on top.