How do I convert a fractional inch like 3 1/4 to millimetres?

Enter '3 1/4' (with a space) or '3-1/4' (with a hyphen) into the converter. The tool will interpret it as 3 and 1/4 inches, convert to millimetres (82.55 mm), and also show the reduced fraction.

Can I enter a decimal like 3.25 instead of a fraction?

Yes, you can type '3.25' and the converter will treat it as 3.25 inches, convert to millimetres (82.55 mm), and also display the equivalent reduced fraction (3 1/4).

What does 'reduced fraction' mean?

A reduced fraction is one where the numerator and denominator have no common factors other than 1. For example, 2/8 is reduced to 1/4. The converter automatically simplifies fractions for you.

单位转换

即时、私密且免费

分数英寸转毫米换算器.

2026年8月4日

将分数英寸转换为毫米，反之亦然。输入3 1/4、3-1/4、1/4或3.25即可得到精确的、已约简的分数。

此计算器尚未完全翻译——部分文本以英文显示。

设备端计算无需注册

输入数值

输入时结果会更新。

02 / 答案

转换方向

英寸可尝试输入 3 1/4、3-1/4、1/4、5/4 或 3.25。

可尝试输入 3 1/4、3-1/4、1/4、5/4 或 3.25。

分数精度

毫米

82.55mm

分数英寸

3 1/4"

明细

小数英寸3.2500

四舍五入至1/16

使用方法

1英寸精确定义为25.4毫米，因此这里的每次换算在您所选的舍入精度内都是精确的。

支持的输入格式：“3 1/4”、“3-1/4”、“1/4”、假分数如“5/4”，以及普通小数如“3.25”。

结果始终为最简形式——您得到的是1/2而非8/16，四舍五入为整英寸的数值会显示为1，而非16/16。

框架施工和普通木工建议选择1/16，橱柜及细木工建议选择1/32，机械加工或精细木工建议选择1/64。

Cite this calculator

Canonical URL: https://mathify.one/zh/conversion/fractional-inches

Updated 2026年8月4日

Cite as: Mathify. (2026). 分数英寸转毫米换算器. https://mathify.one/zh/conversion/fractional-inches

Use Cases

DIY and woodworking projects

When working with imperial measurements on tools or plans, quickly convert fractional inches to millimetres for metric rulers or hardware.

Example: Convert 5/16 inch to mm to drill a hole with a metric bit.

Engineering and design

Switch between imperial and metric dimensions in CAD or technical drawings without manual calculation errors.

Example: Convert 2 3/8 inches to mm for a mechanical part specification.

Frequently Asked Questions

How do I convert a fractional inch like 3 1/4 to millimetres?: Enter '3 1/4' (with a space) or '3-1/4' (with a hyphen) into the converter. The tool will interpret it as 3 and 1/4 inches, convert to millimetres (82.55 mm), and also show the reduced fraction.
Can I enter a decimal like 3.25 instead of a fraction?: Yes, you can type '3.25' and the converter will treat it as 3.25 inches, convert to millimetres (82.55 mm), and also display the equivalent reduced fraction (3 1/4).
What does 'reduced fraction' mean?: A reduced fraction is one where the numerator and denominator have no common factors other than 1. For example, 2/8 is reduced to 1/4. The converter automatically simplifies fractions for you.

Tips & Common Mistakes

Tips

Use a space or hyphen between the whole number and fraction (e.g., 3 1/4 or 3-1/4) for mixed numbers.
For fractions only, just type like 1/4 or 3/16.
Decimals are accepted too – type 3.25 and get the reduced fraction.
Double-check your input format to avoid misinterpretation (e.g., 3-1/4 is not 3 minus 1/4).

Common Mistakes to Avoid

Entering '3 1/4' without a space or hyphen (e.g., '31/4') – the tool may read it as 31/4 inches.
Confusing decimal points with fraction slashes – use '.' for decimals and '/' for fractions.
Assuming the result is rounded – the converter gives exact millimetre values based on 1 inch = 25.4 mm.

Last updated: August 13, 2026