Cost per Nutrient & The Cheaper Source Wins

The price on the bag tells you what you pay for the whole product, much of which is filler and carrier. What you actually buy is the nutrient inside it. A cheaper bag with a low nutrient content can cost more per kilogram of real nitrogen or phosphate than a dearer, more concentrated bag — so the bag price alone can steer you to the worse deal.

Cost per kg nutrient = price ÷ (bag weight × nutrient% ÷ 100). For example a 50 kg bag of urea (46% N) at a given price holds 23 kg of N, so divide the bag price by 23. Doing the same for the second fertiliser puts both on a level footing — the genuine cost of one kilogram of the nutrient you need.

It returns the cost per kg of actual nutrient for both fertilisers, the cost per kg of product, the kilograms of nutrient in each bag, which is the cheaper source, and the saving per kilogram in both currency and percent. That makes the better-value purchase obvious at a glance, whatever the headline bag prices look like.

No — compare like for like. Cost per kg of nitrogen should be set against another nitrogen source, and cost per kg of P₂O₅ against another phosphate source. Comparing the N cost of urea with the P cost of DAP is meaningless because they supply different nutrients; pick the nutrient you are actually buying and compare sources of that.

Urea is usually the cheapest straight source of nitrogen because it is highly concentrated at 46% N. DAP carries nitrogen too (about 18% N) but you buy it mainly for its phosphate (46% P₂O₅), so its N often rides along. Enter both products' prices and analyses and the tool will name the cheaper source for the nutrient you choose.

Not always. Also weigh the application cost, the nutrient form and how well it suits your soil and timing — a slow-release or coated product may justify a higher unit cost, and a foliar grade is priced for a different job. Use cost per nutrient as the first filter, then adjust for agronomy and logistics.

Yes. Because the formula uses the bag weight and nutrient percentage you enter, it normalises everything to one kilogram of nutrient regardless of whether the bags are 25 kg, 45 kg or 50 kg. That is the whole point — it strips away both the bag size and the filler so you compare the actual nutrient cost.

The calculator works in eight currencies, so you can enter prices in your local money and read the cost per nutrient and the saving in the same units. The arithmetic is identical across currencies — it simply keeps the figures in the one you choose for clear, like-for-like comparison.

The same logic applies to any nutrient on the label — sulphur, zinc, boron and so on. Enter the relevant nutrient percentage for each product and the tool gives the cost per kg of that nutrient. Just keep comparing the same nutrient between the two products so the result is meaningful.

It is exact for the prices and analyses you enter — the maths is straightforward. The accuracy of the decision depends on entering the correct bag weight, nutrient percentage and current price for each product. Use up-to-date local prices and the guaranteed analysis from the label for a comparison you can act on.