How to convert w/w% weight concentration to ppm

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).


ppm = (10ab)/18.9271

In the example above,

a=0.8, b=2, so

ppm = (16)/18.9271=0.845


Tropica fertilizer composition

Tropica has 2 brands of liquid fertilizer — Specialised Fertilizer and Premium Fertilizer.

The Specialized fertilizer has N+P (nitrogen and phosphates), while the Premium does not. Unfortunately, not much information is available on the exact ingredients, composition, or percentage of each type of nutrient. For example, it is unclear how much potassium the fertilizers have.

I managed to find the exact composition on the UKAPS forum:

Tropica Fertilizer Composition and Ingredients

Nutrient (w/w%) Premium Fertilizer Specialised Fertilizer
Nitrogen 0 1.34
Phosphorus 0 0.1
Potassium 0.8 1.03
Magnesium 0.39 0.39
Sulfur 0.91 0.91
Zinc 0.002 0.002
Molybdenum 0.002 0.002
Boron 0.004 0.004
Iron 0.069 0.069
Copper 0.006 0.006
Fluoride <0.001 <0.001
Chlorine 0.438 0.5
Manganese 0.039 0.039
The Declaration Certificate showing composition of Tropica fertilizers.


Tropica Specialised Fertiliser – 300 ml

Tropica Plant Growth Premium Fertiliser,300 ml

Tropica Fertilizer ppm

The composition of ingredients given by Tropica is in w/w% which is weight concentration of a solution. Usually, we want to know the concentration in ppm, or parts per million (equivalently, mg/L).

For that, we can use the Rotala Butterfly calculator. The below information is based on 1 pump (2ml) for a 5 Gallon aquarium.

Your addition of 2 ml (equivalent to 1/4 tsp + 1/8 tsp + 1/32 tsp ) Tropica Plant Growth Premium to your 5gal aquarium adds:

Element ppm/degree
K 0.845
Mg 0.412
S 0.962
Fe 0.073
Mn 0.041
B 0.004
Cu 0.006
Mo 0.002
Zn 0.002
Cl 0.463

It is quite a “light” dose. Considering that the recommended target concentration of K is 10-30 ppm (from Tom Barr). Singapore water only has on average 1 ppm for K (potassium). Hence, the once a week dose of Tropica fertilizer Premium is definitely not enough to hit 10-30 ppm of K.

Seachem Root Tabs Review

Just added 1 Seachem Flourish Tab next to my Alternanthera Reineckii Mini (AR Mini). Some “fizzing” or bubbling is observed. Not sure if that is due to the root tab, or maybe I disturbed some gas pockets in the substrate.

I am still continuing the growing of AR Mini in low tech tank (see Alternanthera Reineckii Mini Low Tech).

Seachem Flourish Tabs Growth Supplement – Aquatic Plant Stimulant 10 ct

My dog Charlie curious about the Seachem Root Tab.

Seachem Root Tab Review

The below review video is rather convincing proof that Seachem Flourish tabs are one of the best root tabs in the market, despite having low macro nutrients (nitrogen and phosphate). The purpose of having low macro nutrients is for preventing algae.