Units of Measurement





Parts per billion

Equivalent to µg/L

1 molecule out of 1 billion molecules

Put that into perspective: There are just over 1 billion people in India. If everyone wore a green sweater, and one person wore a red sweater, the person in the red sweater would be representative of 1 PPB.



Equivalent to PPB

Put that in perspective: There are 1000 µg in a mg. Or 1,000,000th of an M&M™!


Parts per million

Equivalent to mg/L

1 molecule out of 1 million molecules

Put that into perspective: Just over 1 million people live in San Diego.



Equivalent to PPM

Put that in perspective: There are 1000 mg in a gram. A regular M&M™ weighs about 1 gram.  1 mg is 1,000th of an M&M™!

Conversion Formula’s

ppb→ ppm

Divide by 1000


100 PPB arsenic = 0.1 PPM arsenic

ppm → ppb

Multiply by 1000


1 PPM arsenic = 1000 PPB arsenic

ppb → µg/L

 ppb are equivalent to µg/L. If arsenic levels are 10ppb, you can easily convert to 10µg/L

mg/L → µg/L

There are 1000 mg to a gram

1000 µg to a mg

Multiply by 1000


10 mg/L x (1000µg/1 mg) = 10,000 µg/L


µg/L  → mg/L

If there are 1000 µg to a mg, then divide by 1000


500 µg/L x (1mg/1000µg) = 0.5 mg/L

Or  written differently,

500µg/L ÷ 1000 = 0.5 mg/L

Real-world example: Susan suspected her well-water was contaminated with arsenic. Using the test she purchased at the Harney County Fair, she sampled her water and received her results. The level of arsenic in her well water is 35 PPB. Alternatively, this could be reported as 35 µg/L. Susan knows this is not safe for drinking or cooking.

Susan’s neighbor had heard that arsenic levels below 0.01 mg/L was a safe level for drinking, cooking and other domestic uses. Susan decides to convert the level of arsenic in her water (35 µg/L) into mg/L to compare between her well water and safe well water. There are two ways to do this.

 1. Convert PPB into PPM, which = mg/L

35 PPB ÷ 1000 = 0.035 PPB = 0.035 mg/L


2. Convert ug/L into mg/L

35 µg/L ÷ 1000 = 0.035 mg/L


0.035 mg/L ÷ 0.01 mg/L (safe levels) = 3.5

The level of arsenic in Susan’s well is 3.5 times higher than the allowable level of arsenic in well water.

To be able to use her water for cooking and drinking, Susan either needs to use only bottled water, or install a treatment system.