Fertiliser labels report phosphorus, potassium, sulfur, calcium, magnesium and several micronutrients either as the free element or as an oxide. Convert either way, instantly.
Australian fertiliser labels state phosphorus and potassium as the elements themselves, P and K. Many imported products, older references and international suppliers state them as oxides, P₂O₅ and K₂O, a leftover convention from how fertiliser analysis was originally standardised. A bag labelled "20-20-20" overseas is not 20% phosphorus and 20% potassium. Converted to the Australian elemental convention it's closer to 20-8.7-16.6, since P₂O₅ and K₂O both contain oxygen that isn't the nutrient itself.
Mixing the two conventions up (reading an oxide number as if it were elemental, or the reverse) overstates or understates what's actually in the bag. This tool converts either direction, for the major nutrients and micronutrients fertiliser labels report.
A label states P₂O₅ 20%, K₂O 10%. Enter those two values with "Oxide → Elemental" selected: P converts to 8.73% (20 × 0.4364), K converts to 8.30% (10 × 0.8301). Those elemental percentages are what a soil test or a nutrient budget in elemental terms is comparing against, not the 20% and 10% printed on the bag.
Each factor is (element mass in the oxide's formula) ÷ (oxide molar mass), from IUPAC standard atomic weights, to 4 significant figures. K₂O → K is the one exception: it uses the published IFA/AAPFCO industry-standard factor (0.8301, from conventionally-rounded atomic weights) rather than a full-precision recompute, which would give 0.8302, a difference of one part in ten thousand.
| Oxide | Element | Factor |
|---|---|---|
| P₂O₅ | P | 0.4364 |
| K₂O | K | 0.8301 |
| SO₃ | S | 0.4005 |
| SO₄ | S | 0.3338 |
| SO₂ | S | 0.5005 |
| CaO | Ca | 0.7147 |
| MgO | Mg | 0.6030 |
| Fe₂O₃ | Fe | 0.6994 |
| FeO | Fe | 0.7773 |
| Fe₃O₄ | Fe | 0.7236 |
| MnO | Mn | 0.7745 |
| MnO₂ | Mn | 0.6319 |
| Mn₂O₃ | Mn | 0.6960 |
| ZnO | Zn | 0.8034 |
| CuO | Cu | 0.7989 |
| Cu₂O | Cu | 0.8882 |
| B₂O₃ | B | 0.3106 |
| MoO₃ | Mo | 0.6666 |
A wrong conversion doesn't just look wrong on paper. If it feeds into a rate calculation, reading an oxide number as elemental overstates the nutrient content, which can lead to under-application when you scale a dose against it, since you've assumed there's more of the nutrient in the product than there actually is. Reading an elemental number as oxide understates the content, the opposite mistake. Getting the direction right before any dosage arithmetic matters more than the arithmetic itself.
No. P₂O₅ is phosphorus pentoxide, the oxide form fertiliser analysis historically uses. It contains phosphorus but also oxygen, so its percentage is higher than the elemental phosphorus percentage. Multiply P₂O₅ by 0.4364 to get elemental P.
Multiply the K₂O percentage by 0.8301. A label showing K₂O 10% is 8.3% elemental potassium.
Both conventions exist internationally; Australian labels state P and K elementally, while many other markets and older references use the oxide convention. Neither is "more correct", they're just different units for the same underlying nutrient content, which is why a conversion is needed when comparing labels from different sources.
Whatever the label itself states. Check the guaranteed analysis section for the exact formula (Fe₂O₃, FeO, MnO, CuO, etc). If the label just says "chelated iron" or similar with no oxide formula, it's reporting the element directly and no conversion is needed.
No. It's free, with no sign-up and no login.
No, it converts a unit of measurement. It doesn't recommend a product, a rate or a dose. For turning a rate into a quantity for your area, use the Label Rate Calculator.
Copy results before leaving; creating an account does not transfer this conversion. LawnSuite has a catalogue of Australian lawn products. Its Free single-product calculator works out a quantity from the application rate you enter from your current product label and your lawn area. An NPK analysis alone does not set the application rate.
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