Originally, exactly as much as a volume of water equal to the cube of the hundredth part of the metre, at the temperature of melting ice. I don't find that terribly helpful. Not many did. That's why, in 1889, the mass of the kilogram was redefined as being equal to the IPK.
Boiling water is also an effective poison ivy killer. If the area where you will be killing poison ivy has no other plants you wish to keep, pour boiling water over the poison ivy plant. Boiling water will kill any part of a plant that it comes in contact with, so be careful using this around desirable plants.
Oatmeal is one of the most popular home remedies for poison ivy. An oatmeal bath soothes the itching related to the rash, but if you do not have time to soak in oatmeal, there are other options. Make some plain oatmeal and make sure it is not hot and that it sits for a few minutes so that it gets really thick.