A replacement window sill takes just three measurements — length, depth, and, on a return sill, the horn returns. Below is exactly how to take each one, with photos, plus the mistakes that trip people up. Every sill is then cut to your numbers, accurate to ±0.5mm.
Three measurements cover most openings. When in doubt, send a photo with your rough numbers and we'll confirm the drawing before anything is cut.
Measure jamb to jamb along the window opening. For a return sill, this is the inside span between the two horns.
From the window frame to the front edge where the sill meets the wall. Add any overhang you want past the wall face.
For return sills, measure how far each horn extends past the opening on the left and right — they can differ.
A straight sill is a single rectangular piece that sits between the two jambs — you only need length and depth. A return sill (also called a horned sill) has ears that wrap back to the wall on each side; in addition to length and depth you'll measure how far each horn returns past the opening, left and right. A single continuous sill can span one, two or three windows, with the openings separated by interior jambs.
Every sill is made to your exact dimensions, accurate to ±0.5mm. You don't have to be a professional to get a good result — enter your best measurements in the online configurator and it draws the sill live as you go, so you can sanity-check the shape. We also review every order and confirm drawings within one business day before fabrication begins.
Not sure about a number? The configurator draws your sill live as you enter dimensions, and we confirm the full spec before anything is cut. You can also email a photo with rough measurements and we'll take it from there. When you're ready, choose a material and start your design.
Eight steps, a live drawing, and a firm quote in one business day.