P to P₂O₅ and K to K₂O Converter
Convert elemental phosphorus and potassium to P₂O₅ and K₂O (and back), plus Ca to CaO and Mg to MgO.
Enter your details
Soil reports sometimes give P and K; fertilizer bags and recommendations always use P2O5 and K2O. Convert between them here. The number changes, but the unit you enter (kg/ha, ppm, %) stays the same.
About the P to P2O5 and K to K2O Converter
Why two ways of writing the same nutrient? Historically, fertilizer chemists reported phosphorus and potassium as their oxides, P2O5 and K2O, and that convention stuck in fertilizer laws and recommendations worldwide – including India's Fertilizer Control Order. Soil and plant scientists, on the other hand, often report the actual elements, P and K.
Neither form exists as such in the bag; they're accounting conventions. But mixing them up is a big error: 30 kg of P is not 30 kg of P2O5 – it's about 69 kg.
Conversion factors
| From | To | Multiply by |
|---|---|---|
| P | P2O5 | 2.2914 |
| P2O5 | P | 0.4364 |
| K | K2O | 1.2046 |
| K2O | K | 0.8301 |
| Ca | CaO | 1.3992 |
| Mg | MgO | 1.6582 |
The factors come from atomic weights. For example, P2O5 weighs 141.94 and contains two phosphorus atoms (2 × 30.97 = 61.95), so 141.94 ÷ 61.95 = 2.2914.
How to use the P to P2O5 and K to K2O Converter
- 1 Choose the nutrient.
- 2 Choose the direction – element to oxide (P → P2O5) or oxide to element.
- 3 Enter the amount in any unit and press Convert. The answer is in the same unit.
The formula we use
P₂O₅ = P × 2.2914 | P = P₂O₅ × 0.4364
K₂O = K × 1.2046 | K = K₂O × 0.8301Worked example
Your soil test shows 27 kg/ha of available P. As P2O5: 27 × 2.2914 = 61.9 kg/ha.
A recommendation of 60 kg K2O/ha is 60 × 0.8301 = 49.8 kg/ha of elemental K.
Things to keep in mind
- Always check whether a number on a report or in a research paper is elemental or oxide before comparing it with a recommendation.
Frequently asked questions
How do I convert P to P2O5?
Multiply P by 2.2914. For example, 10 kg P = 22.9 kg P2O5.
How do I convert K2O to K?
Multiply K2O by 0.8301 (or divide by 1.2046).
Why are fertilizers labelled as P2O5 and K2O?
It is a long-standing international convention. Indian fertilizer grades such as DAP 18-46-0 use it, so recommendations do too.
Guides that use this calculator
Last updated . Spotted a mistake or need a unit we don't support? Tell us.