MM to Inches Converter

What a millimeters to inches converter is for

Millimeters (mm) and inches (in) both measure length, but they come from different measurement systems. Millimeters belong to the metric system, where base-10 steps make it easy to move between mm, cm, and meters. Inches belong to the imperial and US customary systems, where measurements are commonly expressed in inches, feet, and yards — and often marked in fractions like 1/16.

A mm to inches converter helps you switch between these systems quickly and accurately. It’s useful for everything from small hardware and machining measurements to product dimensions, woodworking layouts, and reading specs that were written in a different unit than your tools.

The exact standard: 1 inch equals 25.4 mm

The key reason mm↔in conversions are reliable is that the relationship is defined, not estimated: 1 inch is exactly 25.4 millimeters. That means the conversion itself is exact. If you see slightly different results across websites or apps, it’s almost always because they show a different number of decimal places or apply a different rounding rule.

How to convert mm to inches

The main formula is simple:

Example: 10 mm ÷ 25.4 ≈ 0.3937007874 inches. For many everyday uses, you might round that to 0.394 in or 0.39 in depending on your needs.

How many inches is 1 mm?

Since 1 inch = 25.4 mm, one millimeter is the reciprocal:

A quick mental shortcut some people use is “about 0.04 inches per mm,” but for accurate work — especially small parts — it’s better to use the exact division and round at the end.

Why the “best” rounding depends on the job

Two decimals is not always enough

Inches are larger than millimeters, so small mm changes can look tiny in inches. If you convert a 1 mm difference and round too early, you may lose the detail you actually care about. For example, 1 mm is about 0.03937 inches — rounding that to 0.04 inches can be fine for a rough estimate, but it’s not ideal when you’re matching a drill bit, a bearing, or a tolerance.

When to use more decimals

• General product dimensions: 2–3 decimals is usually enough.
• Hardware and workshop checks: 3–4 decimals helps when comparing close sizes.
• Machining and engineering: use whatever precision matches the specification and tolerance.

Rounding direction matters in real builds

Sometimes you don’t want “nearest.” You might want to round down so a part definitely fits, or round up so you leave enough clearance. That’s why this converter includes round down and round up options, not just standard rounding.

Decimal inches vs fractional inches

A lot of confusion comes from the difference between decimal inches and fractional inches. Many technical drawings and digital calipers show inches as decimals (like 1.375 in). Many tape measures and woodworking plans show inches in fractions (like 1 3/8 in).

Decimal inches are great for calculators and spreadsheets. Fractional inches are great for people using a tape measure. Neither is “more correct”; they’re just different formats for the same length.

How fractional inches are calculated

To convert a decimal inch value into a fraction like the nearest 1/16, you choose a denominator (16), multiply, round, then simplify:

1. Convert: inches = mm ÷ 25.4
2. Scale: scaled = inches × denominator
3. Round scaled to an integer (nearest, up, or down)
4. Convert back: fraction = integer / denominator
5. Simplify the fraction (for example, 8/16 becomes 1/2)

This tool does all of that automatically and returns a clean result like 1 1/16 in or 3/8 in, depending on the size.

Which fraction denominator should you pick?

Nearest 1/16 is the common “tape measure” choice

Most standard tape measures show 1/16 inch marks, so rounding to the nearest 1/16 usually matches what you can physically measure quickly.

Use 1/32 or 1/64 when you need finer marks

If you’re doing fine fitting or precision layout, a finer denominator can preserve more detail. Just remember: a fraction is only as useful as your ability to measure it. If your tape doesn’t show 1/64, the extra precision may not help in practice.

Use 1/2 or 1/4 for quick estimates

For rough planning, larger fractions are often more readable and “good enough.” If you’re sketching a cut list or checking approximate fit, nearest 1/4 can be faster to communicate.

Converting mm to feet and inches

Feet and inches are common for human-scale measurements and construction layouts. Converting mm to feet and inches is a two-step process:

1. Convert mm to inches: mm ÷ 25.4
2. Convert inches to feet: divide by 12

The Feet + Inches tab shows:

• Feet (whole number)
• Remaining inches (rounded as whole inches or a fraction like 1/16)
• Total inches (useful when you’re working with drawings or parts lists)

Common places you’ll see mm-to-inch conversions

Bolts, screws, and drill bits

Hardware sizes often appear in mm (especially metric bolts) while your drill bits may be labeled in fractional inches. Converting accurately helps you choose a compatible bit size or compare close alternatives.

3D printing and fabrication

Many design tools use metric dimensions by default, but some reference parts or enclosures are sold in inch-based dimensions. Conversions make it easier to mix components from different standards.

Automotive and tools

Tool kits may be metric or imperial. When you’re dealing with sockets, bearings, spacers, and fasteners, knowing the inch equivalent of a mm size helps you pick the right tool without trial-and-error.

Product specs and shopping

Online listings may show thicknesses and clearances in millimeters. If your mental model is inches, a converter helps you understand the real size immediately.

Helpful reference values

Here are a few quick conversions that come up often. (Exact conversion uses mm ÷ 25.4, with results shown rounded.)

• 1 mm ≈ 0.03937 in
• 5 mm ≈ 0.19685 in
• 10 mm ≈ 0.39370 in
• 12 mm ≈ 0.47244 in
• 25 mm ≈ 0.98425 in
• 50 mm ≈ 1.96850 in
• 100 mm ≈ 3.93701 in

MM, CM, and inches in one picture

It helps to remember a couple of exact relationships:

• 10 mm = 1 cm
• 1 inch = 25.4 mm
• 1 inch = 2.54 cm

If you already have centimeters, you can convert cm to inches by dividing by 2.54. If you have millimeters, dividing by 25.4 gets you inches directly.

Tips for accurate measuring and converting

Match the output to your measuring tool

If you’re holding a tape measure that marks 1/16, use fractional output to the nearest 1/16. If you’re reading a digital caliper that reports decimals, use decimal inches with a few extra places.

Round at the end, not in the middle

Conversions are exact, but rounding too early can introduce avoidable error. Convert first, then apply rounding once — especially when multiple steps are involved (like converting and then splitting into feet + inches).

Use “round up” or “round down” deliberately

In physical work, rounding direction can matter. For clearance, rounding up can prevent tight fits. For inserts or inner dimensions, rounding down can help avoid oversized parts. Pick a rule and stay consistent.

When you should not use fractions

Fractions are great for tape measures and informal plans, but they’re not always ideal for tight tolerances or technical documentation. If you need a specific tolerance (for example, ±0.01 mm), keep measurements in mm or use decimal inches with enough precision to represent the tolerance accurately.

Quick recap

• Exact rule: inches = mm ÷ 25.4
• 1 mm: ≈ 0.03937 in
• Fractional inches: round to 1/16, 1/32, or 1/64 if you’re using a tape measure
• Feet + inches: convert to inches, then split by 12