you can get perlite at wal-mart, lowes, home depot.....20-25% meaning if you are using lets' say a 1 gallon pot. Mix 25% perlite to 75% potting soil. So you'd have a quart of perlite and three quarts of potting soil (4quarts=one gallon). So, two gallons you would need 2 quarts of perlite. Three gallons = three quarts of perlite and so on. Remember though that when transplanting you will already have previous perlite in also so you may not need to add that much as above. That's just an example of how much is 25%