Glass Size & Area Calculator

Multiply width by height in inches and divide by 144 — a 36 × 48 inch pane is 36 × 48 ÷ 144 = 12 ft². In metric, width × height in centimeters gives cm², and dividing by 929 converts to square feet (or by 10,000 for m²). The area calculator above does all four conversions at once from a single pair of inputs.

Measure the frame rebate, not the visible glass. Run a tape into the recess where the old pane sat, take width and height at three positions each, keep the smallest reading, then subtract 2× the clearance (usually 4–6 mm total for wood, 6–10 mm for metal channels). Only measure the old glass itself if you can remove it undamaged — and then still verify the rebate depth gives enough bite.

Bite is the depth by which the glass edge sits inside the frame rebate on each side — the overlap that holds the pane against wind and its own weight. The practical minimum is glass thickness + 3 mm; window sashes typically aim for 10–15 mm of bite, door lites more. Too little bite and the pane can walk out of the frame; too much and the visible glass shrinks while edge stress rises. The cut-size calculator reports the minimum for your thickness automatically.

Plan on 2–3 mm per edge for wooden frames (paint and putty lines forgive small errors) and 3–5 mm per edge for metal and uPVC glazing channels that use gaskets. Total allowance is therefore 4–6 mm on each dimension for wood, 6–10 mm for gasket systems. Glass needs room to expand and the frame needs room to be imperfect — a pane cut to the exact opening size is a pane that will not go in.

Measure both diagonals of the opening and compare them. Equal diagonals mean a true rectangle; a difference up to 3 mm is within normal tolerance, 3–6 mm calls for shimming or sizing to the smaller diagonal, and more than 6 mm means the frame is racked — template the opening or correct the frame before ordering. The diagonal checker above applies these thresholds to your two readings.

Running (linear) feet measures the perimeter, used mainly for polished-edge pricing: walk around all four edges and add them up. A 36 × 48 inch pane has a perimeter of 168 inches — 14 linear feet. Suppliers quote edgework per linear foot (or per linear inch), which is why a small pane with polished edges can cost more than its glass area suggests. Our glass price calculator uses the same perimeter math for its edge-polish line item.

Use whichever system your supplier cuts in, and stay in it for the whole order. US shops commonly take feet-and-inches with fractional thickness (1/8", 3/16", 1/4"); UK, Australian and European shops work in millimeters. Converting once at the end (inches × 25.4 = mm) is safe; alternating units field by field is where 24-inch widths become 24 cm ones. Thickness is the trap — 6 mm glass is not 1/4" glass (6.35 mm), and channels cut for one will not snugly hold the other.

Subtract twice the per-edge clearance: 4–6 mm total on each dimension for wood frames, 6–10 mm for gasketed metal or uPVC systems. A 600 × 400 mm rebate in a wooden sash therefore wants roughly a 594 × 394 mm pane. The cut-size calculator applies the subtraction live and shows the pane area and perimeter so you can sanity-check the quote that comes back.