Excel Rounddown function

The Excel ROUNDDOWN function returns a number rounded down to a given number of places. Unlike standard rounding, where only numbers less than 5 are rounded down, ROUNDDOWN rounds all numbers down.

Purpose: 
Round down to given number of digits

Return value: 
A rounded number.

Syntax: 
=ROUNDDOWN (number, num_digits)

Examples:

The ROUNDDOWN function rounds numbers down. Unlike standard rounding, where only numbers less than 5 are rounded down, ROUNDDOWN rounds all numbers down.
=ROUNDDOWN(3.9,0) // returns 3

ROUNDDOWN takes two arguments, number and num_digits. Number is the number to be rounded, and num_digits is the place at which number should be rounded. When num_digits is greater than zero, the ROUNDDOWN function rounds on the right side of the decimal point.
When num_digits is less or equal to zero, the ROUNDDOWN function rounds on the left side of the decimal point. Use zero (0) for num_digits to round to the nearest integer. The table below summarizes this behavior:

Round to right of decimal:
To round down values to the right of the decimal point, use a positive number for digits:
 =ROUNDDOWN(A1,1) // Round down to 1 decimal place
=ROUNDDOWN(A1,2) // Round down to 2 decimal places
=ROUNDDOWN(A1,3) // Round down to 3 decimal places
=ROUNDDOWN(A1,4) // Round down to 4 decimal places  

Round to left of decimal:
To round down values to the left of the decimal point, use zero or a negative number for digits:
=ROUNDDOWN(A1,0) // Round down to nearest 1
=ROUNDDOWN(A1,-1) // Round down to nearest 10
=ROUNDDOWN(A1,-2) // Round down to nearest 100
=ROUNDDOWN(A1,-3) // Round down to nearest 1000
=ROUNDDOWN(A1,-4) // Round down to nearest 10000  

Nesting inside ROUNDDOWN:
Other operations and functions can be nested inside ROUNDOWN.
For example, to round down the result of A1 divided by B1, you can use a formula like this: =ROUNDDOWN(A1/B1,0) // round down result to nearest integer

Compare between Round, Roundup and Rounddown functions: