Fertiliser Calculator

Written byOleh Soloninupdated 25 September 2026

Area, m²

What grows there

Nitrogen in the product, %

Phosphorus (P₂O₅) in the product, %

The second number on the bag: phosphorus stated as the oxide P₂O₅.

Potassium (K₂O) in the product, %

The third number on the bag: potassium stated as the oxide K₂O.

Your own nitrogen rate, g/m²

Leave at 0 to use the crop's rate.

Number of plants

Result

Fertiliser required, g3,750
Per square metre, g37.5
Pure nitrogen, g600
P₂O₅ delivered, g600
K₂O delivered, g600
Per plant, genter the plant count
Cropvegetables

The numbers on the bag are percentages

Works out how much fertiliser a plot needs from the nitrogen requirement of the crop and the nitrogen, phosphorus and potassium content of the product.

A marking of 16-16-16 or 10-26-26 is the nitrogen, P₂O₅ and K₂O content in percent by weight. That is what the dose is built from: if a crop wants six grams of nitrogen per square metre and the product is sixteen percent nitrogen, you need nearly six times as much product.

Nitrogen is the reference because it is used up fastest and its shortage shows first. Phosphorus and potassium come along for the ride, and the calculator shows how much of each the nitrogen-based dose delivers.

How to use

Pick the crop — it sets the nitrogen dose per square metre.
Enter the area and the analysis from the bag: the percentages of nitrogen, phosphorus and potassium.
If you have your own nitrogen rate, enter it; for a per-plant dose, give the number of plants.

Nitrogen rates by crop

The calculator works from nitrogen: it takes the rate for the chosen crop, in grams of pure nitrogen per square metre, and divides by the nitrogen share of the fertiliser. That gives the dose — and how much phosphorus and potassium come along with it.

The spread between crops is threefold. A lawn manages on three grams of nitrogen a metre a season; potatoes need nine, so with the same 16-16-16 product the potato dose is three times larger.

The nitrogen rates are built into the calculator; the doses come from it for 100 m² of a 16-16-16 product. If you have your own rate from a soil test or the bag, enter it and the calculator works from that.
Crop	Nitrogen, g/m²	16-16-16 for 100 m²	Per m²
lawn	3	1.88 kg	18.8 g
soft fruit	4	2.50 kg	25.0 g
flowers	4	2.50 kg	25.0 g
fruit trees	5	3.13 kg	31.3 g
vegetables	6	3.75 kg	37.5 g
potatoes	9	5.63 kg	56.3 g

Why a lawn gets a different analysis

The calculator shows not just the dose but how much phosphorus and potassium arrive with the nitrogen. A balanced 16-16-16 delivers them in equal measure, so a lawn gets as much phosphorus as nitrogen — and phosphorus is something grass barely needs.

A 20-5-10 lawn feed at the same nitrogen rate delivers a quarter of the phosphorus and half the potassium, and less of it is needed by weight. Urea, 46-0-0, is 46 percent nitrogen with no phosphate or potash: the smallest dose of all, but it adds no phosphorus or potassium whatsoever.

Every row comes from the calculator on this page. Nitrogen stays the same within each crop; only what comes with it changes.
Fertiliser	Dose for 100 m²	Nitrogen	Phosphorus	Potassium
16-16-16 on a lawn	1,875 g	300 g	300 g	300 g
20-5-10 on a lawn	1,500 g	300 g	75 g	150 g
46-0-0 urea on vegetables	1,304 g	600 g	0	0

What this calculator shows

The mass of fertiliser for the whole plot and per square metre
The pure nitrogen it delivers, plus the phosphorus and potassium that arrive with it
The dose per plant, if you know how many there are

What to keep in mind

The calculator's nitrogen rates are worked examples, not advice: lawn 3 g/m², soft fruit and flower beds 4, fruit trees 5, vegetables 6, potatoes 9. A working rate comes from a soil test and guidance for the crop.
The seasonal figure is split across two or three applications: more nitrogen in spring, more potassium towards the end of summer.

FAQs

What do the NPK numbers mean?

Nutrient content in percent by weight: nitrogen (N), phosphorus stated as the oxide P₂O₅ and potassium stated as K₂O. A 16-16-16 product holds 16% N, 16% P₂O₅ and 16% K₂O.

How much fertiliser per hundred square metres?

For vegetables at six grams of nitrogen a metre and a 16-16-16 product, 3.75 kg a season, split across two or three feeds. Half that for a lawn, half as much again for potatoes.

Can I apply the whole season's dose at once?

Better not. Nitrogen washes out with rain and burns roots at high doses, which is why the seasonal figure is split.

How does feeding a lawn differ from feeding vegetables?

Grass wants nitrogen and almost no phosphorus. A 20-5-10 feed at the same nitrogen rate gives a lawn 75 grams of phosphorus per hundred square metres instead of 300 from a balanced 16-16-16. Vegetables and potatoes need the full set in a more even ratio.

Why does the calculation start from nitrogen?

Because nitrogen drives growth and is the element most often in short supply. The calculator does not tune phosphorus and potassium; it shows how much of each arrives with the nitrogen, so you can see whether the product suits the crop.

How do I get a dose per plant?

Enter the number of plants and the calculator divides the area dose equally between them. That suits fruit trees and shrubs, where the feed goes around the base rather than across the whole area.

Worked example

A hundred square metres of vegetables

Input: 100 m², vegetables, 16-16-16 fertiliser

Output: 3.75 kg of fertiliser, 600 g of pure nitrogen

Note: Along with the nitrogen the beds receive 600 g each of phosphorus and potassium. For vegetables that balance is fine, but a lawn needs nothing like that much phosphorus — it wants a high first number instead.

Useful links

All tools in Home & Utilities

Fertiliser Calculator

Result

Related tools

Pool Volume Calculator

Water Consumption Calculator

Firewood Calculator

Useful links