2017/2018 Tax on Bonus
Threshold Bonus Rate Less
Below or equal 100000 0.00%
More than 100000 4.00% 4000
More than 150000 8.00% 10000
More than 200000 12.00% 18000
More than 250000 16.00% 28000
More than 300000 20.00% 40000
More than 350000 24.00% 54000
Source
http://www.ird.gov.lk/en/Downloads/Lists/Tools/Attachments/6/PAYE%20Tax%20Calculator_1819_V1_E.xlsx