How to Calculate a License Plate Digit Sum in 3 Steps

The shortest formula you need to remember

A plate's digit sum equals the value of every Thai letter in the letter series, plus the leading digit before the series if there is one, plus every digit of the plate number. That is all. There is no multiplication, no weighting, and the order of the characters does not matter.

What confuses people is not the formula. It is the letter values, which you have to look up in a table, and the decision about what to count, which differs between schools. These two things are the source of the popular question of why one website says the sum is 30 while another says 27.

Digit values and letter values

Digits are the easiest. The digits 1 to 9 are worth their own value, and 0 is worth zero. That means plates with several zeros naturally have low sums. The number 1010, for example, has a digit sum of only 2.

Thai consonants are grouped by value. Value 1: ก ด ถ ท ภ. Value 2: ข บ ป ง ช. Value 3: ต ฑ ฒ ฆ. Value 4: ค ธ ร ญ ษ. Value 5: ฉ ณ น ม ห ฮ. Value 6: จ ล ว อ. Value 7: ซ ศ ส. Value 8: ย ผ ฝ พ ฟ. Value 9: ฐ ฏ ฬ.

This is the most common table in Thai numerology texts, but to be frank, some letters are grouped differently from school to school, especially rarely used ones such as ฌ ฎ ฐ. If a plate you are interested in contains one of these letters, always check the breakdown shown by the calculator. Do not memorise a value and argue about it as if it were settled.

Working it out step by step on real plates

Example one: a two-letter plate with the leading digit 1, the letter series ก and ก, and the number 999. Step one, find the letter values: ก is in value group 1, so the two letters give 1 plus 1, which is 2. Step two, add the leading digit 1, for a total of 3. Step three, add the plate number: 9 plus 9 plus 9 equals 27. The final digit sum is 30.

Example two: a plate with no leading digit, the letter series ข and ข, and the number 45. ข is in value group 2, so the two letters give 4. Add the digits 4 and 5, another 9, and the digit sum is 13.

Example three looks at the digits alone to give a quick picture. The number 8888 has a digit sum of 32, while 9999 has a digit sum of 36. If you also count the letter series, the plate's actual sum will be higher by the value of its letters.

A tip for adding several digits: pair up the ones that make ten first, such as 9 and 1 or 8 and 2, then add the rest. This greatly reduces mistakes when you are comparing many plates in a row.

Three choices that make sums disagree

The first choice is whether to count the letter series. Most schools do, but some look only at the digits because they regard the letter series as something assigned by the government rather than chosen by the owner. This choice alone can change the sum by several points.

The second choice is whether to count the leading digit before the series. This digit exists because the letter series were used up in some areas, so a leading digit was added. Some texts treat it as part of the plate and count it; others treat it as merely an administrative code.

The third choice concerns special word series (หมวดคำพิเศษ) that spell a readable word: whether to add up the value of every letter in the word, or to assign a fixed value to the whole word. Both approaches are used in practice. This site counts everything by default, as that is the most common approach, and lets you switch the options yourself when you want to compare.

Second-level sums and reducing to a single digit

Some texts like to keep adding the sum until only one digit remains. A sum of 30, for example, reduces to 3, and a sum of 45 reduces to 9. This is called the base number or second-level sum, and it is used to read the plate's overall character in broad strokes.

This site treats the first-level sum as the main figure for meanings and scoring, and the second-level sum as supplementary information only, because reducing it throws away a lot of information. The sums 24 and 42 both reduce to 6, yet the texts give them different meanings.

How to check your own answer

The quickest way is to enter the plate into this site's calculator and look at the breakdown. It shows, character by character, what value each letter received, whether the leading digit was counted, and how the digits add up to the final sum. If the result differs from your own addition, you will see straight away where the difference lies.

If you compare with another website and get a different result, work through these questions in order before deciding who is wrong. One, do they count the letter series? Two, do they count the leading digit? Three, does their letter value table match yours? Almost every mismatch can be explained by these three points, not by someone adding incorrectly.