Window Calculator – Rough Opening & Cost

A reliable windows takeoff follows one rule: measure twice, order once. The calculator converts your unit size and opening count into dependable quantities using Rough Opening = Unit W + 2 cm × H + 2 cm; Glass Area = Unit Area − 15% frame, then adds the waste margin that experienced builders apply by default. Example: A 1.2 × 1.5 m window needs a 1.22 × 1.52 m rough opening and yields 1.53 m² of glass. Field tip: Order by the three smallest field measurements - old openings are never square. The result is broken down into purchasable units - bags, sheets, rolls or truck loads - so you can order the nearest whole unit without overbuying, and every figure traces back to the formula shown above.

1. 1

   Measure the space

   Record the unit size and opening count at the widest and longest points.

2. 2

   Enter the values

   Type the measurements into the calculator and select your units.

3. 3

   Apply the formula

   The tool computes Rough Opening = Unit W + 2 cm × H + 2 cm; Glass Area = Unit Area − 15% frame for your inputs instantly.

4. 4

   Add waste and order

   Review the total with a 5-10% margin and round up to whole units.

• Ordering exact sheet count

• Compressing insulation

• Vapor barrier on the wrong side