What PPM represents
Parts per million is a dimensionless ratio: it tells you how many parts of one substance are present per million parts of the total mixture. Think of it as a fraction multiplied by one million instead of by 100 (which would give you percent).
PPM is preferred over percent for trace quantities because it avoids tiny decimal values. “50 ppm” is much easier to work with than “0.005%” — both represent the same proportion, but ppm is clearer at that scale.
The key requirement is that both part and whole must be expressed in the same unit. You can’t divide milligrams by liters and call it ppm unless density is exactly 1.
How to Calculate PPM (step by step)
Step One: Ensure matching units
Make sure both values are in the same unit (grams and grams, mg and mg, mL and mL, etc.).
Step Two: Divide part by whole and multiply by 1,000,000
ppm = (part ÷ whole) × 1,000,000
For 5 units in 1,000,000 total: (5 ÷ 1,000,000) × 1,000,000 = 5 ppm.
Practical example
If a 500 g water sample contains 0.025 g of dissolved salt: (0.025 ÷ 500) × 1,000,000 = 50 ppm salt.