How to Measure for a New Picture Window

Prior to installing a new picture window, you have to measure the size of the existing window. This will enable you to buy the right window that can fit on the window opening. The smartest way to get the dimensions for your new window is to simply refer to the window schedule/layout of your house. This is found on your house’s architectural drawing/blueprint. You do not need to do any measurements at all since they are shown on the drawing/window schedule. Look for your architectural plan if you still have it. If not, you can ask the home builder or architect who was involved in the design of your house to provide you with a copy. This copy is the most accurate piece of information that you can ever get. It is even more accurate and reliable than your manual measurements. Once you get the window schedule, simply take it to the hardware store to purchase your new window. If you cannot get the window schedule or architectural drawing for whatever reason or if you have done a lot of renovations over the years, then you should measure the window manually.

Tools and Materials Needed:

  • Tape measure
  • Note pad
  • Laser tape measure (optional)
  • Pencil
  • Calculator

Examine the Walls around Window Frame

Look at the walls surrounding the window frame. Check to see if they are plastered, tiled or just plain bricks. Look at the internal and external sides of window opening (i.e. at the jambs, sills and top).The strip of wall finish that forms a perimeter around the window is known as “narrow widths”. If the wall finish is plaster or tiles, then you should make an allowance for this strip in your measurements. If they are just plain bricks, then there are no adjustments to be made for wall finishes. You only make adjustments for the window sill.

Measure the Sills

Measure the length, width and depth of the existing window sills. To measure the length, place one end of the tape measure at the end of sill, then press down the tape with your finger, making sure that the tape is level and at 90 degrees from the adjacent wall at the jambs. Take down the measurements in your note book. Measuring the depth and width should be simple since these are short dimensions.

Measure the Window Opening

In architectural terms, the window opening is not the glass panel or window frame. It is the opening in a wall without the window frames. To measure this opening, start with the width. Measure the wall-to-wall width from the right to the left using a tape measure, making sure that the tape is level and at 90 degrees to the jamb wall.

Measure the height of the window opening from the top of window opening on the internal side to the top of sills at the bottom. Make sure the tape is level and perpendicular to the flat sill or underside of lintel.

Make Adjustments

To get the true size of the window opening, you should make adjustments for wall finishes and sill in your measurements. The true size of the window opening is its original size without any wall finish, cladding or frame. Depending on what type of wall finish you have, you should add an allowance for the “narrow widths” and sill. As an example, for plastered walls, add 2(10mm) = 20 mm to the measured width. Add 10mm + 75mm to the measured height. Ten (10) mm is the thickness of plaster and 75mm is the depth of window sills. Now you have true size of window opening. Take these to your building material supplier to purchase a new picture window. You must make it clear to the building material supplier that you have adjusted for sills. It is good to bring all your measurements.