If you are an indoor cultivator you may be wondering how to properly add multiple NPK ratios together to calculate the overall ratio of your feedings. When mixing multiple nutrients with varying NPK ratios, you must combine their values to calculate the new ratio. By doing so, you’ll be able to match your feeding’s ratio with your preferred ratio for each stage of plant growth.
The calculation can be described in four steps:
 Add volumes (ml) together to get overall volume
 Divide individual volumes by overall volume
 Multiply result to N, P, and K of each corresponding ratio
 Add Ns, Ps, and Ks
For example, let’s say you are combining two nutrients, 5 ml of 123 and 10 ml of 321. First, add their volumes:
5 ml + 10 ml = 15 ml 
Now that you have the combined volume, divide each volume by the overall volume (These example results have been rounded):
5 ml / 15 ml = 0.3  10 ml / 15 ml = 0.6 
Next, multiply these results to their corresponding nutrient’s NPK ratio:
Nutrient A (1 – 2 – 3) 0.3 [N] 1 x 0.3 = 0.3 [P] 2 x 0.3 = 0 .6 [K] 3 x 0.3 = 0.9 
Nutrient B (3 – 2 – 1) 0.6 [N] 3 x 0.6 = 1.8 [P] 2 x 0.6 = 1.2 [K] 1 x 0.6 = 0.6 
Now, add the Ns together, the Ps together, and the Ks together
[N] 0.3 + 1.8 = 2.1 
[P] 0.6 + 1.2 = 1.6 
[K] 0.9 + 0.6 = 1.5 
The combined N – P – K result is 2.1 – 1.6 – 1.5.
By correctly combining your ratios, you’ll be able to accurately determine what mixture satisfies the plant’s desired ratio. If you’d like to learn more about N – P – K ratios and how they apply to the different stages of cannabis growth, enroll in an Oaksterdam University course today!
Here is the same N – P – K combination example in a more condensed format which you can copy/paste for your own notes!
Nutrient A – 5 ml (1 – 2 – 3)  Nutrient B – 10 ml (3 – 2 – 1) 
5 mL + 10 mL = 15 ml  
5 mL / 15 mL = 0.3 [N] 1 x 0.3 = 0.3 [P] 2 x 0.3 = 0 .6 [K] 3 x 0.3 = 0.9 
10 mL / 15 mL = 0.6 [N] 3 x 0.6 = 1.8 [P] 2 x 0.6 = 1.2 [K] 1 x 0.6 = 0.6 
[N] 0.3 + 1.8 = 2.1 [P] 0.6 + 1.2 = 1.6 [K] 0.9 + 0.6 = 1.5 

Overall NPK – 2.1, 1.8, 1.5 