Water-Soluble Fertilizer Dilution Calculator

Estimate soluble fertilizer mass from target and source-water concentrations.

Inputs

Preset: 5 gal nutrient mix

An arithmetic mixing scenario targeting 100 mg/L of one nutrient with zero starting concentration and a 20% nutrient product. This is not a plant feeding recommendation; use an approved target and product label.

Product mass - Target concentration (mg/L), Starting water concentration (mg/L), Water volume (U.S. gal), Product nutrient fraction (0-1)
Use a target supported by the product label or a trusted source.
Use a water-test result when available.
Enter water volume in U.S. gal.
Enter a decimal from 0 to 1; for example, 0.25 means 25%.
Formula, inputs and conditions

Formula: (Target concentration-Starting water concentration)*Water volume*3.785411784/(1000*Product nutrient fraction)

Decimal results are displayed to two places when needed.

Inputs
Target and starting concentration in mg/L, solution volume in U.S. gallons, and nutrient mass fraction from the label.
Outputs
Fertilizer mass in grams.
Conditions and scope
For dilute aqueous solutions using the mg/L convention, with target at least as high as the starting concentration. It does not recommend crop concentration, compatibility, or spray safety. Results are planning estimates, not engineering, permit, code or contractor-price decisions.

Concentration increase and product mass

A soluble fertilizer calculation starts with the increase in one nutrient concentration. Any concentration already present in the water is subtracted first. The total nutrient mass is then divided by the product’s mass fraction of that nutrient.

Measure and calculate

  1. Enter target and starting concentration for the same nutrient in mg/L.
  2. Enter the water volume in US gallons and product nutrient fraction.
  3. Check that the target is at least the starting concentration before calculating grams of product.
Calculation method

Product grams = (target − starting mg/L) × US gallons × 3.785411784 ÷ 1,000 ÷ nutrient fraction.

A worked example

Inputs
Five US gallons of water start at 0 mg/L; the example target is 100 mg/L using a 20% nutrient product.
Calculation
100 × 5 × 3.785411784 ÷ 1,000 ÷ 0.20 = 9.46352946 g.
Result
9.46 g

The displayed result is 9.46 g of product for this arithmetic scenario. It is not a feeding recommendation and does not model other ingredients or mixing compatibility.

Read the result

Output interpretation and next steps
Output or checkWhat it means for your project
Product mass: gThe calculation uses the difference between target and starting concentration, water volume and the fraction of the selected nutrient.
Concentration boundaryEqual target and starting concentrations give zero added product. A target below the starting level is outside this addition-only calculation; changing concentrations requires a different approach.

Common questions

Why subtract the starting concentration?

The nutrient already in the water contributes to the final target. Ignoring it can overstate the required addition.

Can adding fertilizer reduce a concentration?

Not in this model. A target below the starting concentration is rejected because addition alone cannot achieve that reduction.