Things You'll Need
Blue painter's tape
Circular saw with masonry blade
Water-filled spray bottle
Marble window sills offer higher durability than wood or drywall finished sills. Cutting marble requires a blade capable of removing material without causing the marble to crack. Masonry saw blades that attach to circular saws or angle grinders have an abrasive edge rather than cutting teeth. The abrasive--either diamond chips or a composite material--grinds the material away from the cut. This creates a smooth edge while cutting and eliminates the chances of a marble sill cracking.
Measure the length of a window sill with a tape measure.
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Pull the tape measure along a marble window sill. Mark the center of the sill with a wax crayon.
Divide the window sill measurement by two. Align the end of the tape measure with the center mark made with the wax crayon. Place a mark at the dimension on the tape measure determined by dividing the window sill measurement by two with the wax crayon. Make a second mark on the other side of the center line.
Make a line at the two off center crayon marks by aligning one edge of a carpenter's square with each mark and pulling the wax crayon across the face of the marble sill.
Cover the face of the sill on either side of the crayon line with blue painter's tape.
Align a circular saw with a masonry blade with one of the lines. Pull the trigger of the circular saw. Spray the spinning blade with water from a water-filled spray bottle as you cut to reduce the amount of dust created by the cut. Cut the other marked line as described.
Wipe the water from the marble sill with a rag. Pull the blue painter's tape from the cut area before installing the cut marble sill.
Wear eye, ear and hand protection when cutting marble window sills.