The amount of bitcoin mined in each block is also the percentage of the total supply that will be mined during that era.

50 Btc

10,500,000 Btc

50%

25 Btc

5,250,000 Btc

25%

12.5 Btc

2,625,000 Btc

12.5%

6.25 Btc

1,312,500 Btc

6.25%

3.125 Btc

656,250 Btc

3.125%