Fertilizer Calculator

1. Enter the length and width of the area to feed, in metres.
2. Enter your target nitrogen rate in grams per square metre.
3. Enter the nitrogen percentage, the first number in the NPK analysis on the bag.
4. Read the total fertiliser to apply, the rate per square metre and the nitrogen delivered.

A fertiliser calculator works out how much product to spread to deliver a target amount of a nutrient, usually nitrogen. Because products differ in strength, the same bag weight can deliver very different amounts of nitrogen, so you cannot judge a dose by weight alone.

Feeding by a nutrient rate rather than a guessed handful protects your plants and the environment: too much nitrogen scorches leaves and runs off into waterways, while too little leaves plants pale and slow.

1. Turn the nitrogen percentage into a fraction by dividing by 100.
2. Divide your target nitrogen per square metre by that fraction for the product per square metre.
3. Multiply by the area for the total product in grams.
4. Divide by 1000 for kilograms.

The NPK numbers on a bag are the percentages of nitrogen, phosphorus and potassium. The first, nitrogen, usually drives the dose because it is the nutrient plants use most.

You want to give a 50 square metre lawn 3 grams of nitrogen per square metre, using a 10-5-5 product, so the nitrogen fraction is 0.10. The product per square metre is 3 divided by 0.10, which is 30 grams. Across 50 square metres that is 1500 grams, or 1.5 kilograms, delivering 150 grams of nitrogen in total. Switch to a stronger 20 percent product and you only need half as much.

Nitrogen Percentage

A stronger product delivers the same nitrogen from less material, so a 20 percent feed needs half the weight of a 10 percent one.

Target Rate

Hungry crops and lawns want a higher rate, while a maintenance feed uses less; the target scales the product directly.

Area

Bigger areas need proportionally more, so measure carefully and split odd shapes into rectangles.

Season and Growth

Plants take up more nitrogen in active growth, so rates are usually higher in spring and lower late in the season.

The nitrogen percentage decides how much product delivers a given rate. This is product per square metre to apply 3 grams of nitrogen.

This is why reading the analysis matters: a cheap weak bag can cost more per unit of nitrogen than a concentrated one.

Feeding a Lawn

Hit a precise nitrogen rate so the grass greens up without scorching or surging.

Comparing Products

See how much of a weak versus a strong feed you would need for the same result.

Dosing Beds and Borders

Work out a measured feed for vegetables and ornamentals instead of guessing.

1. Dosing by Product Weight

Two bags of the same weight can deliver very different nitrogen; dose by the nutrient, not the bag.

2. Ignoring the NPK Analysis

The first number sets your nitrogen; skipping it means you never know the real rate.

3. Over-Applying

Too much nitrogen scorches plants and pollutes runoff, so more is not better.

4. Uneven Spreading

Even a correct total gives stripes or burns if it is not spread evenly across the area.

The nutrient maths is exact for the nitrogen you enter, giving a reliable product amount. It focuses on nitrogen and does not balance the full nutrient picture.

Divide your target nitrogen per square metre by the nitrogen fraction, then multiply by the area. For 3 g/m2 of nitrogen on 50 m2 with a 10 percent product, that is 30 g/m2, or 1.5 kg total.

They are the percentages by weight of nitrogen, phosphorus and potassium. A 10-5-5 bag is 10 percent nitrogen, 5 percent phosphorus and 5 percent potassium.

Because products vary in strength, the same weight can deliver very different nitrogen. Dosing by the nutrient rate is the only reliable way.

Many lawn feeds apply around 2 to 5 grams of actual nitrogen per square metre per application, spread across the growing season.

A stronger product needs less material for the same nitrogen. A 20 percent feed needs half the weight of a 10 percent feed for the same rate.

Yes. Excess nitrogen scorches plants, weakens roots and runs off into waterways, so staying near the target rate matters.

The calculator doses by nitrogen, the usual driver. Check that the phosphorus and potassium in your product also suit your soil and plants.

Split the total into halves and apply in two passes at right angles, or use a spreader set to the product per square metre.

No. The calculator runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Fertiliser and NPK (Wikipedia).
• Labelling of NPK values (Wikipedia).
• Nitrogen for plants (Wikipedia).