How to calculate NPK

Hello, I was given a recipe for homemade fertilizer that has worked great. But it is comprised of various types of meals. I have always been curious and wasn’t able to find anything that felt reliable in a basic google search for how to calculate the npk of the fertilizer. Does anyone know the formula for calculating the new npk on homemade ferts?

Good question. NPK ratings are usually listed on the container the product comes in. I have the same question when making JBP fertilizer balls with cottonseed meal, blood and bone meal, garden lime and fish emulsion in different proportions. Their containers list specific amounts of NPK. Is their rating just the addition of the amounts of each the separate items and does that matter! Also: Why is a fall root fertilizer with cottonseed and bone meal listed as 0-10-10 when both items used separately contain nitrogen and some amount of potassium and phosphorus?

A simple search turns up a number of pages that offer basic equations as well as a calculator:

I didn’t try it so will be curious to hear if it’s valuable in this case.

As for Bruce’s question about different listings for seemingly identical fertilizers, I don’t know - would have to see the specifics. Might also be interesting to contact the fertilizer companies directly.