Fertiliser Calculator

How the Fertiliser NPK Calculator works

The formula. Product = nitrogen wanted ÷ (N% ÷ 100); the phosphate and potash it brings = product × P% and K%. For example, 10 g N per m² over 100 m² is 1 kg of N, which is 1 ÷ 0.46 = 2.17 kg of 46-0-0 urea. A second example: the same rate from 20-10-10 takes 5 kg of product and brings 0.5 kg each of P₂O₅ and K₂O. Divide by the bag weight to see how many bags to buy.

Fertiliser instructions are written in a mixture of units: grams per square metre on a garden box, kilograms per hectare in an agronomy guide, pounds per thousand square feet on a lawn product, and the bag itself tells you only the percentage of each nutrient. This calculator does the conversion so that "10 g of nitrogen per square metre" becomes a weight of the product you actually have.

Reading the bag. The three numbers, for example 7-7-7 or 20-5-10, are the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 20 kg bag of 7-7-7 contains 1.4 kg of each. Nitrogen is the nutrient plants use most and run out of first, and it is the one that recommendations are usually expressed in, so the calculator works from a target nitrogen rate and reports the phosphate and potash that arrive with it.

The calculation. Product per square metre = nitrogen target ÷ (N% ÷ 100). At 10 g N/m² with a 7 percent product that is 143 g/m², or 1,430 kg per hectare; with sulphate of ammonia at 21 percent it is only 48 g/m². Multiply by the area for the total and divide by the bag size for the number of bags. Areas in square feet, acres and hectares are converted to square metres, and rates in kg/ha or lb per 1,000 ft² to g/m², so any combination of units works.

Typical rates. A general vegetable dressing is about 5 to 10 g N/m² (50 to 100 kg/ha) before planting, with a top-up for hungry crops such as brassicas and potatoes. Lawns take roughly 10 to 15 g N/m² a year split over two or three feeds, no more than 5 g N/m² at once or you risk scorching. Fruit trees and shrubs want around 5 g N/m² in spring. Farm crops run from 100 to 200 kg N/ha depending on the crop and the soil's own supply, and should follow a soil test.

Phosphate and potash. Most garden soils have enough phosphate, and heavy applications build up and run off into watercourses, so a balanced fertiliser is often more phosphate than you need. Sandy soils and heavy croppers such as tomatoes and potatoes do want potash. The delivered figures let you check whether a balanced product is over- or under-supplying these and whether a straight nitrogen product plus occasional potash would be better.

Spreading evenly. Weigh out the amount for a known area, such as a 10 m² strip, and spread half walking one way and half at right angles. For small quantities mix the fertiliser with dry sand to bulk it up. Water in afterwards unless rain is due, and keep granules off foliage.

Fertiliser calculator: product needed by NPK grade

Enough of each product to deliver 10 g of nitrogen per m² over 100 m².

Product needed and phosphate and potash delivered for 10 g N per m² over 100 m²

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