4 MM to Inches

Quick answer: 4 mm is 0.157 inches and 0.4 cm. That is a small measurement, smaller than the width of most adult fingertips, so it is best pictured against tiny everyday objects rather than against a hand or a room.

Exact Conversion

To convert millimetres to inches, divide by 25.4, because one inch is exactly 25.4 mm. So 4 ÷ 25.4 = 0.157 inches. To get centimetres, divide by 10, because there are 10 mm in 1 cm, giving 0.4 cm. Try the mm to inches converter for other values. You can also use the inches to mm converter to go the other way.

Real-Life Examples

Because 4 mm is small, the best comparisons are small, familiar things you have probably held. These examples give you a real sense of the size.

• The diameter of a small bead used in jewellery making.
• The thickness of a stack of about forty sheets of ordinary paper.
• The width of a small screw head.
• The diameter of a thick pencil lead in some drawing pencils.
• The thickness of a thin phone case, roughly, in some models.

How to Measure It

Millimetres are too small to judge by eye, so use a ruler with millimetre marks, a set of calipers, or a digital measuring tool. Line the zero mark up exactly with one edge of the object and read the small marks: each one is a millimetre. For a bead or a small screw, measure twice and take the same reading both times before you cut, drill, or order a part, because a single millimetre of error can matter at this scale.

Common Uses: Where You See This Size

Four millimetres is a common size for beads, small screws, and the thickness of thin boards and cases. It is also a frequent drill size for pilot holes, where a 4 mm hole is drilled before a larger screw is put in. Because it is just under one sixth of an inch, it is a size that is almost always given in millimetres, not inches, in workshops and craft guides.

Common Mistakes to Avoid

The most common mistake is mixing up millimetres and centimetres, which makes a measurement ten times too large or too small. Another is reading the wrong side of a ruler, since many rulers show inches on one edge and centimetres and millimetres on the other. People also guess instead of measuring: 4 mm looks almost nothing on a screen, but in screws, jewellery, and electronics it is a real, working size.

Putting It in Context

It helps to place this size on a ruler you already know. A metric ruler is marked in centimetres, with ten small millimetre marks between each number, so this measurement is only a few of those small marks. That is why it is easy to miscount by one mark, and why measuring twice is worth the few seconds it takes. Always start from zero, keep the ruler still, and read the number twice. Once the reading is right, the conversion is simple arithmetic, and the result will fit the part you are making or ordering.

Frequently Asked Questions

How many inches is 4 mm?

4 mm is 0.157 inches, because 4 ÷ 25.4 = 0.157.

How many centimetres is 4 mm?

4 mm is 0.4 cm, because there are 10 mm in every centimetre.

Is 4 mm bigger than a centimetre?

No, if the value is below 10. Ten millimetres make one centimetre, so anything under 10 mm is a fraction of a centimetre.

How do I measure 4 mm without special tools?

Use a metric ruler and count the small marks between the numbered centimetre marks. Each small mark is one millimetre, so count carefully from zero.

Why do small parts use millimetres instead of inches?

Because millimetres give whole numbers for small sizes. A part that is 0.197 inches is much easier to describe, make, and check as 5 mm.