How to calculate the npk ratio

Written by thomas bourdin
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How to calculate the npk ratio
Different plants may require different NPK ratios. (Jupiterimages/BananaStock/Getty Images)

The NPK ratio is an important quantity for many garden fertilisers. This number gives the ratio of nitrogen, phosphorus and potassium -- hence the letters; N, P and K are the chemical symbols for nitrogen, phosphorus and potassium, respectively -- in a given fertiliser. Determining the NPK ratio is critical when adding fertiliser to a plot of land, as plants may have different requirements for how much of each element they require for optimal growth. Mixing fertilisers with different NPK ratios can also cause confusion. Finding the NPK ratio of mixed fertilisers can be accomplished in a few steps.

Skill level:
Moderately Easy

Things you need

  • Scale

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  1. 1

    Determine the quantity of each type of fertiliser you would like to use and weigh each sample. For example, assume you want to use 85.1gr. of fertiliser A and 170gr. of fertiliser B. Place each sample on a scale and add or remove fertiliser until the scale reads the desired weight.

  2. 2

    Determine the percentage of each fertiliser used in the final mixture. For example, in the example above, there is twice as much of fertiliser B as fertiliser A. Therefore, the total mixture contains 2/3, or 66.7 per cent, of fertiliser B and 1/3, or 33.3 per cent, of fertiliser A.

  3. 3

    Multiply the NPK ratio of each fertiliser by the per cent contained in the final mixture. For example, if fertiliser A contains a ratio of 1:1:2, multiplying this by the per cent of fertiliser A in the final mixture -- 66.7 -- gives 0.667:0.667:1.333. If fertiliser B contains a ratio of 3:3:1, multiplying this by the per cent of the final mixture -- 33.3 -- gives 1:1:0.33.

  4. 4

    Add the proportion of each amount of nitrogen, phosphorous or potassium within each fertiliser in the final mixture together. Concluding the example, adding the mixture amounts 0.67:0.67:1.33 and 1:1:0.33 together gives 1.67:1.67:1.67. Therefore, there is a ratio of 1.67 for each of nitrogen, phosphorus and potassium in the final mixture.

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