1,000,000,000,000,000,000,000,000,000,000,000,000,000,000 a
1 Y (yotta) is equal to 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000 a (atto). To convert from yotta to atto, multiply by 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000 or divide by 0.0000000000000000000000000000000000000000009999999999999999.