Conversion from Millimeter to Fractional and Milesimal Inch – Calculation and Chart’s Interactive Program

Convert millimeter - gractional inch or milesimal inch - interactive conversor

This MM-to-inch Interactive Simulator (and vice versa) is designed to make it easier to understand, to allow you to evaluate your results and anticipate them. Its use is very simple, scroll the page to the converter, drag and drop the red line horizontally, aligning the values in the scales (millimeter ‘mm’ and inch ‘in’). Just below them, the values in millimeter, fractional inch (fractionated) and corresponding thousands of millimeters will appear and the calculation formula. After you interact, scroll the page to understand the mechanics of the calculation.

Before going on on this page, I recommend you  interact with the Fraction topic on page: Using the fractional inch understanding and measuring , come back here later, it’s worth it!

Transformation of millimeter to inch it has no mistery


Inch (polegada / polegadas in portuguese). Its abbreviation is in, sometimes we see “(double quotes) or in. It is a unit of linear measure of the English System, Imperial System, among others. It was based on the dimensions of the human body and the fractional (fractional) numerical system.

Some people maintain that its controversial origin is associated with the extent of the human thumb, another argues that its origin comes from the Latin word uncia that means “one twelfth” so, inch is an uncia , or 1/12, the foot.

Later, beyond the fractional system, it was then divided by the decimal system, so there are two systems of division of the inch that we must know: fractional (eg 1/128 “) and thousandsimal (eg 0.025in) . The thousandsimal system is also known as thousandth of an inch (thousandth) the notation of the previous example is 25,000 (twenty-five thousandths of an inch).

Nowadays, and by definition, the yard was standardized as 0.9144m. A yard is aproximatelly to 3 foot -table 1, a inch, that is 1/12 of a foot, and it’s aproximatelly to 2.54cm or 25.4mm (unit used in the metrology, mechanical engeneering and in the metal mechanic ofices). 0.9144/(3*12)

Thus, 1in = 25.4 mm. So we have 25.4 mm/in (25.4 millimeters by “for each” inch)-Figure 1 (note that in Interactive conversor (simulador) there are two parallel horizontal lines of the same length. They are equivalent to 1.000in and 25.4mm – 1 “) (bounded by 0 and 1).

figure 1

Conversion of inch to millimeter

Inch to millimeter Transformation? There’s nothing easier!

Fractional inch to milesimal inch

The fractional inch is that represented as a fraction, for example: 1/2 “(1/2 inch); 1/4 “(one quarter inch); 7/16 “(seven sixteen-eighths of an inch); 3.5 / 128 “(three inches and five hundred twenty eight avos).

To convert to milesimal inch form simply divide the fraction and add the integer part. See:

1/2″ -> one divided by two -> 0.5″

observe that in the english system the point used as separator of the integer part (zero) of the decimal part (five) of number

1/4″ -> one divided by four -> 0.25″

7/16″ -> seven divided by sixteen -> 0.4375″

3.5/128″ five divided by one hundred and twenty eight and added three -> 3.0391″

Milesimal inch to millimeter

Just multiply the measure in milesimal inch by 25.4mm/in. See:

0.5in * 25.4mm/in = 12.7mm

0.25in * 25.4mm/in = 6.35mm

0.4375in * 25.4mm/in = 11.112mm

3.0391in * 25.4mm/in = 77.192mm

Fractional inch to millimeter

Just transform the fractional inch in milesimal inch and multiplicate the meausre in milesimal inch by 25.4mm/in. See this example: 1.3/8″ (or 1.3/8in or also a three-eighths inch). We will divide three by eight, to transform the fraction in mils of inch, will add the integer part and multiply by 25.4 mm/in.

3/8 = 0.375in

three hundred seventy-five mil

observe that in the english system the point is used as separator of the integer decimal part of the number

0.375in + 1in = 1.375in

1.375in x 25.4mm/in =34.92mm

other example: we will transform 5.1/8in (five and one-eighth inch) in millimeter: we will divide one by eight (1/8 = 0.125), and add five and multiplicate by 25.4mm/in-figure 2.

figure 2

Conversion of millimeter to fractional inch or milesimal

It’s a little bit more difficult, but it’s also easy. Just divide the measure in millimeter by 25.4 mm/in. See this example: 70 mm. We will divide 70mm by 25.4mm/in obtainning 2.756in (or two thousand seven hundred fifty-six mils) already is the milesimal inch.

To convert to fractional inch, subtract the integer part and keep it: 2.756in – 2in=0.756in. Multiply this value by 128/128 (that is the lower fraction of inch used in the mechanical engineering, besides being equivalent to one -that os the neutral element of multiplication) and obtain as result 96.77/128. Round the numerator 97/128in and add the whole number that was reserved: 2.97/128in. With no intention of alienating anyone, the fact that the numerator is an odd number indicates that it is necessary to simplify the fraction. If the numerator is even simplify the fraction – Figure 3.

70mm / 25,4mm/in = 2.756in (milesimal form of inch)

2.756in – 2in = 0.756in

0.756in x 128/128 = 97/128in

2in + 97/128in = 2.97/128in


88.9mm / 25.4mm/in = 3.500in

3.500in – 3in = 0.500in

0.500in x 128/128 = 64/128 (64 is an even number)

64/128 = 32/64 = 16/32 = 8/16 = 4/8 = 2/4 = 1/2

3in + 1/2in = 3.1/2in

figure 3

See! There is nothing more than this.

Now, solve the lists of exercises and check the results:

exercises of conversion of millimeters to fractional and milesimal inch

exercises of conversion of fractional inch to millimeters and milesimal inch

exercises of conversion of milesimal inch to millimeters and fractional inch

Practise a little bit with the Interactive Conversor and check the results at:
Program to calculate linear dimensions in millimeter, fractional inch and milesimal

Any problems? Take a look at the page:
Use of measures in fractional inch – understanding and applying

table 1

Millimeter – a thousandth of a meter

Meter , from greek metron = measure. Its abreviation is m (minuscule). That is a unit of linear measure of International System. Was based on the size of the earth and the decimal number system. Its thousandth part is millimeter abreviation mm (minuscules), widely used in mechanical engineering.

Names and symbols to prefixes of Meter

Virtual Interactive Conversor: conversion of linear measures between millimeter, fractional inch and milesimal

How to use the interactive conversor millimeter to fractional inch or milesimal

The utilization is very simple, drag and drop the vertical line in the horizontal, aligning the values in the scales (millimeter ‘mm’ and inch ‘in’). Just below them, the corresponding values in millimeters, fractional inch and milesimal inch and the formula to the calculation will appear.

This Interactive Conversor of millimeter to inch (and vice-versa) was designed to facilitate comprehension, allow you to evaluate your results and anticipate.

Eduardo Stefanelli

Engenheiro por profissão, professor por vocação

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