Why do I need more product than the nitrogen figure?

Because the product is not pure nitrogen. Urea is 46% nitrogen, so every kilogram of nitrogen needs a bit over two kilograms of urea.

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Lawn and Garden Fertilizer Calculator

Fertiliser quantity from a nutrient application rate. Rates are given as the nutrient, not the product, so the concentration on the bag is what converts one into the other.

Also called: fertiliser rate calculator, urea calculator.

Fertiliser needed (kg)

5.43

5.43 kg of product delivers 2.5 kg of nitrogen across 500 m². That is 0.11 bags, or 10.9 grams per square metre.

Nitrogen needed (kg)

2.5

Bags

0.11

Grams per square metre

10.9

Area in hectares

0.05

Kg per 100 m²

1.09

How this is calculated

Method and background

How this is calculated

Agronomic advice is given as kilograms of nitrogen per hectare, but you buy a product that is only partly nitrogen. Urea is 46% nitrogen, so delivering 50 kg of nitrogen needs about 109 kg of urea. Confusing the two is the most common fertiliser error and it is always in the direction of underapplying, since the product number is the larger one.

product = area in hectares * nutrient rate / the nutrient fraction in the product

A_ha: Area in hectares (hectares)
R: Nutrient rate wanted (kg/ha)
n: Nutrient share of the product (decimal)

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

urea at fifty kilos of nitrogen a hectare

Area (m²): 10,000
Nitrogen wanted (kg per hectare): 50
Nitrogen in the product (%): 46
Bag size (kg): 50

Fertiliser needed (kg)108.7

50 / 0.46, worked by hand

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a pure nutrient needs no conversion

Area (m²): 10,000
Nitrogen wanted (kg per hectare): 50
Nitrogen in the product (%): 100
Bag size (kg): 50

Fertiliser needed (kg)50

boundary

Open this example

Method and limits

What it assumes

A uniform application across the whole area.

What it deliberately does not model

Only one nutrient is calculated. A compound fertiliser delivers several and the limiting one governs.
Soil testing should set the rate. This converts a rate into a quantity, it does not choose the rate.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Why do I need more product than the nitrogen figure?: Because the product is not pure nitrogen. Urea is 46% nitrogen, so every kilogram of nitrogen needs a bit over two kilograms of urea.