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The amount payable for Additional State Pension or Second State Pension (S2P) is related to your earnings and amount paid within the limits for National Insurance Contributions NICs.
The amount payable is based on NICs paid for earnings between £155 and £770 per week. The amount of years you have "contracted out" will be factored in as well which means that your pension may be lower.
When you make your claim for a State Retirement Pension, any S2P due will also be calculated.
Further Information
Visit NI Direct, for more information on the additional state pension.