Use the OpenSpending Tax Calculator API (at the bottom of the page):

http://openspending.org/api

Example output from http://openspending.org/api/mytax?income=20000:

{
 alcohol_tax: 217.26799007444168
 explanation: [ "This household income falls between national average
income decile 3 (which has average gross household income of 17204.00,
and pays 1939.00 in direct tax, 1125.00 in VAT, 1295.00 in smoking
taxes, 317.00 in alcohol-related taxes, 182.00 in car-related taxes,
and 373.00 in other indirect taxes), and decile 4 (which has average
gross household income of 22040.00, and pays 3108.00 in direct tax,
1262.00 in VAT, 1562.00 in smoking taxes, 320.00 in alcohol-related
taxes, 243.00 in car-related taxes, and 505.00 in other indirect
taxes).",
 "Therefore, a household with an income of 20000.00 pays
approximately 2614.87 in direct tax and 3638.90 in total indirect
tax."
 ]
 tax: 6253.771712158808
 tobacco_tax: 318.7344913151365
 car_related_tax: 449.317617866005
 total_direct_tax: 2614.8734491315136
 vat: 1449.3697270471464
 total_indirect_tax: 3638.898263027295
}

More details in their documentation: http://wiki.openspending.org/Tax_Calculator