* Know the conversion: 1 inch equals 25.4 millimeters.

* Set up a proportion: 20 mm / x inches = 25.4 mm / 1 inch

* Solve for x: x inches = (20 mm * 1 inch) / 25.4 mm

* Calculate: x inches ≈ 0.787 inches

Therefore, 20 mm is approximately 0.787 inches.