How To Measure For Vertical Blinds | A Complete Guide

You’ve picked out the vertical blinds. You’ve measured the window once. Then you look at the size you wrote down and wonder if one measurement is really enough. The answer, as anyone who has installed blinds before knows, is that windows are rarely perfectly square.

This guide walks through measuring for vertical blinds step by step, covering both inside and outside mounts. You’ll learn where to measure, what to record, and the common mistakes that lead to blinds that don’t fit.

Inside Mount Or Outside Mount — The First Decision

The very first choice determines how you measure. An inside mount fits the blinds inside the window frame, flush with the wall. An outside mount covers the entire window and part of the frame, extending past the edges.

Inside mounts generally look clean and modern, which is why many designers prefer them. But inside mount tolerances are much tighter, making accurate measurement more critical. A mistake of even ⅛ inch can mean the difference between a snug fit and a wobbly one.

Outside mounts are more forgiving and offer better light control since extra overlap covers gaps. For sliding glass doors or patio doors, an outside mount is often the most practical choice because you can measure the full frame and add overlap as needed.

Why Measuring One Point Is Never Enough

It’s tempting to measure the window once, write down the number, and order. But windows are rarely perfectly square — the width at the top can differ from the width at the bottom by a quarter inch or more. That single measurement sets you up for a poor fit.

A common mistake is measuring at only one point on the window. This almost guarantees that at least one side will be off, leaving gaps or requiring force to install. Measuring in multiple places catches these variations before you order.

• Use a steel tape measure: Cloth or flexible tape measures can stretch over time, introducing small errors. A steel tape measure stays accurate and records to the nearest ⅛ inch, which is the standard for blind orders.
• Ignore obstructions: Window handles, cranks, and other hardware can interfere with the blinds’ operation. Measure around them or note their position so your blind length clears them.
• Check window squareness: Measure the diagonal points of the window. If the diagonals are not equal, the frame is out of square, and an outside mount is typically required for a proper fit.
• Account for clearance: Failing to leave enough space can result in blinds that scrape against walls, windowsills, or other surfaces. Always check for natural obstructions before finalizing dimensions.

These four checks alone eliminate most fitting problems. They take an extra two minutes per window but can save a week of returning and reordering.

Measuring For An Inside Mount

For an inside mount, you’re working within the window recess. The goal is to get the smallest width that will fit inside the frame and the longest height to cover the full drop. Start with the width first.

Measure the width at the top, middle, and bottom of the recess. Record all three numbers, then use the smallest one as your width. This ensures the blind fits even if the frame narrows at one point. For height, measure the left, center, and right side of the drop. Use the longest measurement here, so the blind reaches the sill or floor without a gap.

Measure the depth of the window frame from the opening to the glass. The mounting brackets need enough space behind the frame — typically about 2 inches — to install properly. If the depth is shallow, an inside mount may not be possible.

As steel measuring tape accuracy notes, always record to the nearest ⅛ inch and double-check each measurement. A small error in a single reading can cascade into a blind that sits crooked or scrapes the wall.

Inside Mount Height Rule

For height, measure inside from the top of the window casing down to the top of the sill (or to the floor for patio doors) in three places. Record the shortest measurement for height, not the longest. This prevents the blind from binding against the sill when fully extended.

These five checks cover the essential measurements for a correctly fitting inside mount. Skip any one, and you risk ordering a blind that needs to be returned or modified.

Measuring For An Outside Mount

An outside mount gives you more flexibility because you control the dimensions. The key decision is how much overlap you want. For standard light blocking and privacy, adding 3 inches on each side (6 inches total overlap) is the recommended starting point.

For width, measure the full width of the area you want covered — typically the window frame plus the extra inches on each side. For height, measure from the top of the mounting surface to the desired bottom point. That could be the floor for patio doors or the bottom of the sill for standard windows.

Unlike an inside mount, for an outside mount you record the longest measurement for both width and height. This ensures the blind fully covers the window opening without gaps. If the window is slightly skewed, the overlap will hide the imperfection.

1. Decide on overlap amount: 2–3 inches per side is standard; add more if you want near-total light blockage.
2. Mount above the window: Measure from above the frame if you want the blind to hang clear of handles or cranks.
3. Check the mounting surface: Ensure the wall or frame above the window is sturdy enough to hold the brackets. Drywall may require anchors.
4. Account for floor clearance: If the blind will touch the floor, check for carpet thickness or baseboard height that could interfere.

Following these four steps prevents the most common outside mount mistakes: insufficient overlap, mounting too close to the window, and forgetting to clear hardware.

Common Mistakes And How To Avoid Them

Even with a good measuring guide, people make the same errors. The most frequent is using a flexible tape measure. Cloth tape can stretch, and flexible plastic can warp over time. A steel tape measure eliminates this variable and provides consistent accuracy.

Another common mistake is measuring only once. Even careful measurers can misread a mark or write down the wrong number. Measure each dimension twice in the same spot and confirm the reading. If the two numbers differ, re-measure a third time to find the correct value.

Failing to account for necessary clearance can lead to blinds that scrape against walls, windowsills, or other surfaces. Always check the area around the window for protrusions — light switches, thermostat covers, door handles — that could limit the blind’s range of motion.

Co’s guide on measure width three places highlights that for inside mounts, the width variation across a single window can be as much as ¼ inch. If you only measure at one point, you miss that variation entirely.

What To Do If Your Window Isn’t Square

If the diagonal measurements differ by more than ¼ inch, the window frame is out of square. An inside mount may fit poorly, leaving noticeable gaps on one side. In this case, an outside mount is recommended because the overlap covers the uneven frame.

These five mistakes account for the majority of returns and exchanges for vertical blinds. Taking the extra few minutes to avoid them saves time and frustration.

The Bottom Line

Measuring vertical blinds comes down to two choices and three measurements: decide inside or outside mount, then record width, height, and depth. For inside mounts, use the smallest width and the appropriate height. For outside mounts, add 2–3 inches of overlap on each side and measure the full drop.

A certified window treatment installer or your local blinds retailer can double-check your measurements before you place an order, especially if your window frame is out of square or has unusual hardware that could affect clearance.