A fertiliser grade turned into what is actually in the bag — with the oxide convention undone, the bag weighed out, and rates in both systems.
Calculator
Products:
The second and third boxes are oxides, because that is what bags declare almost everywhere. If your label is Australian or New Zealand it is already elemental, and the conversion below has been done for you.
10.0-10.0-10.0 as printed
10.0-4.4-8.3
as actual N, P and K · 30.0% of the bag is declared nutrient, the rest is carrier, coating and moisture
What the three numbers actually mean
The same bag, in both conventions
Nutrient
As declared
As the element
Factor
Nitrogen
10.0% N
10.0% N
1 — already elemental
Phosphorus
10.0% P₂O₅
4.36% P
0.43643
Potassium
10.0% K₂O
8.30% K
0.83015
Ratio
1.00 : 1.00 : 1.00
1.00 : 0.44 : 0.83
normalised to nitrogen
The middle number is the one that moves. P₂O₅ is only 43.6% phosphorus by mass, so a grade that looks balanced is not: 10.0% P₂O₅ is 4.4% actual phosphorus. Neither compound is even present in most of these products — the oxide convention is a nineteenth-century analytical habit that outlived the analysis.
What is in the bag
Nitrogen5.00 kgthe same either way
Phosphorus2.18 kgdeclared as 5.00 kg of P₂O₅
Potassium4.15 kgdeclared as 5.00 kg of K₂O
Everything else35.00 kg70.0% of the bag
The last row is not waste. It is the counter-ion the nutrient arrived attached to, plus granulation aids, coating and moisture — and in some products it carries sulphur or calcium that the three-number grade has no room to declare. Ammonium sulphate’s 21-0-0 leaves out about 24 per cent sulphur, which is often the reason it was chosen.
Rates, in either system
1 lb per 1000 sq ft is48.82 kg/haone pound per 1000 sq ft is 48.82 kg/ha
Product needed for 100 kg N/ha1,000.0 kg/ha20.48 lb per 1000 sq ft
Lawn products are rated in pounds per thousand square feet and agronomy in kilograms per hectare, and the two worlds rarely meet on a label. The factor is close to fifty, which makes a rate quoted in one system look absurd in the other and is worth checking before spreading anything.
Common products, both ways round
As printed, and as elements
Product
Grade
As N-P-K
Note
Urea
46-0-0
46.0-0.0-0.0
The densest common nitrogen source. 46% is close to the theoretical maximum for urea.
Calcium ammonium nitrate
26-0-0
26.0-0.0-0.0
Lower analysis than urea, and much less prone to volatilising on the surface.
Ammonium sulphate
21-0-0
21.0-0.0-0.0
Also supplies about 24% sulphur, which the three-number grade does not show.
DAP — diammonium phosphate
18-46-0
18.0-20.1-0.0
The most-traded phosphate. Its 46% P₂O₅ is only 20% actual phosphorus.
MAP — monoammonium phosphate
11-52-0
11.0-22.7-0.0
Higher phosphate than DAP, less nitrogen, and more acidifying in the band.
SSP — single superphosphate
0-16-0
0.0-7.0-0.0
Low analysis, and carries around 11% sulphur and calcium that the grade omits.
MOP — muriate of potash
0-0-60
0.0-0.0-49.8
Potassium chloride. 60% K₂O is 49.8% actual potassium.
SOP — sulphate of potash
0-0-50
0.0-0.0-41.5
Chloride-free, for salt-sensitive crops, and supplies sulphur.
Balanced blend 10-10-10
10-10-10
10.0-4.4-8.3
Balanced only in the oxide convention. As elements it is 10-4.4-8.3.
Lawn blend 20-10-10
20-10-10
20.0-4.4-8.3
A 2-1-1 grade that is closer to 2-0.44-0.83 as elements.
DAP is the row worth looking at twice. Sold everywhere as 18-46-0, it carries 20.1 per cent actual phosphorus — so a tonne of DAP delivers 201 kg of phosphorus, not 460. Nutrient budgets and export figures are quoted both ways, and the difference between them is a factor of 2.3.
Which convention your label uses
Labelling practice by region
Region
Convention
Note
United States & Canada
Oxide (P₂O₅, K₂O)
Guaranteed analysis is N, available P₂O₅ and soluble K₂O, set by state fertiliser law.
European Union & United Kingdom
Oxide (P₂O₅, K₂O)
N, P₂O₅ and K₂O, with secondary nutrients as CaO, MgO and SO₃. Elemental values may be shown in addition.
India
Oxide (P₂O₅, K₂O)
The Fertiliser Control Order specifies N, P₂O₅ and K₂O — so DAP is sold as 18-46-0.
Australia & New Zealand
Elemental (P, K)
Labels give elemental N, P, K and S. The same product carries a visibly smaller middle number than it would elsewhere.
An Australian bag reading 10-4.4-8.3 and an American, European or Indian bag reading 10-10-10 hold the same fertiliser. Reading one as the other in either direction is a real and repeated error in trade data, in recommendations copied across borders, and in comparing a bag against a soil test — which is nearly always reported elementally, whatever the bag says.
What this converter covers
Any grade converted between oxide and elemental, ten common products both ways, bag quantities, and rates in kg/ha and lb per 1000 sq ft.
P₂O₅ and K₂O to actual phosphorus and potassium, from atomic masses
Nutrient ratios in both conventions, which are not the same ratio
Kilograms of each nutrient in a bag of any size
Application rates in kg/ha and lb per 1000 sq ft, and product needed for a target
Which regions label elementally and which label oxides
Oxide to elemental 10-10-10 is 10-4.4-8.3 kg/ha and lb/1000 ft² Both directions
Free, no signup — exact by definition, not an estimate.
Updated 8 September 2026
At a glance
Formula shown
P = P₂O₅ × 0.4364 · K = K₂O × 0.8301 — both derived from atomic masses
Scenario support
10-10-10 is really 10-4.4-8.3 · DAP's 46% P₂O₅ is 20.1% actual phosphorus
Educational estimate
Planning support from the values you enter — not professional advice.
The middle number is an oxide
Three numbers on a fertiliser bag, and only the first one means what it appears to mean.
Nitrogen is reported as elemental N. Phosphorus is reported as P₂O₅ and potassium as K₂O — compounds that are not present in most of these products at all. They are reporting conventions, and the actual element is a fraction of the number printed.
What the printed figure contains
Printed as
Element
Fraction
So 10% is
N
N
1.0000
10% N
P₂O₅
P
0.4364
4.36% P
K₂O
K
0.8301
8.30% K
So a “balanced” 10-10-10 is 10% nitrogen, 4.4% phosphorus and 8.3% potassium. It was never balanced; it only looked that way in a convention that inflates one number by 2.3 and another by 1.2.
The factors above are not looked up anywhere. They are the mass fraction of the element in the oxide, computed from atomic weights: two phosphorus atoms in a molecule that also carries five oxygens come to 43.64% of its mass, and two potassium atoms with one oxygen come to 83.02%. Anyone can check them with a periodic table, which is the point of deriving rather than quoting them.
This matters most at the moment a bag meets a soil test. Soil tests report phosphorus and potassium elementally, essentially everywhere. Comparing a 46% phosphate product against a soil phosphorus figure without converting is comparing two different quantities that share a name.
Where the convention came from
The obvious question is why anyone would report a compound that is not in the bag, and the answer is that it used to be in the crucible.
Nineteenth-century agricultural chemistry analysed materials by ignition: burn the sample and weigh what is left. Phosphorus came out of that process as the pentoxide and potassium as the oxide, so those were the quantities that could actually be measured, and the analysis was reported as what the balance showed.
Analytical methods moved on more than a century ago. The reporting convention did not, because it had been written into fertiliser law, into trade contracts, into recommendation tables and into the habits of everyone who buys the stuff. Changing it would invalidate every historical figure and every printed recommendation at once.
So the oxide numbers survive as a kind of legacy unit — a measurement of something that is no longer measured, kept because too much is denominated in it. It is worth knowing this, because it explains why the convention feels arbitrary: it is arbitrary now, and was not when it started.
The same history is why sulphur appears as SO₃ on European labels, and calcium and magnesium as CaO and MgO. Ignition analysis produced those oxides too, and the declarations followed the crucible.
Two countries label it differently
Australia and New Zealand broke with the convention and label elementally. That single fact produces a genuinely dangerous ambiguity in anything that crosses a border.
The same fertiliser, two labels
Region
Printed grade
Convention
United States, EU, India
10-10-10
Oxide
Australia, New Zealand
10-4.4-8.3
Elemental
Those are the same bag. Read an Australian label with American expectations and the product looks badly deficient in phosphorus; read an American label with Australian expectations and it looks more than twice as strong as it is.
The practical rule is to look at the middle number and ask whether it is plausible. Elemental phosphorus percentages above about 25 are rare, because even DAP — the most concentrated widely traded phosphate — is only 20.1% phosphorus. A middle number of 46 is therefore an oxide figure with near certainty.
The same check works on recommendations copied from overseas extension material, which is where this most often goes wrong: a rate given in “kg P per hectare” and a rate given in “kg P₂O₅ per hectare” differ by a factor of 2.3, and both are written the same way when someone drops the subscript.
A ratio is not a ratio
Fertiliser recommendations are often given as a ratio rather than a grade — a “3-1-2 lawn feed”, a “1-2-2 for root crops” — and the ratio inherits the same ambiguity as the grade, invisibly.
Take a 3-1-2 in the oxide convention: 21-7-14, say. As elements that is 21-3.1-11.6, which is a ratio closer to 3-0.4-1.7. The phosphorus share of the recommendation has more than halved simply by changing how it is written down.
Neither number is wrong. But a ratio quoted without its convention cannot be acted on, and unlike a grade it carries no units to give the game away. A bag says “P₂O₅” somewhere in the guaranteed analysis; a ratio in a magazine article says nothing at all.
The calculator above prints both ratios side by side for whatever grade you enter, which is usually enough to identify which convention a remembered recommendation came from — the oxide version is the one where the numbers are rounder.
What the three numbers leave out
A final thing the grade does not tell you: what else came in the bag, and it is often the reason the product was chosen.
Ammonium sulphate is sold as 21-0-0 and carries about 24% sulphur. Single superphosphate is 0-16-0 and carries roughly 11% sulphur plus calcium. Muriate of potash brings chloride, which some crops object to and others do not notice. Nothing in the three-number grade has room to say any of this.
The rest of the bag is not filler in the pejorative sense either. Most of it is the counter-ion the nutrient arrived attached to — the sulphate in ammonium sulphate, the chloride in muriate of potash — plus granulation aids, anti-caking coating and moisture. A 10-10-10 is 70% “something else”, and the something else is mostly chemistry rather than sand.
Which is worth weighing when comparing a low-analysis product against a high one on price. A tonne of urea delivers 460 kg of nitrogen and a tonne of ammonium sulphate delivers 210, so urea is cheaper per unit of nitrogen at anything like similar prices — but the ammonium sulphate also delivered 240 kg of sulphur, and on a sulphur-deficient soil that is not a rounding error in the comparison. It is the comparison.
Related calculators
Other concentration, soil and solution tools:
PPMppm to percent, mg/L and µg/m³ — asking which liquid or which gas, because without that the conversion has no answer.
EC to TDSµS/cm, mS/cm, dS/m and ppm on all three scales at once — because the ppm factor is a meter convention, not a property of the water.
Water Hardnessppm, gpg, °dH, °fH and °e — every factor derived from molar masses, plus the calcium and magnesium a lab actually reports.
Moles to GramsGrams, moles, molarity, molality and normality — asking for the molar mass and the density, because neither conversion exists without them.
Densitykg/m³, lb/ft³, lb/in³ and lb/gal, with specific gravity against any water reference and the API scale that runs backwards.
Volume to WeightGallons to pounds, cubic yards to tons, litres to kilograms — by substance, with an honest range rather than one invented number.
The first two are the regulations that require the oxide form on North American and European labels; the third is why the same distinction shows up in trade statistics. The conversion factors themselves are not cited to anyone — they are computed from atomic masses in the code, so they can be checked against the chemistry.
Association of American Plant Food Control OfficialsAAPFCO · verified 2026-09-08 · The guaranteed-analysis format used across US state fertiliser law — total nitrogen, available phosphate as P₂O₅ and soluble potash as K₂O — which is the reason the second and third numbers on a North American bag are oxides
Regulation (EU) 2019/1009 on EU fertilising productsOfficial Journal of the European Union · verified 2026-09-08 · The European declaration requirements, including nutrients expressed as P₂O₅, K₂O, CaO, MgO and SO₃, and the provision for stating elemental values alongside the oxide forms
FAOSTAT — fertilizers by nutrientFood and Agriculture Organization of the United Nations · verified 2026-09-08 · That international fertiliser statistics are compiled in nutrient terms, which is why a tonnage figure has to state whether it is oxide or elemental before it can be compared across sources
Conversion note
This converts units and weighs out a bag. It is not a fertiliser recommendation and does not know your soil, crop, climate or history. How much of any nutrient to apply depends on a soil test, the crop and yield target, what the previous crop removed, organic matter, irrigation water quality and local regulation — several of which override arithmetic entirely. Nutrient availability is not the same as nutrient content: phosphorus availability in particular depends strongly on soil pH and on the form applied, so applying the elemental figure calculated here does not mean the crop receives it. Many jurisdictions cap nitrogen and phosphorus application rates by law, sometimes seasonally, and those limits are stated in specific units that must be read carefully for the oxide-or-elemental distinction this page is about. Product analyses vary between manufacturers and batches; the guaranteed analysis on the bag in front of you governs. Consult a local agronomist or extension service before changing a fertiliser programme.
Published the NPK Fertilizer Converter: any grade between the oxide convention and elemental N, P and K, with bag quantities, nutrient ratios in both conventions, and application rates in kg/ha and lb per 1000 square feet.
States what the three numbers on a bag actually are. Nitrogen is elemental, but phosphorus is reported as P2O5 and potassium as K2O -- compounds not present in most of these products -- so a 'balanced' 10-10-10 is 10 per cent N, 4.4 per cent P and 8.3 per cent K.
Derives both factors from atomic masses in the code rather than quoting a rounded 0.44, so they can be checked against a periodic table instead of against another calculator.
Explains where the convention came from: nineteenth-century ignition analysis produced those oxides on the balance, and the reporting outlived the method because it had been written into fertiliser law and trade contracts.
Flags the cross-border ambiguity. Australian and New Zealand labels are ELEMENTAL, so their 10-4.4-8.3 and everyone else's 10-10-10 are the same bag, and a recommendation copied between the two systems is out by a factor of 2.3 on phosphorus.
Shows that a nutrient RATIO carries the same ambiguity invisibly: a 3-1-2 in oxide terms is nearer 3-0.4-1.7 as elements, and unlike a grade a ratio has no units to give the convention away.
Weighs out the bag, including the portion that is not declared nutrient, and notes that the remainder is largely the counter-ion the nutrient arrived attached to rather than inert filler -- ammonium sulphate's undeclared 24 per cent sulphur being the common example.
Converts between lb per 1000 square feet and kg per hectare, a factor of 48.8, because lawn products and agronomy are rated in different systems that rarely meet on a label.
Verified by 106 automated cases, asserting the factors against atomic masses rather than decimals, that a 10-10-10 is emphatically not balanced as elements and that its middle number more than halves, that the oxide and elemental ratio triples genuinely differ, and that a grade declaring more than a full bag of nutrient is refused.
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