1 a (atto) is equal to 0.000000000000000000000000000000000000000001 Y (yotta). To convert from atto to yotta, multiply by 0.0000000000000000000000000000000000000000009999999999999999 or divide by 1,000,000,000,000,000,000,000,000,000,000,000,000,000,000.