Each letter takes the sum of the values from aleph to itself: aleph 1, bet 3, gimel 6, dalet 10 … kaf 75, lamed 105, mem 145 … tav 1495. אמת = 1 + 145 + 1495 = 1641.
Meaning
The preceding value makes every letter carry the whole alphabet behind it, so the series is the series of triangular numbers and tav contains all twenty-two letters. The tradition uses it to show that the alphabet is cumulative — that a late letter is heavy with everything before it — and the same triangular arithmetic is what gives the 231 gates their count.