In this post, we describe how to convert w/w% (weight concentration of a solution) to ppm (parts per million, or mg/l), with applications to liquid fertilizers.

This method is an approximation for **dilute solutions**.

We will focus on converting 0.8 w/w% to ppm, with 2ml of fertilizer in a 5 gallon container. This is based on Tropica fertilizer Potassium concentration. We will then verify it with the Rotala Butterfly calculator (answer is 0.845 ppm).

1) 0.8 w/w% = 8g per 1000ml

**Here we have made the assumption that 1000ml of solution weighs 1000mg, which is valid for dilute solutions.**

2) This is equivalent to: 0.016g per 2ml (in the fertilizer).

3) Now after pouring the fertilizer into the 5 gallon container, the concentration is 16mg per 5 gal = 16mg per 18.9271 liters.

**Here we have made the approximation that the amount of fertilizer (2ml) is negligible compared to the volume of the container (5 gal).**

4) This is equivalent to 0.845 mg per l or 0.845 ppm as calculated by the Rotala Butterfly calculator!

## Rotala Butterfly Calculator Formula

Let a be the w/w% concentration.

Let b be the amount of fertilizer to use (in ml).

Then,

ppm = (10ab)/18.9271

In the example above,

a=0.8, b=2, so

ppm = (16)/18.9271=0.845