# N-P-K ratio

> The N-P-K ratio is the three numbers on a fertiliser label giving percent nitrogen, phosphate (P2O5) and potash (K2O) by weight, not elemental P and K.

- Canonical: https://genetixbay.com/knowledge/glossary/npk-ratio.html
- Also written: NPK, N-P-K, NPK ratio, fertiliser ratio, fertilizer grade
- Glossary category: Nutrients · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

The N-P-K ratio, or fertiliser grade, is the three numbers printed on a fertiliser: percent total nitrogen (N), percent available [phosphate](https://genetixbay.com/knowledge/glossary/phosphate.md) expressed as P2O5, and percent soluble potash expressed as K2O, all by weight. A 10-5-20 product contains 10% N, 5% P2O5 and 20% K2O.

## In practice

Because P and K are given as oxides, the numbers overstate the elements: 5% P2O5 is only about 2.2% P, and 20% K2O about 16.6% K. Comparing bottles by their N-P-K says little without knowing the dose and dilution. A liquid concentrate at 2-1-3 used at 5 mL/L can deliver more than a 20-20-20 powder at 0.2 g/L. The numbers also leave out calcium, magnesium, [sulfur](https://genetixbay.com/knowledge/glossary/sulfur.md) and the [micronutrients](https://genetixbay.com/knowledge/glossary/micronutrient.md).

## Units and formula

P = P2O5 × 0.436. K = K2O × 0.830. [mg/L of an element](https://genetixbay.com/knowledge/glossary/elemental-ppm.md) = (element % ÷ 100) × dose in mg/L.

## Don't confuse with

- **Elemental analysis**: P and K as elements, used in hydro recipes and research.
- **Ratio vs grade**: 10-10-10 and 20-20-20 have the same ratio but different strength.

## Related terms

[Guaranteed analysis](https://genetixbay.com/knowledge/glossary/guaranteed-analysis.md), [Macronutrient](https://genetixbay.com/knowledge/glossary/macronutrient.md), [Nitrogen](https://genetixbay.com/knowledge/glossary/nitrogen.md), [Phosphorus](https://genetixbay.com/knowledge/glossary/phosphorus.md), [Potassium](https://genetixbay.com/knowledge/glossary/potassium.md)

## Read more

- [Building a cannabis feed by stage: ppm targets per element and how to hit them](https://genetixbay.com/knowledge/nutrients/feed-by-stage.md)
- [Hydroponic salts explained: what each fertiliser salt supplies](https://genetixbay.com/knowledge/nutrients/hydroponic-salts.md)

## Sources

1. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6