Fertiliser Calculator

Area, m²

What grows there

Nitrogen in the product, %

Phosphorus in the product, %

Potassium in the product, %

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
Phosphorus delivered, g600
Potassium 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, phosphorus and potassium content in percent. 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.

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

Seasonal nitrogen: lawn 3 g/m², soft fruit and flower beds 4, fruit trees 5, vegetables 6, potatoes 9.
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?

The percentage of nitrogen, phosphorus and potassium. In a 16-16-16 product each element is sixteen percent and the rest is filler and accompanying compounds.

How much fertiliser per hundred square metres?

For vegetables at six grams of nitrogen per metre and a sixteen percent product, about 3.7 kg per hundred square metres a season, split across two or three feeds.

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, so lawn feeds look like 20-5-10. Vegetables and potatoes need the full set in a more even ratio.

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