An NPK calculator for mixing, ratios and fertigation ppm
The three numbers on a fertilizer bag are its N-P-K grade: the percentage by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O). This NPK calculator does the three jobs those numbers are needed for. It finds the grade and NPK ratio of any mix of fertilizers, works out how many kilograms of urea, DAP, MOP or other products — plus filler — make a target grade such as 10-10-10, and converts grams of a water-soluble fertilizer into ppm (parts per million, mg per litre) for drip fertigation, hydroponics or liquid feeding.
The fertilizer list uses Indian Fertiliser Control Order grades, including the 100% water-soluble grades sold for fertigation, and any other bag can be entered as a custom N-P-K. The arithmetic is shown in kilograms and percentages so you can check it by hand, and all of it runs in your browser.
How to use the NPK calculator
- Choose NPK of a mix, pick up to four fertilizers and type how many kg of each go into the mix. The grade, ratio and total nutrients appear at once.
- Choose Make a grade, type the target N, P₂O₅ and K₂O percentages and the batch size, and select two or three fertilizers. The table gives the kg of each and the filler needed.
- Choose Fertigation ppm, select a water-soluble fertilizer, type the grams dissolved, the litres of water and your injector ratio to see ppm of N, P₂O₅, K₂O and elemental P and K.
- Optionally enter a target ppm to get the grams per 1,000 litres needed to reach it.
- Press Copy result to paste the recipe into a notebook or message.
NPK Calculator features
Grade and NPK ratio
Blends up to four fertilizers and reduces the result to a ratio such as 1 : 2.67 : 1.33.
Target-grade solver
Finds non-negative amounts of two or three fertilizers for a grade and batch size, adds filler, and says when a grade is impossible or too concentrated.
Fertigation ppm both ways
From grams to ppm, and from a target ppm back to grams per 1,000 litres, including injector dilution.
Oxide and elemental
Shows P₂O₅ and K₂O as on the bag, and elemental P and K as many hydroponic recipes use.
Custom grades
Any fertilizer can be typed in as its own N-P-K percentages.
How the NPK numbers are calculated
Grade of a mix. Nutrient in each fertilizer = kg × percentage ÷ 100. Add the nutrients and divide by the total weight. 20 kg urea (46% N) + 50 kg DAP (18-46-0) + 30 kg MOP (0-0-60) contains 9.2 + 9 = 18.2 kg N, 23 kg P₂O₅ and 18 kg K₂O in 100 kg, so the mix is 18.2-23-18. Its ratio, dividing by the smallest number, is 1.01 : 1.28 : 1.
Making a grade. For 100 kg of 10-10-10 from urea, DAP and MOP: DAP for phosphate, 10 ÷ 0.46 = 21.74 kg, which brings 3.91 kg N; MOP for potash, 10 ÷ 0.60 = 16.67 kg; urea for the remaining 6.09 kg N, 13.23 kg. These total 51.64 kg, so 48.36 kg of filler completes the batch. With more complex choices the calculator solves the three equations together.
ppm. One ppm in water is one milligram per litre. ppm = grams × nutrient % ÷ 100 × 1,000 ÷ litres. One kilogram of 19-19-19 in 1,000 L gives 190 ppm each of N, P₂O₅ and K₂O. With a 1:100 injector the stock tank is 100 times stronger than what reaches the plants.
Oxide to element conversion
The factors come from atomic weights (P 30.974, K 39.098, O 15.999). Bags in India, the US and most countries state P₂O₅ and K₂O; many hydroponic recipes give elemental P and K, so check which one a recipe means. For field doses in kg per acre, use the fertilizer calculator.
| Convert | Multiply by | Example |
|---|---|---|
| P₂O₅ → P | 0.4364 | 46% P₂O₅ in DAP = 20.1% P |
| P → P₂O₅ | 2.2914 | 50 ppm P = 114.6 ppm P₂O₅ |
| K₂O → K | 0.8301 | 60% K₂O in MOP = 49.8% K |
| K → K₂O | 1.2046 | 200 ppm K = 240.9 ppm K₂O |
Mixing safely
- Mix dry fertilizers just before applying them; urea mixed with SSP or DAP becomes sticky as it absorbs moisture.
- Keep calcium nitrate in a separate stock tank from phosphates and sulphates (19-19-19, MAP, MKP, magnesium sulphate) to avoid insoluble precipitates that clog drippers.
- Fill the tank with water first, then dissolve one fertilizer at a time with stirring.
- For a mixing ratio of liquids rather than nutrients, the dilution calculator handles C₁V₁ = C₂V₂.
NPK Calculator FAQ
What does an NPK ratio mean?
It is the grade divided by its smallest number. 12-32-16 has an NPK ratio of 1 : 2.67 : 1.33 — for every kg of N it supplies 2.67 kg of P₂O₅ and 1.33 kg of K₂O.
How do I calculate the NPK of a fertilizer mix?
Multiply each fertilizer’s weight by its N, P₂O₅ and K₂O percentages, add the results and divide by the total weight. The NPK calculator does this for up to four products at once.
How do I make 10-10-10 with urea, DAP and MOP?
For 100 kg: 21.74 kg DAP, 16.67 kg MOP, 13.23 kg urea and 48.36 kg filler. Without filler the same three fertilizers would give a stronger grade.
How do I calculate ppm of NPK in water?
ppm = grams of fertilizer × nutrient % ÷ 100 × 1,000 ÷ litres of water. 500 g of 19-19-19 in 1,000 L gives 95 ppm of N, P₂O₅ and K₂O.
Is ppm the same as mg/L?
For dilute solutions in water, yes: 1 ppm = 1 mg per litre.
Why is my target grade impossible?
Either a nutrient has no source among the fertilizers you picked, or the grade needs more fertilizer than the batch weight. Choose a more concentrated source (for example DAP instead of SSP) or a lower grade.