Note; the denominator is always a one with as many 0's as there are digits behind the decimal point.

To get the fraction's numerator, multiply the decimal part of the number by the denominator to get a whole number. This whole number is the numerator of the fraction.

Example conversions

0.875

First, get the denominator:
the third digit, the 5, is in the third spot of the decimal part of the number. The denominator consists of a one plus three zero's

1000 is the denominator

Get the numerator by multiplying the decimal amount of the number by 1000

0.875 * 1000 = 875

875 is the numerator of this fraction.

0.875 = 875/1000 simplified to 7/8

12.3685

First, get the denominator:
the fourth digit, the 5, is in the fourth spot of the decimal part of the number. The denominator consists of a one plus four zero's

10000 is the denominator

Get the numerator by multiplying the decimal amount of the number by 10000

0.3685 * 10000 = 3685

3685 is the numerator of this fraction.

12.3685 = 12-3685/10000 simplified to 12-737/2000

Check out: How to convert a fraction to a usable fraction!