A pretty basic question, don`t you think? Like everything associated with the soapberry project, we started from zero in knowing the answer to this important question. We compared the berries by weight with what we know about their Asian cousins. Our soapberries tend to be smaller, so you need to use 6 or 7 rather than 4-5 but by weight it comes out about the same.
How many loads those 6 or 7 berries will do depends on a lot of factors, like the temperature and hardness of the water. Basically, when they turn grey and mushy, you have used up the soapiness and can compost what is left. That is usually at least 5 loads, but could be more. I have seen some berries advertised as being good for 10 loads. Maybe, sometimes.
When we started making powder, the question came up again. We tried different ways of figuring it out. We weighed the whole berries and compared that with the weight of the powder. We experimented with different quantities of powder. It is very concentrated, so using too much can leave your clothes feeling stiff. The powder is also a blend of the different varieties of soapberries that we buy as wild harvest, so no doubt there is some slight difference from batch to batch, but not much.
What we found is that one teaspoon is sufficient for 5 to 7 gallons of water. That means a small load (12-15 pieces) will use one teaspoon and a large load might need 2 or 3.
It was a different adventure to find a spoon that we could include with the powder to help a customer know how much to use. But, that is a different story. Hope that helps answer the question. Happy laundry day!