1/4 Inch to MM
Fractional inch to millimeters
1/4 inch (1/4″)
6.35 mm
= 0.635 cm · 1 inch = 25.4 mm exactly
💡 Commonly used for air hose, water line tubing.
Nearby fractional inches
| Fraction | mm | cm |
|---|---|---|
| 1/8″ | 3.175 mm | 0.3175 cm |
| 3/16″ | 4.7625 mm | 0.4762 cm |
| 1/4″ | 6.35 mm | 0.635 cm |
| 5/16″ | 7.9375 mm | 0.7937 cm |
| 3/8″ | 9.525 mm | 0.9525 cm |
Also written as
1/4 to mm · 1/4 in mm · 1/4" in mm · 1/4 inch in mm · 1/4 inches to millimeters · 1/4 en mm · 0.25 inch to mm
All of these mean the same measurement. The double-prime mark (″) and the plain quote (") both stand for inches, and en mm is simply the French and Spanish wording of the same question.
Closest metric drill size
6.5 mm
1/4″ = 6.35 mm · difference 0.15 mm
Metric drills step in 0.5 mm, so a fractional inch size rarely lands exactly on one. A 0.15 mm gap is usually invisible in a clearance hole but changes the fit on a press fit or a tapped hole.
FAQ
How many mm is 1/4 inch?
1/4 inch equals 6.35 mm. Since one inch is defined as exactly 25.4 mm, the result is 25.4 × 1 ÷ 4.
Why is the value exact?
The international yard and pound agreement of 1959 fixed the inch at exactly 25.4 mm, so fractional inch conversions terminate rather than repeat.