How to Measure for Replacement Windows from Inside – Step by Step Guide

When it comes to home renovations, replacing old or inefficient windows can significantly enhance your living space’s comfort, aesthetic appeal, and overall value. However, the cornerstone of a successful window replacement project lies in the precision of your measurements. While many homeowners might think of this as a task for professionals, measuring for replacement windows from the inside is a skill you can master with the right guidance. This approach not only saves you time but also allows for a more convenient and adaptable measurement process, especially in scenarios where exterior access might be limited or impractical.

In this comprehensive guide, we will walk you through the meticulous process of measuring your windows from the inside. Whether you’re dealing with hard-to-reach windows or simply prefer the DIY approach, this article will equip you with the knowledge and confidence to ensure your replacement windows fit perfectly, function flawlessly, and look fantastic.

Check out our video on the topic here!

What’s Inside

Why Measure from the Inside?

Measuring for replacement windows from the inside of your home isn’t just a matter of convenience; it’s often a necessity. Various situations, such as living in a high-rise building, having windows that are difficult to access from the outside due to landscaping or architectural features, or simply dealing with inclement weather conditions, can make external measurements challenging, if not impossible.

Interior measurements offer a viable alternative, providing several benefits that go beyond mere convenience. Firstly, they allow you to undertake the task regardless of external weather conditions, ensuring that your home improvement projects can proceed without delay. Secondly, measuring from the inside is often safer, particularly for windows located on upper floors, eliminating the need for ladders or scaffolding. Lastly, this method gives you a unique perspective on the window’s structure and how the new units will integrate with your interior space, allowing for more informed decisions regarding style, function, and aesthetics.

According to The World Health Organization “There are more than 164,000 emergency room-treated injuries and 300 deaths in the U.S. each year that are caused by falls from ladders.”!


Tools and Materials Needed

Before diving into the measurement process, it’s essential to gather the right tools and materials to ensure accuracy and efficiency. Fortunately, the list is short and straightforward, comprising items you likely already have around the house:

  • Tape Measure: A retractable metal tape measure is ideal for its rigidity, which makes it easier to measure long distances accurately. Ensure it’s at least 10 feet long to accommodate various window sizes.
  • Notepad and Pen: You’ll need to record your measurements immediately to avoid confusion or errors. A notepad dedicated to this task helps keep everything organized.
  • Level: A small spirit level can help ensure your measurements are perfectly horizontal or vertical, which is crucial for accuracy.
  • Laser Measure (Optional): For an even more precise measurement, especially in hard-to-reach areas, a laser measure can be a handy tool, though not strictly necessary.

With these tools in hand, you’re ready to proceed. Remember, the key to successful measurements is not just the tools but also the care and attention to detail you apply during the process.

Understanding Window Terminology

Before we start with the actual measuring, it’s crucial to familiarize yourself with some basic window terminology. This knowledge will not only help you understand the instructions better but also ensure you’re measuring the correct parts of the window.


The part of the window that moves and holds the glass panes. In double-hung windows, both the upper and lower sections can be sashes.

Sliding window vent


The outer structure that encases the window sash and holds it in place. It’s fixed and does not move.

sliding window frame


The bottom part of the window frame that often protrudes outward and can be used as a shelf.

sliding window sill


The vertical sides of the window frame. The sash slides or closes against these when the window is shut.

sliding window jamb

Understanding these parts will help you identify where to place your tape measure and what to look out for during the measuring process.

Step-by-Step Guide to Measuring

1. Preparing the Area

First, ensure that the window area is ready for measurement. Clear any decorations, plants, or obstacles from the window sill and surrounding areas to provide unobstructed access. If your windows have removable grilles or security bars, take these off for a clearer measuring path. Cleaning the window can also help, as it allows you to see the edges and corners more clearly, reducing the risk of measurement errors.

2. Measuring Width

Start by measuring the width of your window. Place the end of your tape measure on the inside surface of one jamb and extend it across to the inside surface of the opposite jamb. Measure at three points: the bottom, middle, and top of the window. This is particularly important for older homes, where windows may have settled and may not be perfectly square. Record the smallest of these three measurements to ensure the replacement window will fit.

3. Measuring Height

After width, the next step is to measure the height of your window. This is done from the high point of the sill, which is the surface closest to the inside of your home, to the top of the window opening. Again, it’s advisable to take this measurement from three different points: the left side, the center, and the right side of the window. Windows, especially in older homes, may not be perfectly square due to settling over time, so measuring at multiple points ensures you account for any irregularities. Use the smallest of these measurements to guarantee a fit that’s snug and secure.

4. Measuring Depth

Depth measurement is crucial to ensure your new window fits into the existing opening without any issues. To measure the depth, open the window and measure from the outermost point of the window frame to the innermost point. This measurement is typically taken at the top of the window where the lock is located, as this tends to be the deepest point. However, ensure there’s no part of the window protruding further into the room elsewhere. It’s important to note that you need a minimum depth to accommodate the replacement window, which is usually about 3.25 inches, but this can vary depending on the window type and manufacturer.

Double-Checking Measurements

The adage “measure twice, cut once” is particularly pertinent when it comes to ordering replacement windows. Given the cost and logistical challenges of correcting errors, it’s crucial to double-check your measurements. After recording the initial set of width, height, and depth measurements, take a break, then return to measure everything again. If you find discrepancies, measure a third time to confirm. This meticulous approach helps avoid costly mistakes and ensures that your replacement windows will fit perfectly.

Considering Window Types and Styles

While measuring, it’s also essential to consider the type and style of the replacement window. The measurements for a casement window, for example, might differ slightly from those of a double-hung window due to the operational mechanics and how they fit within the frame. Familiarize yourself with the specific requirements of the window type you’re considering and adjust your measuring technique accordingly. This is also an excellent time to think about any changes you’d like to make in the window style, as some alterations might be possible without significant modifications to the opening.

Common Mistakes to Avoid

Measuring for replacement windows is a straightforward process, but it’s prone to a few common pitfalls:

Ignoring Irregularities

Assuming all sides and corners of your window are perfectly square can lead to incorrect measurements. Always measure at multiple points and use the smallest measurements.

Forgetting to Check for Obstructions

Consider any potential obstructions (like window handles, locks, or mullions) that might interfere with the replacement window’s operation.

Overlooking the Depth

Failing to measure the depth correctly can result in a window that doesn’t fit or has issues with insulation and functionality.

When to Call a Professional

While DIY measurements can be accurate and satisfying, certain situations call for a professional touch. If your windows are in a hard-to-reach place, if you’re dealing with a particularly large or uniquely shaped window, or if you simply doubt your measurements, don’t hesitate to call in a professional. Utah Window Experts offer free measurement services as part of their quote, providing peace of mind and ensuring precision.


Measuring for replacement windows from the inside is a manageable task that can save you time and offer flexibility. With careful attention to detail, the right tools, and a methodical approach, you can ensure your new windows fit flawlessly and enhance your home’s comfort, efficiency, and aesthetic appeal. Remember, precise measurements are the foundation of any successful window replacement project, so take your time, double-check your work, and look forward to the transformation new windows will bring to your home.


Q1: What if my window openings are not square?

  • It’s common for older homes to have window openings that are not perfectly square due to settling over time. Measure at multiple points for both width and height, and use the smallest measurements to ensure the replacement window will fit. The new window can be shimmed during installation to account for any minor discrepancies and ensure it operates correctly.

Q2: Can I measure for replacement windows without removing the existing ones?

  • Yes, you can measure for replacement windows without removing the existing ones. The measurements should be taken from inside the frame where the window sash sits, not from the trim or part of the wall surrounding the window.

Q3: How do I measure for windows with existing storm windows or screens?

  • If your window has an existing storm window or screen, measure the primary window’s dimensions, not the storm window or screen. You’ll need to remove these add-ons to access the main window frame for accurate measurements.

Q4: How much clearance should I allow for when ordering a replacement window?

  • Typically, it’s recommended to subtract about 1/4 inch from both your width and height measurements to provide a bit of clearance for the new window. This allows for easier installation and accounts for any minor irregularities in the opening.

Q5: What should I do if I’m replacing a non-standard size window?

  • If your window measurements don’t match standard sizes, you might need to order a custom window. Many manufacturers offer custom sizes to fit unique openings, though this option may come at a higher cost. It’s crucial to provide the exact measurements to ensure a proper fit.

Q6: Should I measure the window trim or molding as well?

  • No, you should not include the trim or molding in your measurements. The focus should be on the window’s operational area — the part that holds the glass and moves (if applicable) — and the frame it sits within.