Measure a window carefully before ordering blinds.

Replacement blinds can look simple to order until a window is slightly out of square or its handle blocks the headrail. A careful sketch and three measurements can prevent an expensive sizing mistake. Read the measuring guide for the exact blind or shade you plan to buy before placing an order.

What you’ll need

• Metal tape measure
• Pencil
• Paper or phone notes
• Step stool only if it can stand level and you can work safely
• Product measuring guide

Take it step by step

1. Label the room and window on your sketch. Decide whether you are considering an inside mount within the frame or an outside mount over the trim.
2. For an inside mount, measure the opening width at the top, middle, and bottom. Record all three exact measurements rather than assuming the sides are parallel.
3. Measure the opening height at the left, center, and right. Record each number, then measure the available frame depth.
4. Note handles, locks, trim, screens, and other obstacles that could interfere with the blind or shade. Compare the depth with the product’s minimum-depth requirement.
5. For an outside mount, mark the exact area you want the treatment to cover. Follow the manufacturer’s overlap guidance and check that the mounting surface is suitable.
6. Measure every window separately, even when two appear identical. Enter measurements exactly as the ordering guide requests and let the manufacturer make only the deductions it specifies.

When to ask for help

Ask an installer for help if the window is unusually shaped, the mounting surface is damaged, the treatment is heavy or motorized, or safe measuring requires leaning from a ladder. Do not drill until hidden utilities and the proper fasteners have been addressed.