Inside Mount or Outside Mount: Measuring Interior Blinds Without Costly Errors

Inside Mount or Outside Mount: Measuring Interior Blinds Without Costly Errors

Short answer: inside mount (recess fit) sits inside the window opening for a clean, built-in look, and it requires enough depth for the headrail — products differ, but shallow reveals rule it out. Outside mount (face fix) mounts on the wall or frame above the opening, needs a flat mounting surface with clearance above and to the sides, and is the answer for shallow reveals, out-of-square openings and maximum light blocking. On made-to-measure orders the mounting decision must be made before measuring, because the two methods use opposite conventions: inside mount is measured to the opening and the factory makes the deduction, outside mount is measured to the finished blind size and you add the overlap.

### The two methods

#### Inside mount (recess fit)

The blind sits within the window recess. It looks integrated, keeps sills usable, and suits deep reveals and architectural openings.

Requirements and trade-offs:

• Depth. The recess must accept the headrail. Very shallow reveals cannot take an inside mount at all — retailer guidance commonly puts the floor around ¾ inch of depth before an inside mount becomes unviable, with more required for larger headrails and for specialty shapes (arched inside mounts commonly need around 1½ inches).

• Light gaps are inherent. The manufacturing deduction that lets the blind operate freely leaves a small gap at each side. This is normal, not a defect — but it means an inside-mounted blind alone will never be a true blackout.

• Squareness matters. Recesses are rarely perfectly square. Measuring at three points and using the smallest dimension is standard practice for exactly this reason.

#### Outside mount (face fix)

The blind mounts on the wall or window frame outside the opening.

Requirements and trade-offs:

• Mounting surface. You need flat, solid surface above the opening — commonly cited as at least about 2 inches of flat surface above for the brackets, and on doors, measuring at least 2 inches above and below the glass (3 inches above is often recommended) to give room for hardware.

• Overlap is your friend. Extending beyond the opening on both sides and above dramatically reduces light leakage — this is why outside mount is the standard approach when darkness matters.

• It occupies wall space and projects into the room, which can conflict with handles, furniture and adjacent openings.

### The measuring conventions that cause remakes

This is where most made-to-measure losses occur, and the cause is almost always a mismatch of convention rather than a bad tape measure.

| |Inside mount|Outside mount| | ---|---|---| | What you measure|The opening itself|The finished size you want the blind to be| | Who deducts|The factory applies its standard deduction|Nobody — you specify final size| | If you deduct too|Blind ends up undersized with large gaps|—| | Width rule|Measure at top, middle and bottom; use the smallest|Opening width plus overlap each side| | Height rule|Measure at left, centre and right; conventions vary — confirm whether the factory uses the longest or the shortest|Top of bracket position to desired bottom|

The single most expensive mistake is deducting from an inside-mount measurement because a previous supplier expected it. Confirm in writing, per supplier, whether they want the opening size or the finished blind size — the phrases "recess size" and "blind size" mean specific and opposite things in this trade.

The second most expensive is measuring in the wrong unit or rounding the wrong way. Convention in most factories is to record to the millimetre and never round up on an inside mount.

### Product-specific depth considerations

Different interior products need different recess depth for a flush inside mount:

• Roller blinds are generally the shallowest — the tube and bracket set the requirement.

• Venetian blinds need depth for the headrail and for the stacked slats at the top when raised.

• Cellular/honeycomb shades need depth for the headrail plus the stack height of the compressed cells.

• Vertical blinds need track depth and clearance for the louvres to rotate without hitting the reveal.

• Plantation shutters need the most: the frame, the panel and the louvres' rotation arc all sit in the opening, which is why shutters are often specified with a face-fix frame on shallow reveals.

Always ask the supplier for minimum recess depth for a flush fit and for a partial-projection fit, since many products can be inside-mounted with a small projection when depth is marginal.

### Obstructions: the checklist that prevents surprises

Before committing to inside mount, check for:

• Window handles and hinges — do they clear the blind when opening the window?

• Tilt-and-turn windows — inward-opening sashes usually force outside mount or a blind fitted to the sash itself.

• Trickle vents at the head of the frame.

• Deep sills or protruding tiling at the base.

• Adjacent walls — a blind directly beside a return wall may not have room for controls.

• Alarm sensors and blind-spot fixtures in commercial reveals.

### What to ask any supplier

1. "Do you want the recess size or the finished blind size?" — the question that prevents the most common remake.

2. "What deduction do you apply on inside mounts, and where?"

3. "What is the minimum recess depth for a flush fit on this product?"

4. "What tolerance do you work to, and what is your remake policy on measurement disputes?"

5. "What overlap do you recommend on outside mounts for light control?"

6. "Who is responsible for measuring — us or your surveyor?" — on contract projects this should be explicit in the contract.

### Frequently asked questions

Which mount blocks more light? Outside mount, by a wide margin, because it overlaps the opening rather than sitting inside it with operating gaps. Inside mount plus side channels is the alternative when the built-in look is required.

Can I inside-mount if my recess is only slightly too shallow? Often yes, as a partial-projection fit where the headrail protrudes a little. Ask for the projection dimension so you can judge the appearance.

How do I handle an out-of-square recess? Measure at three points and use the smallest width; consider outside mount if the discrepancy is large, because an inside mount will show a visibly tapering gap.

Should I measure before or after the windows are installed? After. Measuring from architectural drawings is a reliable way to generate remakes — reveals move during construction.

Written by the engineering team at GRAND HOMEDECOR (Guangdong Grand Homedecor Technology Co., Ltd.). Depth and clearance figures cited reflect common industry guidance; our measuring guide states the exact deductions, minimum depths and tolerances applied to each product line, and is supplied with every order confirmation.