Blind and shade sizing: inside versus outside mount

What actually decides the mount#

Four questions, answered in order. The first one that says no ends the argument.

1. Is there enough flat depth? Measure from the innermost obstruction, which is usually the face of the sash, a handle, a lock or a bead, not the glass. Cranked casement handles are the usual killer: a handle projecting 2 in (5 cm) into a 3 in (7.6 cm) reveal leaves 1 in.
2. Is the opening square? Measure the width at top, middle and bottom. If they differ by more than about 1/2 in (13 mm), an inside mount will show a wedge of daylight down one side.
3. Do you need darkness? An inside mount cannot give it. See below.
4. Where does the stack go when the shade is up? This is the question almost nobody asks, and it decides more inside-versus-outside arguments than the other three combined.

Outside mount also does something inside mount cannot: it makes an undersized window look bigger. A shade 6 in (15 cm) wider and 12 in (30 cm) taller than the opening changes the proportion of the wall, as a high rod does in curtain length and hanging height.

Measuring: three places, and which number to use#

Steel tape, not fabric. Record to the nearest 1/8 in (3 mm).

Width: measure at the top, the middle and the bottom of the opening. Height: measure at the left, the centre and the right.

• Inside mount: use the smallest width and the smallest height. The shade has to pass through the tightest point of the opening.
• Outside mount: use the largest width and height, then add the overlap on top.

Never deduct anything from an inside mount width yourself. Give the fabricator the raw smallest measurement and let their deduction do the work. Deducting twice is the most common ordering error there is, and it produces a shade with an inch of daylight down each side that no adjustment will fix.

Mount depth, deduction and stack, by type#

Shade type	Flush inside mount depth	Partial mount minimum	Width deduction	Stack at 72 in (183 cm) drop
Roman, flat fold	3 to 3.5 in (7.6 to 8.9 cm)	2 in (5 cm)	1/4 to 1/2 in (6 to 13 mm)	11 to 13 in (28 to 33 cm)
Roman, relaxed	3.5 in (8.9 cm)	2.5 in (6.4 cm)	1/4 to 1/2 in (6 to 13 mm)	9 to 11 in (23 to 28 cm)
Roller, 1.5 in tube	2 to 2.5 in (5 to 6.4 cm)	1 in (2.5 cm)	1/4 in (6 mm)	2.5 to 3.5 in (6 to 9 cm)
Roller in a cassette	3 to 3.5 in (7.6 to 8.9 cm)	2 in (5 cm)	1/4 in (6 mm)	3.5 to 4.5 in (9 to 11 cm)
Dual or zebra roller	3 to 3.5 in (7.6 to 8.9 cm)	2 in (5 cm)	1/4 in (6 mm)	3.5 to 4.5 in (9 to 11 cm)
Cellular, honeycomb	1.5 to 2 in (3.8 to 5 cm)	5/8 in (16 mm)	1/4 to 3/8 in (6 to 10 mm)	2.5 to 3.5 in (6 to 9 cm)
Venetian, 2 in wood	2 in (5 cm)	1 in (2.5 cm)	1/4 to 1/2 in (6 to 13 mm)	12 to 15 in (30 to 38 cm)
Aluminium slat, 1 in	1 in (2.5 cm)	5/8 in (16 mm)	1/4 to 1/2 in (6 to 13 mm)	5 to 7 in (13 to 18 cm)
Woven wood	2 to 2.5 in (5 to 6.4 cm)	1.5 in (3.8 cm)	1/4 to 3/8 in (6 to 10 mm)	10 to 14 in (25 to 36 cm)

"Flush" means the headrail sits entirely inside the reveal with the face of the shade level with the face of the frame. "Partial" means the headrail projects into the room, which is structurally fine and visually obvious.

The deduction is not optional and it is not a manufacturing tolerance. A shade cut to the exact opening width binds on the first frame that is 1/16 in out of square, and every frame is. Roller shades take the smallest deduction, typically 1/4 in (6 mm), because the fabric is a single plane. Venetians take the largest because the slats have to tilt inside the opening without their ends catching.

Light gaps, and what to do instead#

An inside mount leaves 1/4 to 1/2 in (6 to 13 mm) of gap per side, plus a gap at the head where the fabric hangs off the roller or headrail, plus whatever the bottom rail does not seal against the sill. On a 36 in (91 cm) window that is roughly 60 sq in (390 sq cm) of daylight, enough to read by under a bright sky.

There are three honest answers.

• Outside mount with 3 in (8 cm) of overlap each side and a wrapped or cassette headrail. The cheapest real fix.
• Side channels, aluminium U-sections fixed to the jambs that the shade edges run inside. They work, and they cost 1 to 1.5 in (2.5 to 4 cm) of opening width each and are visible.
• Layering: an inside-mounted shade for privacy under a lined or blackout curtain for darkness, with the curtain returning to the wall at each end. See curtain width, fullness and stack-back for the return and overlap dimensions that make that actually dark.

Stack height: the top of your window, permanently#

A raised shade has to go somewhere. For a roman, the working figure is 1 in of stack for every 8 in of drop, plus 2 to 3 in (5 to 8 cm) of headrail. A 72 in (183 cm) drop therefore stacks 11 to 12 in (28 to 30 cm).

Roller tubes: diameter against drop and width#

A roller's tube has two jobs, and they set two different minimum diameters.

Rolled-up bulk. The finished roll diameter is sqrt(d² + 4tL/π), where d is the tube diameter, t the fabric thickness and L the drop. A 1.5 in (38 mm) tube with a 72 in (183 cm) drop of ordinary 0.012 in screen fabric finishes at about 1.8 in (46 mm). The same tube with a 96 in (244 cm) drop of 0.020 in blackout finishes at about 2.2 in (56 mm), so the shade that fitted a 2.5 in reveal on paper does not fit once it is rolled up.

Stiffness. A tube deflects under its own weight like a simply supported beam, and beam deflection scales with the fourth power of the span. Doubling the width from 36 in (91 cm) to 72 in (183 cm) multiplies the sag by sixteen for the same tube. This is why tube size is quoted against width, not against price:

• Up to 42 in (107 cm) wide: 1.25 to 1.5 in (32 to 38 mm) tube
• 42 to 72 in (107 to 183 cm): 1.75 to 2 in (44 to 51 mm)
• 72 to 96 in (183 to 244 cm): 2.5 in (64 mm)
• Over 96 in (244 cm): 3 in (76 mm), or split the opening into two shades

Two shades on one wide window is not a compromise. A 120 in (305 cm) opening split into two 60 in (152 cm) rollers sags less, weighs less per bracket, and takes a smaller tube, so it fits a shallower reveal than one wide shade ever would.

Cordless and motorised clearances#

ANSI/WCMA A100.1 requires stock window coverings to have no accessible operating cords, so cordless lift and motorisation are now the default rather than the upgrade. Both cost depth.

A cordless roman or cellular headrail carries its spring or clutch inside, which is why the flush depths above are what they are. A motorised roller needs a tube big enough to swallow the motor: 1.75 in (44 mm) minimum for a battery tube, 2 in (51 mm) or more for most mains motors, plus bracket clearance. Budget 3 in (7.6 cm) of inside depth for any motorised roller, and check where the charging port or the mains lead lands: a battery wand adds 1 to 1.5 in (2.5 to 4 cm) at one end and needs to be reachable a few times a year.

Cordless also has a reach constraint. You raise the shade by lifting the bottom rail, and over a deep sill or a radiator, on a drop past about 84 in (213 cm), that rail sits at head height with nothing to pull. Specify a wand or motorisation on any cordless shade over an 84 in drop.

Sill, apron and layering under curtains#

An inside mount ends at the sill by definition. An outside mount has three sensible bottom lines and one bad one.

• Above a projecting sill: bottom rail 1/2 in (13 mm) clear of the sill nose. The tidiest option on a window with a deep sill.
• Past the apron: 2 in (5 cm) below the bottom edge of the apron. Use this where you want the shade to read as a single rectangle covering the whole window assembly.
• To the floor: for a full-height window or a door.
• Mid-apron: never. A shade that stops halfway down a piece of trim looks like a measuring error, because it is one.

Layering a shade under curtains is mostly a projection problem. The bracket has to carry the curtain past the shade:

Shade condition	Depth off the wall	Rod projection needed
Inside-mounted roller or cellular	0 to 1 in (0 to 2.5 cm) past the casing	3 to 3.5 in (7.6 to 8.9 cm)
Outside-mounted roller in a cassette	3 to 3.5 in (7.6 to 8.9 cm)	4.5 to 5 in (11 to 13 cm)
Outside-mounted roman	3 to 3.5 in (7.6 to 8.9 cm)	4.5 to 5 in (11 to 13 cm)
Outside-mounted 2 in venetian	2.5 to 3 in (6.4 to 7.6 cm)	4 to 4.5 in (10 to 11 cm)

Also set the curtain rod at least 2 in (5 cm) above the shade headrail so the two sets of brackets do not fight for the same fixings, and remember that the projection you choose is added to the return the curtains need at each end, which is worked through in curtain width, fullness and stack-back.

Ordering without a mistake#

1. Find the innermost obstruction in the reveal, with the window closed and then with it open. Measure depth to that, not to the glass.
2. Take three widths and three heights. Write down all six.
3. Decide the mount from the depth and from the squareness, not from a preference.
4. If inside: send the smallest width and smallest height, unadjusted, and say "inside mount" so the deduction is applied once.
5. If outside: send the largest width plus 4 in (10 cm) total overlap as a minimum, and a height that covers the stack above plus your chosen bottom line.
6. Check the stack figure from the table against the wall you have above the window, before you order anything.
7. If curtains are going over the top, fix the rod projection and rod height now. The curtain size calculator will size the panels once the shade is settled.

Step 6 is the one that gets skipped, and it is the one that cannot be corrected after delivery.