Will I be fined for overstaying my visa exempt stamp in Thailand by 1 day?

When you fly out on an overstay of under 24 hours you don't get fined for it, and you only get a stamp in your passport in thai saying overstay under 24 hours fine waived.

You'll be okay..

You're not the first person to have miscounted the days in Thailand.

Your arrival Day counts as DAY #1

So you will be in Thailand for 31 days.

When you pass thru Thai Immigration on departure, you will be directed to another Thai Immigration Officer desk. Where they will stamp you out of Thailand and waive the usual 500 Thai baht fee as you are Only on OVERSTAY one day.

If your departure flight is after midnight..i.e 2 am ...in the early morning hours, then if you clear Thai Immigration before midnight you are not on OVERSTAY.