In the USA
1,000,000 ppm / 17.1181 US grains/ppm = 58,417.7 grains
58,417.7 grains / 15.43236 grains/gram = 3,785.4 grams
3,785.4 grams / 453.6 grams/pound = 8.345 pounds
8.345 pounds = the weight of 1 USA gallon of water
In the UK
1,000,000 ppm / 14.2538 UK grains/ppm = 70,156.73 grains...