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NPK Converter (P₂O₅ to P, K₂O to K)

NPK converter: change P₂O₅ to P and K₂O to K, see the nutrients in a weight of 15-15-15, and find how much fertilizer gives a target amount.

How to use

  1. Enter the three N-P-K numbers from your fertilizer bag, for example 15-15-15.
  2. Section 2: enter a product weight in grams to see how much N, P₂O₅ and K₂O (and elemental P and K) it contains.
  3. Section 3: choose a nutrient and a target amount in grams to see how much product supplies it.
  4. Section 4: convert any single value between oxide and element form.

Why there are two ways to write P and K

Fertilizer labels in Malaysia, as in most countries, state phosphorus as P₂O₅ (phosphate) and potassium as K₂O (potash). This is a historical convention — the fertilizer does not actually contain these oxides. Scientific papers, soil tests and some hydroponic recipes use the elemental forms, P and K. To compare them you need to convert.

How it's calculated

The factors come from atomic weights:

  • P = P₂O₅ × 0.4364 (and P₂O₅ = P ÷ 0.4364 ≈ P × 2.29)
  • K = K₂O × 0.8301 (and K₂O = K ÷ 0.8301 ≈ K × 1.20)
  • Nitrogen is already given as N, so it needs no conversion.

Nutrients in a product:

  • Nutrient (g) = product weight (g) × label % ÷ 100

Product for a target:

  • Product (g) = target nutrient (g) ÷ (label % ÷ 100)

Worked example

You have 100 g of 15-15-15:

  • N = 100 × 15% = 15 g
  • P₂O₅ = 15 g → P = 15 × 0.4364 ≈ 6.55 g
  • K₂O = 15 g → K = 15 × 0.8301 ≈ 12.45 g
  • So on an elemental basis, 15-15-15 is about 15-6.5-12.5.

You want to give a bed 10 g of nitrogen using the same 15-15-15:

  • 10 ÷ 0.15 ≈ 67 g of product, which also supplies 10 g of P₂O₅ and 10 g of K₂O.

Converting 10 g of P₂O₅ gives 10 × 0.4364 = 4.364 g of P.

Tips

  • Recommendations from agricultural sources are usually in oxide form (kg P₂O₅ or K₂O per hectare), the same as bag labels. Convert only when a source uses elemental P and K.
  • To work out a target for your bed, multiply the recommended grams per m² by the bed area.
  • Granular fertilizer should be scattered around the plant, not against the stem, then watered in.
  • Several small feeds are safer than one big dose, especially in pots, where salts build up quickly.
  • Organic compost adds nutrients more slowly and improves the soil; it works well together with small amounts of NPK.

CalculatorsFertilizer Dilution CalculatorFertilizer dilution calculator: how many ml or grams of liquid or soluble fertilizer to mix for your watering can or sprayer, from ml/L or 1:N ratio.

CalculatorsCompost C:N Ratio CalculatorCompost C:N ratio calculator: mix kitchen scraps, dry leaves, grass, manure and more and see your carbon-to-nitrogen ratio against the 25–30:1 target.

FAQ

Is 15-15-15 the same as 15% of each element? It is 15% N, 15% P₂O₅ and 15% K₂O by weight. As elements that is about 15% N, 6.5% P and 12.5% K.

Why does the calculator say the product contains none of a nutrient? If the label number is 0 (for example 0-0-60), the product cannot supply that nutrient, however much you use.

Which number do I use for a hydroponic recipe? Hydroponic recipes often give elemental ppm (P and K). Use section 4 to convert, then compare with your label.

Does this tell me how much fertilizer my plant needs? No. It converts and does the arithmetic. How much a crop needs depends on the crop, the soil and the label instructions.