How to Calculate the NPK Ratio

Determine the quantity of each type of fertilizer you would like to use and weigh each sample. For example, assume you want to use 3 oz. of fertilizer A and 6 oz. of fertilizer B. Place each sample on a scale and add or remove fertilizer until the scale reads the desired weight.

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

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

Add the proportion of each amount of nitrogen, phosphorous or potassium within each fertilizer 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.