I flew into Bkk and was stamped the 30 day visa which expires 18th sept. If I go to the immigration on the 18th sept does the 7 day ext start from the 18th or from the 19th sept? I need to know which day I should buy a flight out of LOS ie the 24th or 25th

many thanks

It used to be day of application but have seen many reports that they now count from end of current stay. But I would buy ticket on 24th myself or just cross a border and return for 15 days at about the same price from most locations in Thailand and not worry about it (and stay a bit longer if you want).

Extensions are calculated from the end date of your current stay as mentioned on the stamp in your passport.

In your case from the 18.

If you come after this date you have to pay the overstay fine ( 500 baht a day ) and pay for the 7 day extension which then will be calculated from the day of the extension application.

For example if you come at 20 Sept you pay 1000 Baht for the overstay and 1900 Baht for the extension which will be calculated from 20 Sept.