A basic standalone UK National Insurance calculator. See http://www.hmrc.gov.uk/rates/nic.htm for more details on rates.
Usage options:
var NationalInsuranceCalculator = require('uk-national-insurance');
var annualNationalInsuranceRepayment = NationalInsuranceCalculator(40000, '2013_14');
Resulting in annualNationalInsuranceRepayment:
4119.84