Useful conversions are:

• ppm = mg/litre            i.e.  5 ppm = 5 mg / litre
• mg / litre  x 3.785 = mg / gall (US)           i.e  5 mg / litre = 18.9 mg / gall (US)
• mg/ litre x 4.546 = mg / gall (UK)            i.e  5 mg / litre = 22.7 mg / gall (UK)
• To convert imperial gallons to US gallons multiply by 1.2

Other useful figures: 

• 1 ounce = 28.35 grams

A 1% solution = 

• 10 ml per litre
• 10 gram per litre
• 38 gram per gall  (US) 
• 45 gram per gall (UK)