1) get the HS code from supplier.
2) If you cannot get though supplier, google it. but keep in mind that there are many HS Codes marching to your article according to interpretation. so it might be wrong.
3) Search HS Code in custom tariff guide.
4) Get Rate
5) Calculate as follows.
https://www.customs.gov.lk/wp-content/uploads/2021/07/15.formula_2011.pdf
v= Value.
v+15% = CIF Value *1.15
Other Abbreviations stated with formula
if it comes though DHL or similar, based on documents, they will give you estimate.
if you clearing though agency, they can give you estimated duty calculation.
View attachment 151662