This fertilizer calculator (using American measurements and the metric system) should show the resulting NPK when you mix two fertilizers together, before any water is added. (I don't guarantee it's bug-free. ChatGPT wrote most of the code using the logic that I supplied.) Be warned that you don't want to mix just any two fertilizers; for instance, if you mix calcium nitrate and potassium sulfate, you might lose water solubility. See the source code for the copyright/licensing.