Calculating With PPM (parts per million) in Two Easy Steps

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Reading a hay analysis or puzzling over the ingredients in feed or supplements can be a chore, yet when considering particular elements—selenium, for example—some minor math can make a major difference to your horse’s health.

Feed tags and hay analysis often list ingredients as “ppm” or parts per million. Does this confuse you? You aren’t alone. The best way to think of ppm is as milligrams per kilogram (mg/kg) of feed (since a mg is a one millionth “part” of a kg). Therefore, when using mg/kg, you have to make sure you’re dealing with kg of feed (instead of lbs) in order to make your calculations.

Consider this example: Let’s say your hay contains 0.2 ppm (mg/kg) of selenium. How many mg of selenium does 10 lbs of hay contain?

First, you need to convert the lbs to kg. Since there are 0.454 kg in one lb, make the conversion by multiplying lbs by 0.454. So, 10 lbs multiplied by 0.454 equals 4.54 kg (10 x 0.454 = 4.54).
Now you’re ready to calculate mg of selenium. Multiply 4.54 kg of hay by 0.2 ppm (or mg/kg) (4.54 x 0.2 = 0.91). That gives you 0.91 mg of selenium in your 10 lbs of hay.

Formulas to remember:

  • Convert lbs to kg: lbs x 0.454 = kg
  • Calculate to find mg: kg X ppm (or mg/kg) = mg

Juliet M. Getty, PhD, is an internationally respected, independent equine nutritionist who believes that optimizing horse health comes from understanding how the horse’s physiology and instincts determine the correct feeding and nutrition practices. She is available for private consultations and speaking engagements. Getty provides a world of useful information for the horseperson at Sign up for her informative, free monthly newsletter, Forage for Thought; browse her library of reference articles; search her nutrition forum; and purchase recordings of her educational teleseminars. And for the growing community of horse owners and managers who allow their horses free choice forage feeding, Getty has set up a special forum as a place for support, celebrations, congratulations and idea sharing. Share your experiences at Reach Getty directly at






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