Could create your own Function to convert it

Create Function
F to C

Math in Fields

Thanks Alex

Excel has the built-in feature of converting decimals to fractions and vice versa, but sadly Access doesn't have it, so you have to create a custom function yourself as Alex suggested.  To code this for every conceivable fraction would be a daunting task, so I propose a simpler solution: create a "lookup table" that contains your most often used fractions:

FracFormat     DecValue
0              0
1/2            0.5
1/3            0.333333
2/3            0.666667
1/4            0.25
2/4            0.5
3/4            0.75
.
.
1/8            0.125
2/8            0.25
3/8            0.375
.
.
14/16           0.875
15/16           0.9375
Etc.

Then you create a custom function to look up the table (using OpenRecordset) to find the fraction of a decimal, or the decimal of a fraction.

You also need to take the user entry's precision into account.  Whether the user enters 0.333 or 0.333333, your function should return 1/3.  But if the user enters 0.3, you need to return 3/10 instead.  So you need to control this precision.

And if you need more fractions, you can always add to the lookup table.

Give this a go (see screenshot). Modify for your needs Table Name, Field Name(s))

Note: I used an unbound field for the conversion then wrote the converted value to the table.

I added a Fraction field to my table for illustration purposes.

How about converting decimals to fractions?  The original poster did say he wanted conversion both ways.  That could be a lot more complicated, and may even be consultant-level work.  Excel has this built-in, as I said.  Maybe automation between Access and Excel could be considered.

These two functions should do what you want.

Details

Function FractionToDecimal(fraction As String) As Double
    Dim parts() As String
    Dim wholeNumber As Double
    Dim numerator As Double
    Dim denominator As Double
    Dim decimalValue As Double
    
    ' Split the fraction by space to separate whole number and fraction part
    parts = Split(fraction, " ")
    
    If UBound(parts) = 1 Then
        ' There is a whole number part
        wholeNumber = Val(parts(0))
        fraction = parts(1)
    Else
        wholeNumber = 0
        fraction = parts(0)
    End If
    
    ' Split the fraction part by "/" to separate numerator and denominator
    parts = Split(fraction, "/")
    
    numerator = Val(parts(0))
    denominator = Val(parts(1))
    
    ' Calculate the decimal value
    decimalValue = wholeNumber + (numerator / denominator)
    
    FractionToDecimal = decimalValue
End Function

Function DecimalToFraction(decimalValue As Double) As String
    Dim wholeNumber As Long
    Dim fractionPart As Double
    Dim numerator As Long
    Dim denominator As Long
    Dim gcdValue As Long
    Dim fraction As String

    ' Separate the whole number part
    wholeNumber = Fix(decimalValue)

    ' Calculate the fraction part
    fractionPart = decimalValue - wholeNumber

    ' If the fraction part is zero, just return the whole number
    If fractionPart = 0 Then
        DecimalToFraction = CStr(wholeNumber)
        Exit Function
    End If

    ' Set an initial precision level
    Dim precision As Double
    precision = 0.00001
    
    ' Find numerator and denominator by approximation
    Dim tempNumerator As Double
    Dim bestNumerator As Long
    Dim bestDenominator As Long
    Dim minError As Double
    minError = 1 ' Initialize with a large error

    For denominator = 1 To 10000 ' Limit the maximum denominator for practical purposes
        tempNumerator = fractionPart * denominator
        tempNumerator = Round(tempNumerator)
        If Abs(fractionPart - (tempNumerator / denominator)) < minError Then
            minError = Abs(fractionPart - (tempNumerator / denominator))
            bestNumerator = tempNumerator
            bestDenominator = denominator
        End If
    Next denominator

    numerator = bestNumerator
    denominator = bestDenominator

    ' Combine the whole number and the fraction part
    If wholeNumber <> 0 Then
        fraction = wholeNumber & " " & numerator & "/" & denominator
    Else
        fraction = numerator & "/" & denominator
    End If

    DecimalToFraction = fraction
End Function

' Helper function to find the Greatest Common Divisor (GCD) using the Euclidean algorithm
Function GCD(a As Long, b As Long) As Long
    Do While b <> 0
        Dim temp As Long
        temp = b
        b = a Mod b
        a = temp
    Loop
    GCD = a
End Function

Copy

Another approach may be to store the whole number, numerator, and denominator in the table.  Use a query to pull in table data and calculate the decimal expression for calculations.  Then you can design the form to show the fractions or decimal as needed.  Without the look-up table option or some VBA, you would be limited to entering data in fractional form.

Here's a couple examples of using the conversion functions I provided above.

Kevin Works Great! All the other Fraction to Decimal VBA I tried would not work because the would not return just the whole number without the fraction. I tried Gary James's to but it would not return just a whole number.