Step-by-Step Guide: How to Measure a Door for Replacement

Whether you’re dealing with drafts, damage, or just want a fresh look, accurate measurements are the key to finding the right fit for a replacement door. A door that’s too small or too large can leave you with an imperfect fit. Getting the measurements right will save you time, effort, and potential headaches, so you can shop around with confidence and find the perfect replacement. Whether it’s a front door, patio door, or even a storm door, here’s how to measure a door for replacement. 

Measurements You’ll Need

Before you can learn how to measure for a door replacement, you need to know what to measure. Here are some of the terms you’ll need to know:  

• Rough opening: The space in the wall where the door frame will be installed. It’s slightly larger than the door frame to allow for adjustments. 
• Slab: The actual door without hinges or frame. 
• Frame: The structure that surrounds and supports the door, including the jambs, head, and sometimes the threshold. 
• Jamb: The vertical parts of the frame on either side. 
• Head: The horizontal part at the top of the door frame that connects the vertical jambs. 
• Threshold: The bottom part of the door frame. 
• Sill: The horizontal part below the threshold. 
• Casing: Trim around the door that covers gaps. 

Measuring Prehung Doors vs Slab Doors 

When shopping for a replacement door, you’ll come across two main options: prehung doors and slab doors. Which is the right choice for you?   

• A prehung door comes already attached to its own frame, hinges, and sometimes even with weatherstripping. Prehung doors are a great option if your existing door frame is damaged, warped, or rotting.  
• A slab door is just the door. If your current door frame is in good shape, a slab door is a more affordable option, and the measurements are a little simpler. 

In general, prehung doors are easier to install but require more accurate measurements. Slab doors are cheaper but can be harder to fit correctly. 

How to Measure for a Replacement Door

Accurate measurements are essential to make sure your door fits and functions perfectly. Here’s how to measure a door for replacement in five steps:  

1. Measure the width: Use a tape measure to find the width of your door frame at three points—top, middle, and bottom. Write down the smallest measurement so you don’t order a door that’s too wide to fit. 
2. Measure the height: Measure from the threshold (bottom) to the top of the door frame. Take this measurement on both the left and right sides and use the smaller of the two measurements.  
3. Measure the thickness: Measure along the edge of the door, starting from one face to the other (front to back). Most doors are around 1 ¾ inches thick, but older doors or custom installations can vary.  
4. Measure the jamb width: The jamb is the thickness of the door frame. Standard jambs are around 4 ⅝ inches, but it’s always good to measure. If your jamb width doesn’t match, the door won’t fit properly in the frame. 
5. Double-check everything: Before you go shopping, double-check your measurements. A small error can mean big problems when it’s time to install your new door. 

Measuring Sidelites

If your front door has sidelites—narrow windows on either side of the door—you may need to measure them for replacement as well.  

1. Measure the sidelite width: Sidelites vary in width, so be sure to measure each one separately. Start from the inside frame on one side to the inside frame on the other. 
2. Measure the sidelite height: Measure from the threshold to the top of the sidelite frame, just like you did with the door height. Go with the smaller number. 
3. Total width: Add the width of your sidelites to the width of your door to get the full width of the entire entry system. This step is important because sidelites and the door are often ordered as a single unit. 
4. Check the frame: Measure the overall frame dimensions to ensure your new door system—including sidelites—will fit properly into the rough opening. 

How to Measure a Storm Door for Replacement

Learning how to measure a storm door for replacement is slightly different because it sits outside your existing door frame. Here’s how to measure for a perfect fit: 

• Measure the width: Just like with regular doors, measure the width of the storm door frame at the top, middle, and bottom and use the smallest measurement. 
• Measure the height: Measure from the threshold to the top of the door opening on both sides and use the smallest measurement. 
• Check the frame and trim: Be aware of any obstructions around the door frame, like decorative trim or molding. They can impact how the storm door sits, so make sure to measure their depth as well. 
• Measure the jamb depth: Finally, check the depth of the door jamb. A storm door needs to sit flush against the frame, so double-check that it will fit snugly. 

Ready for Your New Front Door?

Now that you know how to measure a door for replacement, it’s time to upgrade that old door. At West Shore Home, we make front door installation fast, easy, and stress-free. Whether you need a prehung door, slab door, or a full entry system with sidelites, our team can help you find the perfect match. After your free, no-obligation design consultation, we’ll handle the installation from start to finish, making sure your new door fits perfectly. Contact us today and let us transform your entryway with a stylish new door. 

FAQs

How do I measure the width of a prehung door for replacement?

To measure a prehung door, start by measuring the rough opening, which is the space inside the door frame, from stud to stud. Take three measurements, at the top, middle, and bottom. Use the smallest measurement to ensure the new door fits snugly. Don’t forget to measure the depth of the frame, as prehung doors include the door jamb, and double-check for obstructions like trim. 

How do I measure the height of a door for replacement?

Measure the height from the threshold (bottom) to the top of the door frame. Take this measurement on both the left and right sides of the door opening, as door frames can settle over time and become uneven. If there’s a difference, use the smaller number to avoid ordering a door that’s too tall. Be sure to measure from the finished floor, not from any carpeting or flooring. 

How do I measure the thickness of a door and door jamb for replacement?

Most front doors are about 1 ¾ inches thick, but it’s important to confirm by measuring the current door. For the door jamb, measure from the inside edge of the door frame to the outer edge. This measurement is typically around 4 ⅝ inches, but this can vary depending on the construction of your home.  

What are the normal dimensions for a front door?

A standard front door size is 36 inches wide, 80 inches tall, and 1 ¾ inches thick. However, some homes feature different sized front doors, especially custom or older homes. And if your door has sidelites or transom windows, the total dimensions of the entryway could be much larger. Always measure your specific door and frame before ordering.