From The Nation newspaper:

Getting the numbers straight

So, you’ve learnt how to count in Thai.

After the obvious: sawat dii – the all-purpose Thai “hello” and “goodbye”; sabai dii mai? – “how are you?”; and khop khun (“thank you”) tackling the numbers is the next step most foreign visitors and residents take.

Tones aside, they’re not so difficult to master. And once you’ve made it to 20, the rest is logical, with only a few exceptions:

sip-et – “11”, not sip-neung

yi-sip – “20”, not sorng-sip.

The “11” exception repeats itself all the way up to 101, as in:

sam-sip-et – “31”, not sam-sip-neung

ha-sip-et – “51”, not ha-sip-neung.

roi-et – “101”.

However, once you get to 1,000, the et disappears:

neung-phan-neung – “1,001”, not phan-et

All well and good, but there will still be moments when all the effort you’ve put into learning the mostly logical Thai counting system will be thrown into confusion. The first is a common contraction of the number “20”, in which yi-sip becomes simply yip. Thus: yip-sorng – “22”.

The other source of confusion is the tendency of Thai speakers to play fast and loose with the number “1” when it indicates amounts of “100”, “1,000” or more. Thus, when a Thai speaker says to you, roi-neung, they’re actually saying “100”, not “101”, and the same goes for “1,000”: phan-neung – “1,000”. If you do want to say “1,001”, it’s neung-phan-neung, and if you want to say “101”, as we’ve already noted, it’s roi-et.