18 inches. Very heavy original aluminum construction in excellent condition with the back flange intact so fully reusable back on a boat. Can also be anodized to any color and used for interior applications such as a window or on a door. 3-4 pieces in stock from the same source, you are bidding on 1 piece. Use buy now for additional pieces and avail of automatic shipping discounts. Great for architectural applications such as using on hardwood doors, beachfront homes etc Also can go right back on the boat. No scratches on the glass. Each window will be meticulously polished and cleaned before despatch. Pay safely with Paypal.