Computes the expected decoded byte size of the Base64 encoded data. Reference: https://tools.ietf.org/html/rfc4648

Base64Util.java

import static java.util.Objects.requireNonNull;

public class Base64Util {
    private static final double BASE64_SIZE_RATIO = 4.0d / 3;

    private Base64Util() {
    }

    /**
     * Computes the expected decoded byte size of the base64 encoded data.
     * <p>
     * NOTE: This code assumes it is being passed a valid base64 encoded string.
     *
     * @param base64Text valid base64 encoded text
     * @return expected decoded byte size
     */
    public static int computeDecodedByteSize(String base64Text) {
        return (int) (trueLength(requireNonNull(base64Text)) / BASE64_SIZE_RATIO);
    }

    private static int trueLength(String base64Text) {
        return base64Text.length() - lineFeedsAndPaddingCount(base64Text);
    }

    private static int lineFeedsAndPaddingCount(String base64Text) {
        return (int) base64Text.chars().filter(c -> c == '\n' || c == '\r' || c == '=').count();
    }
}