Excel VBA Rnd Function to generate random number

This Excel tutorial explains how to use Excel VBA Rnd Function to generate random number and create custom Function for random decimal number.

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Excel VBA Rnd Function to generate random number

Rnd Function is for use in Access, Access VBA, Excel VBA, it is used to generate random number larger than 0 and smaller than 1 (1 and 0 exclusive).

Excel worksheet use RAND Function and RANDBETWEEN Functions, which have quite different behavior.

Syntax of Excel VBA Rnd Function

Rnd[(number)]

Number	Rnd generates
Not supplied or any positive number	Generate a new random number
0	Return the last generated number
Any negative number	Generate a new random number and use it every time

Example of Excel VBA Rnd Function – generate random number

It is not useful to use Excel VBA Rnd Function alone, because we ultimately want to generate a random number between two specific numbers, but not a random number between 0 and 1.

Use the below formula to generate a number bounded by lowerbound and upperbound (both inclusive).

Int ((upperbound - lowerbound + 1) * Rnd + lowerbound)

Formula	Explanation
Int((10-1+1)*Rnd()+1)	Generate random number between 1 and 10
Int((200-100+1)*Rnd()+100)	Generate random number between 100 and 200

VBA custom Function – generate random decimal number

If you think the formula is too complicated, you can use the below custom Function. The purpose of this custom Function is to simulate Excel worksheet Function RandBetween, which allows you to simply use upperbound and lowerbound as Function arguments.

This custom Function offers an optional parameter that allows you to generate random integer or decimal number.

VBA code of custom Function – generate random decimal number

Public Function wRandomNumber(lowerbound, upperbound, Optional rndType = 1) As Double
    Randomize
    rndVariable = Rnd
    If rndType = 1 Then
        wRandomNumber = Int((upperbound - lowerbound + 1) * rndVariable + lowerbound)
    ElseIf rndType = 2 Then
        If (upperbound - lowerbound + 1) * rndVariable + lowerbound <= upperbound Then
            wRandomNumber = (upperbound - lowerbound + 1) * rndVariable + lowerbound
        Else
            Do While (upperbound - lowerbound + 1) * rndVariable + lowerbound > upperbound
                rndVariable = Rnd
                If (upperbound - lowerbound + 1) * rndVariable + lowerbound <= upperbound Then
                    wRandomNumber = (upperbound - lowerbound + 1) * rndVariable + lowerbound
                End If
            Loop
        End If
    End If
End Function

Syntax of custom Function – generate random decimal number

wRandomNumber(lowerbound, upperbound, [rndType])

lowerbound	The minimum random number
upperbound	The maximum random number
rndType	Optional, indicate whether the random number is decimal or integer value description 1 (default) Return a random integer 2 Return a random decimal number

Example of custom Function – generate random decimal number

Formula	Explanation
=wRandomNumber(1,100)	Return a random integer between 1 and 100 (both inclusive)
=wRandomNumber(100,200,2)	Return a random decimal number between 100 and 200 (both inclusive)

Outbound References

https://support.office.com/en-us/article/Rnd-Function-503CD2E4-3949-413F-980A-ED8FB35C1D80