About

Ordering blinds with incorrect measurements is the single most common reason for returns and refits. A blind that is 5 mm too wide will not fit into the recess; one that is 10 mm too narrow leaves visible light gaps. This calculator applies manufacturer deduction and overlap constants for four blind types - roller, venetian, vertical, and roman - across inside-mount and outside-mount configurations. Inputs are validated against practical minima (200 mm) and maxima (6000 mm) that reflect standard production limits.

The tool assumes a level window frame with square corners. For frames out of plumb by more than 3 mm, measure at the narrowest point and note that outside mount is generally safer. Fabric area output is a gross estimate and does not account for pattern repeat waste. Always confirm deduction values against your specific manufacturer's data sheet before placing an order.

Formulas

For inside-mount installation, the ordered blind width and height are reduced from the measured recess dimensions to ensure mechanical clearance:

W_blind = W_recess − 2 × d_side

H_blind = H_recess − d_top

For outside-mount installation, overlap is added to cover the window opening and prevent light leakage at edges:

W_blind = W_window + 2 × o_side

H_blind = H_window + o_top + o_bottom

Gross fabric or material area per blind (for cost estimation):

A = W_blind × H_blind1000000 m²

Where: W_recess = measured recess width mm, H_recess = measured recess height mm, d_side = side deduction per edge mm, d_top = top deduction mm, o_side = side overlap mm, o_top = top overlap mm, o_bottom = bottom overlap mm, A = material area m².

Reference Data

Blind Type	Mount	Width Adjustment	Height Adjustment	Min Width mm	Max Width mm	Typical Bracket Depth mm	Notes
Roller	Inside	−3 mm each side	−3 mm top	300	3000	40 - 70	Check tube diameter fits recess
Roller	Outside	+75 mm each side	+75 mm top, +25 mm bottom	300	3600	N/A	Side channels optional
Venetian (25 mm)	Inside	−5 mm each side	−5 mm top	250	2400	30 - 50	Slat count ≈ height ÷ 22
Venetian (25 mm)	Outside	+50 mm each side	+50 mm top, +25 mm bottom	250	2700	N/A	Valance recommended
Venetian (50 mm)	Inside	−6 mm each side	−5 mm top	300	2700	45 - 65	Heavier headrail; check recess depth
Venetian (50 mm)	Outside	+60 mm each side	+60 mm top, +30 mm bottom	300	3000	N/A	Wood or faux-wood common
Vertical (89 mm)	Inside	−5 mm each side	−10 mm top	300	5000	55 - 75	Floor clearance 15 mm
Vertical (89 mm)	Outside	+100 mm each side	+100 mm top, +50 mm bottom	300	6000	N/A	Louvres should clear sill
Vertical (127 mm)	Inside	−5 mm each side	−10 mm top	300	5000	55 - 75	Wider louvres suit large windows
Vertical (127 mm)	Outside	+100 mm each side	+100 mm top, +50 mm bottom	300	6000	N/A	Common for patio doors
Roman	Inside	−5 mm each side	−5 mm top	300	2400	35 - 60	Fabric stacking adds ≈200 mm
Roman	Outside	+75 mm each side	+75 mm top, +25 mm bottom	300	2700	N/A	Pattern matching adds waste
Pleated	Inside	−4 mm each side	−4 mm top	250	2300	20 - 35	Minimal stack height
Pleated	Outside	+50 mm each side	+50 mm top, +25 mm bottom	250	2500	N/A	Tensioned option for tilt/turn

Frequently Asked Questions

For inside mount, measure the recess opening (the space between the inner edges of the frame) at three points horizontally and three points vertically, then use the smallest value. For outside mount, measure the outer frame or the wall area you want to cover. Never measure the glass pane itself.

Every blind mechanism requires a minimum recess depth for brackets. Roller blinds need 40-70 mm, venetian blinds 30-65 mm, and vertical blinds 55-75 mm. If your recess depth is less than the required bracket depth, you must use an outside mount or a face-fix bracket extension.

Each mechanism has different bracket widths, end-cap sizes, and operating chain/wand clearances. A roller blind has a tube that extends nearly the full width so the deduction is small (3 mm per side). A vertical blind headrail has wider end caps requiring 5 mm per side. These values come from manufacturer installation guides and account for thermal expansion of PVC components.

The blind will not physically fit into the recess. The brackets will press against the frame, preventing level mounting. In the case of venetian blinds, the slats will scrape the sides and jam. Most manufacturers will not accept returns on custom-cut blinds, resulting in a total financial loss on the order.

For effective blackout, increase side overlap to at least 100 mm per side and top overlap to 100 mm. Standard overlap values (50-75 mm) reduce light ingress but do not eliminate it. Side channels or light-block strips can be added for near-total darkness without increasing blind width further.

PVC and aluminium components expand at different rates. Aluminium venetian blinds expand approximately 0.024 mm per metre per degree Celsius. In a conservatory reaching 50°C above ambient, a 2000 mm aluminium blind could expand by about 2.4 mm. The standard deduction values already account for this within normal indoor temperature ranges (15-35°C).