HM Revenue and Customs (HMRC) publish estimates of the exchequer impact of various illustrative tax changes in the publication series “Direct effects of illustrative tax changes”. This includes illustrative effects for the following National Insurance changes:
Change Class 1 employee main rate by 1 percent point
Change Class 1 employee additional rate by 1 percent point
Change Class 1 employer rate by 1 percentage point
Change Class 2 rate by £1 per week
Change Class 4 main rate by 1 percentage point
Change Class 4 additional rate by 1 percentage point.
The latest bulletin, published in April 2019, is linked below: https://www.gov.uk/government/statistics/direct-effects-of-illustrative-tax-changes