Short answer: the three numbers are the percentages by weight of nitrogen (N), phosphate and potash in the product, always in that order. A label reading 8-15-36 contains 8% nitrogen, 15% phosphate and 36% potash.
Phosphate and potash are not pure P and K
By long-standing convention, labels in the United States report phosphorus as phosphate (P2O5) and potassium as potash (K2O). The actual elemental amounts are lower:
- Phosphorus = phosphate figure × 0.44
- Potassium = potash figure × 0.83
So 15% phosphate is about 6.5% phosphorus, and 36% potash is about 30% potassium. This only matters if you compare products in detail or calculate concentrations yourself.
The rest of the label matters more
For hydroponics, the lines below N-P-K are what separate a complete nutrient from an incomplete one. Look for:
- Calcium (Ca) and magnesium (Mg)
- Sulfur (S)
- Iron (Fe), usually in a chelated form so it stays available
- Manganese, zinc, copper, boron and molybdenum
Two-part products
A two-part nutrient has a separate label for each part, and neither part is complete on its own. One part usually carries the calcium and nitrogen; the other carries the phosphorus, potassium, magnesium and sulfur. The complete formula is the two parts used together at the stated rate.
Higher numbers are not better
A product with bigger numbers is simply more concentrated, so you use less of it. What matters is the balance between elements and mixing it to the right strength, which you check with an EC meter.
Related guide: what a Kratky nutrient solution contains.
