Replace spaces with "%20" in C

// Implement the method of replacing all the spaces in the string with substring "%20"
char* replace_spaces(const char *s)
{
    const char SPACE = ' ';
    const char REPLACEMENT[] = "%20";
    unsigned int numOfSpaces = 0;
    for(int i = 0; i < strlen(s); i++)
    {
        if(s[i] == SPACE)
        {
            numOfSpaces++;
        }
    }
    // we have found number of spaces in the string
    // it helps us to define the exact size of modified string
    char* modifiedString = new char[strlen(s) - numOfSpaces + numOfSpaces*strlen(REPLACEMENT) + 1];
    modifiedString[strlen(s) - numOfSpaces + numOfSpaces*strlen(REPLACEMENT)] = '\0';
    for(int sourceStrIdx = 0, modifiedStrIdx = 0; sourceStrIdx < strlen(s); sourceStrIdx++)
    {
        if(s[sourceStrIdx] != SPACE)
        {
            modifiedString[modifiedStrIdx] = s[sourceStrIdx];
            modifiedStrIdx++;
        }
        else
        {
            for(int k = 0; k < strlen(REPLACEMENT); k++)
            {
                modifiedString[modifiedStrIdx] = REPLACEMENT[k];
                modifiedStrIdx++;
            }
        }
    }
    return modifiedString;
}