From Innerpedia - Encyclopedia of Torah's Inner Dimension - Chassidut and Kabbalah
The gematria (גמטריה) of a letter is its numerical value.
Gematria assigns a numerical value to each of the letters of the Hebrew alphabet. The complete assignment is called a mapping, a term borrowed from mathematics and information theory.
The most common mapping is called normative value. The numerical values of the Hebrew letters in this mapping is:
א = 1 ב = 2 ג = 3 ד = 4 ה = 5 ו = 6 ז = 7 ח = 8 ט =9 י = 10 כ = 20 ל = 30 מ = 40 נ = 50 ס = 60 ע = 70 פ = 80 צ = 90 ק = 100 ר = 200 ש = 300 ת = 400
Normally, the numerical value of the final letters (ך, ם, ן, ף, ץ) is the same as their equivalent simple letters (כ, מ, נ, פ, צ) however, sometimes they are mapped differently as:
ך = 500 ם = 600 ן = 700 ף = 800 ץ = 900
The letters may be used with an apostrophe, indicating thousands. E.g. א' = 1000.