I made this recipe because I use 5 ounce bars of soap (Dr. Bronner’s, since you ask) and I had to do the math every time, since the directions I followed used 3 oz.
So here ya go.
5 oz soap
3/4 cup borax
3/4 cup washing soda
3 gal water
Chop soap and put in a big bucket, about 5 gal. Add 1 gal hot water. Let soap dissolve (I usually just let it sit for a day or two). In sep. container add borax and washing soda to 1 gal hot water. (You can do this over the stove if you like.) Stir til it dissolves. Add to soap bucket. Add 1 gal water. Use 1/2 cup per load.
This makes about a hundred loads. Not bad!
ETA: I feel compelled to add that you must use a lid that the kids can’t get off 1. because it would make a terrible mess and 2. because if you use a big bucket, a toddler/baby can drown in it. So, you know, be careful. I keep mine in the garage next to the washer.