(Inside Mount vs Outside Mount + Mistakes to Avoid)

Buying blinds or shades feels easy until you reach the “Enter your measurements” step. That is where most returns, light gaps, and “why does this look crooked?” moments begin.

The good news: measuring is not complicated. It just needs a repeatable method, a few simple rules, and the discipline to measure every window like it is slightly different (because it usually is). Visit Better blind plus website and contact us.

This guide walks you through:

• What tools you need
• How to measure inside mount and outside mount step by step
• What changes for different blind and shade types
• Special cases (sliding doors, bay windows, crank windows, etc.)
• The most common measuring mistakes and how to avoid them
• A copy-paste worksheet you can use for every room

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What you’ll need before you start

Tools

• A steel tape measure (avoid cloth tape measures)
• A pen + paper (or a notes app)
• Step stool (if needed)
• Optional: flashlight (to check depth and obstructions), small level

Before you measure, decide this first

• Are you doing an inside mount (inside the window frame) or an outside mount (covering the window from outside the frame)?

That single decision changes your entire measuring method.

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Quick definitions so you don’t mix things up

Inside mount
The blinds/shade sits inside the window opening. Clean, built-in look. Requires enough depth in the frame and works best when the window opening is fairly square.

Outside mount
The blinds/shade is mounted outside the window opening (on the wall, trim, or above the frame). Great for shallow frames, uneven openings, and better light control because you can add overlap.

Window opening vs trim/casing

• Opening = the “hole” inside the frame
• Trim/casing = the decorative border around the opening

Inside mount measures the opening. Outside mount measures the coverage area you want to block.

Depth
This matters mainly for inside mount. It is the usable flat surface inside the frame where brackets can sit. Depth is often reduced by window cranks, locks, or decorative edges.

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The three numbers you’ll record for most windows

1. Width
2. Height
3. Depth (inside mount only)

And one more thing you should always note: obstructions (handles, cranks, alarms, tile ledges, etc.).

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Step-by-step: Measuring for an inside mount

Inside mount is all about precision. Window openings are rarely perfectly rectangular, so you measure multiple points and use a consistent rule.

Step 1: Measure the width in 3 places

Measure inside the frame at:

• Top
• Middle
• Bottom

Rule: Use the smallest of the three widths.

Why? If the top is slightly wider than the bottom, the product must still fit the tightest point. Using the smallest width prevents scraping, jamming, and forced installs.

Example:
Top 48 1/4″
Middle 48 1/8″
Bottom 48″
➡ Use 48″

Step 2: Measure the height in 3 places

Measure inside the frame at:

• Left
• Center
• Right

Now the “height rule” can vary based on how the product is made and how the supplier wants measurements.

A safe approach for planning is:

• Record all three heights
• Use the height recommended in your product’s instructions (some ask for the smallest; some want the largest; some want exact and they make deductions)

Best practice: If you are ordering custom blinds or shades, check whether the supplier wants:

• Exact opening size (they will make deductions), or
• Finished blind size (you provide the final size)

If you are unsure, record all three and keep your notes clean so you can adjust later.

Step 3: Measure the depth (inside mount only)

Depth is the most ignored measurement, and it is the one that decides whether inside mount will look clean or look awkward.

Measure depth from:

• The inside front edge of the frame (where brackets would sit)
• To the glass or the back surface

Also check if the depth is flat and usable. A decorative lip or slanted edge can reduce usable depth.

Tip: If your window has a crank or bulky handle, note how far it sticks out. Even if you have depth, the handle can interfere with shade operation.

Step 4: Check for obstructions

Inside mount needs clearance. Note things like:

• Window cranks and handles
• Locks that protrude
• Alarm sensors
• Tile or stone ledges (kitchen windows)
• Deep sills that change bracket placement

Step 5: Label the window

Do not write “Bedroom window.” Write:

• “Bedroom: Left of bed”
• “Bedroom: Right of bed”
• “Bedroom: Facing street”

This saves you from mixing up similar-looking windows later.

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Step-by-step: Measuring for an outside mount

Outside mount is more forgiving and often gives better light control because you can add overlap around the window.

Step 1: Decide how much coverage you want

Outside mount is not just “measure the window.” It is “measure the coverage area.”

Most people want outside mount for one of these reasons:

• The frame is too shallow for inside mount
• The opening is not perfectly square
• They want better privacy and fewer light gaps

Step 2: Measure the width of the area you want to cover

You typically want the treatment to extend past the opening on both sides.

Instead of giving a one-size-fits-all number, use this practical method:

• Decide how much you want to extend beyond the opening on the left and right
• Add that to your opening width
• Confirm you have a stable mounting surface (wall/trim)

Example:
Opening width = 48″
You want extra coverage on both sides
➡ Final coverage width is bigger than 48″ (your exact choice depends on space and mounting surface)

If you have limited space because of corners, cabinets, or adjacent walls, measure what is actually possible.

Step 3: Measure the height of the area you want to cover

Decide:

• Where the top should start (above the opening or on the trim)
• Where the bottom should end (past the sill or to a specific point)

For better light blocking, outside mount is often mounted slightly higher than the top of the opening, but again, the real answer depends on your wall space and aesthetics.

Step 4: Note trim and surface constraints

Outside mount only works well if the brackets mount securely. Note:

• Thin trim
• Uneven walls
• Narrow areas where a bracket might not sit flat

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Inside mount vs outside mount: quick comparison

Feature	Inside Mount	Outside Mount
Look	Sleek, built-in	More coverage, can look larger
Measuring difficulty	Higher	Medium
Light gaps	More likely	Less likely (with overlap)
Works with shallow frames	Sometimes no	Yes
Best for	Clean finish, modern look	Better light control, uneven openings

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Measuring differences by product type (what actually changes)

You measure width and height for all treatments, but a few types have extra “gotchas.”

Blinds (horizontal slats)

• Inside mount: check depth so the headrail fits cleanly
• Outside mount: add enough side coverage to reduce light gaps around slats

Roller / solar shades

Roller shades are fantastic, but side gaps are more noticeable because it is one continuous sheet of fabric.

Key things to know:

• Some systems have a difference between bracket width and fabric width
• Small measurement errors show up as visible light lines on the sides

Best practice:

• Be extra precise with width
• Choose outside mount if you want maximum glare control or privacy

Cellular (honeycomb) shades

Cellular shades are slightly more forgiving visually, but depth still matters for inside mount if you want a flush look. Also note:

• Handles and cranks can interfere more with cellular shades because the shade comes close to the glass

Roman shades

Roman shades often stack/fold at the top. That changes how they sit visually:

• Outside mount is common when you want a softer, decorative look
• Measure height with the stack in mind (you still use width and height, just plan for the visual)

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Special windows and tricky situations

Sliding glass doors / patio doors

For wide openings, you will usually want something designed for that width and frequent use. When measuring:

• Measure the full width of the opening you want to cover
• Note handle depth and where the door slides
• Decide whether you need the treatment to stack left, right, or split

Bay windows

Bay windows are almost never “one measurement.”

• Measure each section separately
• Label each section clearly
• Note angles, protruding ledges, and any crank windows

Crank windows

Cranks are the #1 reason people regret inside mount shades.

• Measure how far the crank sticks out
• Consider outside mount if the crank will hit the shade

Kitchen windows with tile or stone ledges

A tile ledge can block bracket placement or reduce usable depth.

• Measure usable depth where the bracket would sit
• Note ledge height and any uneven surfaces

Odd shapes (arches, angled tops)

These are usually best handled with professional guidance because measuring rules vary by solution type. If you do measure, include:

• Full width at the base
• Height at the tallest point
• Photos and notes for shape details (for your own reference)

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The 10 most common measuring mistakes (and how to avoid them)

1. Using a cloth tape measure
   Use a steel tape for accuracy.
2. Measuring width only once
   Always measure top, middle, bottom.
3. Not deciding mount type first
   Inside mount measurements are not “convertible” to outside mount without rethinking coverage.
4. Ignoring depth
   Depth decides whether inside mount looks clean or looks forced.
5. Forgetting about handles and locks
   A perfect fit on paper can still hit a crank in real life.
6. Mixing up width and height in your notes
   Write “W x H” every time.
7. Assuming all windows in the room are identical
   Measure each one. Even new builds vary.
8. Rounding too aggressively
   Be precise. If you must round, do it consistently and only after understanding how your supplier expects measurements.
9. Measuring the trim when you meant the opening (or vice versa)
   Inside mount = opening. Outside mount = coverage area.
10. Not labeling windows clearly
    “Living room window” turns into confusion fast.

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Copy-paste measurement worksheet (use this for every room)

Copy this into Notes/Google Docs and fill it out window by window:

Room / Window label:
Mount type: Inside / Outside
Product type (if known): Blinds / Roller / Solar / Cellular / Roman / Other

WIDTH

• Top:
• Middle:
• Bottom:
• Width to use (smallest for inside mount):

HEIGHT

• Left:
• Center:
• Right:
• Height to use (follow product instructions):

DEPTH (inside mount only)

• Usable depth:
• Notes (slanted edge, tile lip, etc.):

OBSTRUCTIONS / NOTES

• Crank/handle size:
• Locks/sensors:
• Trim limitations:
• Anything unusual:

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When to stop DIY and get a second set of eyes

Measuring yourself is totally doable, but it is smart to ask for help when:

• Your openings are visibly out of square
• You have lots of crank windows
• You are covering a wide opening (sliding doors, large picture windows)
• You want a tight light-blocking setup (bedrooms, media rooms)
• You are unsure whether inside or outside mount is better

If you are ordering through a window treatment provider, share your worksheet and notes. A good team will spot issues like depth conflicts or handle interference before you buy.

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Final checklist before you submit measurements

• ✅ You chose inside or outside mount first
• ✅ You measured width in 3 places
• ✅ You measured height in 3 places
• ✅ You checked depth for inside mount
• ✅ You noted handles, locks, sensors, and ledges
• ✅ You labeled each window clearly
• ✅ You confirmed how the supplier wants measurements submitted (exact opening vs finished size)

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FAQs

1) Should I measure in inches, and should I round?
Measure in inches and be as precise as you can. If rounding is needed, follow the product’s measuring instructions. Different suppliers handle deductions differently, so precision plus the correct measurement method wins.

2) What if my window isn’t perfectly square?
That is common. For inside mount, take three widths and use the smallest width to ensure it fits. If the difference is large or the window looks visibly uneven, outside mount may give a cleaner result.

3) How do I know if I have enough depth for inside mount?
Measure usable flat depth inside the frame and compare it to the product requirements for your chosen blind/shade type. Also confirm that handles and locks will not interfere with operation.

4) How can I reduce light gaps on the sides?
Outside mount with good overlap typically reduces light gaps the most. Roller shades and blinds can show side gaps if measured too tightly to the opening.

5) Do roller shades measure differently than blinds?
The measurement process is similar (width and height), but roller shades are more sensitive to small width errors because the fabric is continuous and side gaps are easier to see.

6) Can I reuse measurements for multiple windows that look the same?
It is risky. Even “identical” windows can vary slightly. Measure each window and label it.

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