It takes very little of the powder to dechlorinate water. 35mg per gallon to be exact. You will need one of those small jewelry scales to accurately measure it out. You can get one pretty cheap on eBay. If you measure out other salts to treat your water, you'll need it anyway. Check out BrunWater at some point to learn more about it. One final piece of advice. Potassium metabisulphite powder off gases especially if it has a chance to absorb moisture. The resulting gas will knock you back and burn your nostrils. I'd recommend opening your container of powder outside to air it out for a minute or two before working with it.