Project Guide

How to Install a Window

Measure Opening and Determine Your Window Needs
Man measuring a window.

The first step in learning how to install a window is by measuring them. In order to get the exact height and width of your window, you need to measure the pocket opening. Taking your time and getting an exact measurement will save you from having to sand the frame and add filler strips.   


  • Start by measuring inside the existing frame, including the height in the middle and left and right sides.  
  • Next, measure the width at the top, middle and bottom. 
  • Write down the measurements and use the smallest measurements for width and height to buy your new window. It's important that your new window be about 3/4-inches smaller than your window opening to keep you from reframing the opening.  

Tip: When learning how to remove windows, always focus on safety. Be careful to avoid jagged glass. When performing residential window replacement or window repair, keep the old windows in place until you have inspected the new ones for damage and verified their size. 

Remove the Existing Window
Man installing a window.

Instructions for how to replace a window may vary based on the type of window you have. The following steps will cover how to remove a double hung sash window that is common is most homes.


  • If you are replacing a damaged window, remember to wear eye protection, work gloves and closed-toe shoes.  
  • Use painter's tape to cover any cracks to prevent shards from falling out during the removal process. 
  • Score around the window casing and the interior wall with a utility knife. 
  • Use a hammer and chisel to separate the window casing from the wall.  
  • If your windows are held in place with nailing fins, you may have to remove some siding on the exterior of your home to access it. Remove the siding carefully with a pry bar, as you will have to replace it once the new window is installed. 
  • Remove old caulk from the window. 
  • Pry up the nails holding the window in place. You can also cut through the nails with a reciprocating saw if necessary. 
  • Remove the bottom sash, then remove the rest of the window from the wall. 
Prep Area and Dry Fit the New Window
Man tests new window.
  • Now that the window is out of the wall, check the window jamb and the window stop for structural damage.  
  • Replace any damaged areas before installing windows. 
  • Clean the area. 
  • Test fit the new window into the space. 
Set and Level the Window
Man uses a level to make sure the window is even.
  • Set the window into the opening and push the brick mold or nailing fin tight against the sheathing. 
  • Center the new window in the opening, supporting it with wood blocks and shims placed below the horizontal jambs.  
  • Use a level to check how much adjustment will be necessary to balance and plumb the window unit. Adjust the shims if necessary.  
  • Clamp or have a helper hold the moulding in place.  
  • If you have vinyl or aluminum siding, you may need to install a J-channel to hold the trim. 
Double Check the Window and Pre-Drill Holes
Man double checks that the window is square using a tape measure.
  • Measure the diagonals of the window before installing windows.  
  • If they are equal, the window is square. If not, adjust the shims until the window is square. Always double check that the window is level and square after you adjust the shims. 
  • If the window has a brick mold, pre-drill holes in it and drive 16d nails through it and into the framing.  
  • If the window has a nailing fin, nail through it into the framing.  
  • In either case, start at the corners and space the remaining nails as recommended by the manufacturer.  
  • Drive all nail heads below the wood surface with a nail set. 
Cut Siding and Apply Caulk
Man applies caulk along the window frame.
  • Use a circular saw adjusted so that the blade depth equals the thickness of the siding.  
  • Start the cut with the toe of the saw plate on the siding, but with the blade and heel of the plate above the surface.  
  • Slide the saw guard back, start the saw and ease the blade into the wood.  
  • To avoid splintering, stop before the corner.  
  • Complete the corner cuts with a sharp chisel. 
  • Cut a length of drip edge to fit over the top of the window, then slide it between the siding and the building paper. 
  • Next, caulk around the moulding. 
  • If it has brick mold, apply a continuous bead of caulk around the opening.  
  • If it has a nailing fin, apply the caulk to the back of the fin. 
Install Window
Man installs new window.
  • Place the window into the opening, pressing it tightly against the caulk. 
  • Put shims where pre-drilled holes are to secure the window.   
  • Drive screws into holes through to the shims. 
  • Check that sashes are even and the window is level. 
  • Add shims where needed. 
Trim Shims and Add Insulation
Man adds insulation to the window.
  • Trim the shims flush with the framing by scoring them with a utility knife and then snapping off the excess. 
  • Fill the gaps between the window jambs and the framing members with loosely packed fiberglass insulation
Man paints the window frame.
  • When the caulk dries, paint to match the trim on the rest of your house.
Window Replacement Tips
A person installing a replacement window.

Windows will differ by material and design so it's important to know what type of window you currently have and what type of window you are installing. Here are a few tips to keep in mind when buying replacement windows:


  • Homeowners often opt for vinyl and metal windows because they have a nailing flange that attaches to your home to keep water out. Because the flange must be attached to the rough opening of a wall, the process of how to install a vinyl window with nailing flanges is a bit different than other window install projects.  
  • If you need to replace a smaller bathroom or larger bay window, the process of how to remove windows will involve the same steps; however, your measurements, power tools and supplies may change.  
  • How to install a replacement window differs slightly from how to install a new window. If you are installing a new construction window in an existing home, keep in mind that these windows often have a nailing flange. Replacement windows do not.  
  • If you are planning on installing a new construction window in an existing home, a little more work will be involved, including cleaning out the window reveals, using more insulation and sealing. You can opt to take this project on yourself, or one of our trusted professionals can complete your window installation.  
  • Each manufacturer has specific window installation instructions for the wide range of shapes, styles and construction types available. However, windows are mostly installed in the same manner. Cross reference these steps with your window’s manufacturer instructions as you work. 
  • Keep the old windows in place until you have inspected the new ones for damage and verified their size. 
  • Pre-hung windows come complete with finished frames. You can insert them in one piece into the rough opening left by the old window.  

Once you know how to install a window yourself, you can replace damaged windows and update older style windows as desired. Don't worry if you don't own all the tools needed to complete this DIY project. Enjoy tool and truck rental from The Home Depot. And if you'd rather leave how to install a window to a professional, trust our expert window installation services to complete the job for you.