Predict the byte size of a base64 encoded byte[]

I'm trying to predict the size string representation of a base64 encoded byte array.

I've come up with the formula below, however the length of the actual encodedString is 4 larger than the base64EncodedSize.

The whole idea here is to calculate/predict what the encoded string size would be for a given byte[]. I would prefer not to convert the byte[] to a base 64 string just to determine it's length.

FileInfo pdfFile = new FileInfo(@"C:\TEMP\1.pdf");

long originalSizeInBytes = pdfFile.Length;

String encodedString = Convert.ToBase64String(File.ReadAllBytes(pdfFile.FullName));

long base64EncodedSize = (originalSizeInBytes / 3) * 4;

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- Results -
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  originalSizeInBytes                           913663  long
  base64EncodedSize                             1218216 long
  encodedString.Length                          1218220 int
  base64EncodedSize                             1218216 long
  encodedString.Length - base64EncodedSize      4       long