MM → Inches
4.2 mm to inches
4.2 millimeters equals 0.165354 inches — or about 3/16" (nearest 1/16″).
Decimal
0.1654 in
Fraction
3/16"
Centimeters
0.42 cm
Feet
0.0138 ft
mm
0.165354
in≈ 3/16" (nearest 1/16)
1 in = 25.4 mm · tolerance ±0.0001
Formula
To convert mm to inches, divide by 25.4:
inches = mm ÷ 25.4
inches = 4.2 ÷ 25.4 = 0.165354
Why 4.2 mm = 0.1654 in
Since 1 inch is defined as exactly 25.4 millimeters, 4.2 mm converts to 0.165354 inches. Rounded to the nearest 1/16″ that's 3/16", which is what you'd read off a standard imperial ruler.