So here is the formula ...
Here is sample code to calculate gunning fog index written in VB.Net
//some variables
Dim inputText As String = txtInput.Text
Dim ltr As String
Dim wordCount As Integer
Dim sentenceCount As Integer
Dim syllableCount As Integer
Dim complexCount As Integer
Dim i As Integer
Dim inWord As Boolean = False
Dim vowels As String = "aeiouy"
TextBox1.Text = ""
' First make the input string case insensitive.
inputText = inputText.ToLower()
For i = 0 To inputText.Length() - 1
ltr = inputText.Substring(i, 1).ToLower
If ltr = "'" Then
' do nothing in case of apostrophe
ElseIf (ltr >= "a" And ltr <= "z") Then ' check for alphabetic input
If Not inWord Then ' new word
wordCount += 1 ' so add 1 to count
End If
inWord = True
Else ' could be space, full stop, comma etc
inWord = False ' no longer in word
' check if end of sentence
If (ltr = "." Or ltr = "?" Or ltr = "!" Or ltr = ":" Or ltr = ";") Then
sentenceCount += 1 ' add 1 to count
End If
End If
Next
Dim input As String = inputText.Length() - 1
Dim ch As Char
Dim inVowel As Boolean = False
' syllable count is zero if no input
If (input <>Then
syllableCount = 0
complexCount = 0
End If
' e at end of word doesn't count as a vowel
ch = inputText.Substring(inputText.Length - 1)
If (ch = "e") Then
input -= 1
End If
'For Each ch In inputText
' If (vowels.IndexOf(ch) >= 0) Then
' If Not inVowel Then
' syllableCount += 1
' End If
' inVowel = True
' End If
'Next
Dim word As String
Dim pieces As String()
'Replace all what is not needed here
inputText = inputText.Replace(".", " ")
inputText = inputText.Replace(":", " ")
inputText = inputText.Replace(";", " ")
inputText = inputText.Replace("?", " ")
inputText = inputText.Replace("!", " ")
inputText = inputText.Replace(",", "")
inputText = inputText.Replace("'", " ")
inputText = inputText.Replace(" ", " ")
'Now make pieces
pieces = inputText.Split(" ")
syllableCount = 0
complexCount = 0
For index As Integer = 0 To pieces.Length - 1
word = pieces(index)
word = word.ToLower().Trim()
Dim pattern As String = "[aeiouy]+"
'Handle vowels
Dim count As Integer = Regex.Matches(word, pattern).Count
If word.EndsWith("e") Then
count -= 1
End If
'some special cases
If word.EndsWith("cial") Or word.EndsWith("tia") Or word.EndsWith("cius") Or word.EndsWith("cious") Or word.EndsWith("giu") Or word.EndsWith("ion") Or word.EndsWith("iou") Then 'Or word.EndsWith("sia") Or word.EndsWith("ely") Then
count = 1
End If
syllableCount += count
If count <>Then
count = 1
End If
If count >= 3 Then
complexCount += 1
End If
Next
'Now display all results on GUI
TextBox1.Text += "Words: " & wordCount.ToString() & Environment.NewLine
TextBox1.Text += "Sentences: " & sentenceCount.ToString() & Environment.NewLine
TextBox1.Text += "Syllable: " & syllableCount.ToString() & Environment.NewLine
TextBox1.Text += "Complex Words: " & complexCount.ToString() & Environment.NewLine
TextBox1.Text += "Fog index: " & (0.4 * ((wordCount / sentenceCount) + (100 * (complexCount / wordCount)))).ToString() & Environment.NewLine
TextBox1.Text += "Flesch-Kincaid Reading Age: " & (206.876 - (1.015 * (wordCount / sentenceCount)) - (84.6 * (syllableCount / wordCount))).ToString() & Environment.NewLine
