This fertiliser calculator works out how much product to apply to deliver the nutrients your plants or lawn need, using the N-P-K percentages on the label and your target application rate. It helps you feed correctly — enough to benefit the plants without wasting product or harming them.
Understanding N-P-K
Every fertiliser is labelled with three numbers — its percentage of nitrogen (N), phosphorus (P), and potassium (K) by weight. A 10-10-10 product is 10% of each; a 20-5-10 is richer in nitrogen. To deliver a target amount of a nutrient over an area, you divide the desired nutrient quantity by that nutrient's percentage to find how much product to spread. The calculator does this so you apply the right weight for the feed you want.
Because the percentages differ, two products giving the 'same' feed need very different application weights. A high-analysis fertiliser is applied more sparingly than a low one for the same result, which is exactly the kind of arithmetic that is easy to get wrong by hand.
Applying safely
More is not better: over-applying nitrogen scorches lawns and pollutes waterways through run-off. Spreading the calculated amount evenly, and often splitting it into smaller doses through the season, gives steadier growth than one heavy application. Water in granular feeds and follow the product's own guidance alongside the calculated rate.
You want to apply 4 kg of actual nitrogen using a 20-5-10 fertiliser.
- The product is 20% nitrogen (0.20 by weight).
- Product needed = desired nitrogen ÷ nitrogen fraction = 4 ÷ 0.20.
- Product = 20 kg of fertiliser.
Spread 20 kg of the 20-5-10 product to deliver 4 kg of actual nitrogen.
