NPK Fertilizer Mixing Calculator

How NPK Fertilizer Mixing Works

Fertilizer labels express nutrient analysis as N-P-K percentages by weight: nitrogen (N), phosphorus as P₂O₅, and potassium as K₂O. A target N-P-K ratio like 4-2-1 means the nutrient blend should contain four parts nitrogen for every two parts phosphorus and one part potassium. Precision farming often requires blending multiple fertilizer sources—such as urea, monoammonium phosphate, or potash—to reach the ratio that matches your soil test and crop uptake goals. This calculator treats the ratio as a proportional target and solves for the weights of each source in a fixed batch size.

The math is a linear system. Each fertilizer source contributes a percentage of N, P, and K to the final batch. When you multiply those percentages by the unknown weights of each source, you obtain the total pounds or kilograms of each nutrient in the mix. We also introduce a scaling factor that represents the total “parts” of the ratio so the system can satisfy the ratio and the total batch size at the same time. Solving the system gives the exact weights that best match the target ratio using the sources you chose. If the system produces negative weights, it means the selected products cannot exactly satisfy the target ratio—swap a source or adjust the ratio until all weights are realistic.

After solving for the blend, the calculator reports the resulting N-P-K analysis and the deviation from the target. This is critical for agronomy decisions: small deviations may be acceptable for field crops, while hydroponics and controlled-environment agriculture often require tighter tolerances. Always verify your mixing plan against local extension guidance, crop nutrient demand, and product labels, especially when micronutrients or sulfur content are important. The tool is designed to support informed decisions, not replace soil testing.

Formula

Let the target ratio be \(R_N : R_P : R_K\), the batch size be \(W\), and the source nutrient percentages be \(N_i, P_i, K_i\) for sources \(i=1..3\). We solve:

\(w_1 + w_2 + w_3 = W\)

\(N_1 w_1 + N_2 w_2 + N_3 w_3 = R_N \cdot t\)

\(P_1 w_1 + P_2 w_2 + P_3 w_3 = R_P \cdot t\)

\(K_1 w_1 + K_2 w_2 + K_3 w_3 = R_K \cdot t\)

where \(t\) is a scaling factor. The resulting mix analysis is: \(\text{N\%} = 100 \cdot (N_1 w_1 + N_2 w_2 + N_3 w_3)/W\) and similarly for P and K.

Example Calculation

Suppose you want a 4-2-1 ratio in a 50 lb batch using 10-10-10, 0-46-0, and 0-0-60 sources. The solver computes the exact weights of each source that satisfy the ratio while keeping the total batch at 50 lb. The resulting analysis might read around 9.6-4.8-2.4 (by percent), which preserves the 4:2:1 proportion. If the deviation is near zero, your blend is on target; if it is high, consider adjusting the ratio or swapping sources.

FAQs

What does N-P-K ratio mean?

It is the relative proportion of nitrogen, phosphorus (P2O5), and potassium (K2O) in the mix.

Does this calculator work with two sources?

Yes. Set the third source to 0-0-0 to let the solver use only two fertilizers.

Why might I see negative weights?

Negative weights indicate the sources cannot match the target ratio; choose a different blend.

Is this a fertilizer recommendation?

No. It is a math tool; always follow soil tests and local extension guidance.

Are results private?

Yes, everything runs locally in your browser.

How it works

This calculator solves a 4-variable linear system that includes total batch size and ratio scaling. All computation runs client-side for privacy and speed.

Disclaimer

Always follow product labels, soil test results, and local agronomy guidance. This calculator provides a mathematical blend and does not account for micronutrients, soil pH, or crop-specific nutrient timing.