The above calculator is based on information from Inland Revenue Authority of Singapore (IRAS).
http://www.iras.gov.sg/irasHome/uploadedFiles/Quick_Links/Copy%20of%20Tax_Cal_Final_170211_IRAS.xls
Even though the maximum individual tax rate is 20%, most of us will be charged at a (much) lower rate due to the progressive tax structure. For example, if you are a resident and your chargeable income is $100k, your tax rate is only 5.65%.