Fertigation Calculator
Calculate fertilizer injection rates, stock solution concentration, and nutrient ppm for fertigation systems.
About this calculator
Fertigation calculators exist to answer two coupled questions at once: what concentration of nutrient should be flowing through your irrigation water, and at what rate should your injector be pumping fertilizer to hit that concentration? This calculator starts from your target application rate in pounds of nutrient per acre and your field size to get total nutrient pounds needed, then divides by the fertilizer product's nutrient percentage (so a 46%-nitrogen product like urea requires more than twice the product weight of pure nitrogen) to get total product pounds, and by product density to convert that into gallons of liquid product. Separately, it computes the target concentration in parts per million by comparing total nutrient mass to the total volume of irrigation water moving through the system during the run — this is the number that matters for avoiding fertilizer burn or under-fertilizing.
The injection rate in gallons per hour comes from your injector ratio (the fraction of system flow the injector pulls as stock solution), and the calculator backs out the stock solution's required concentration in pounds of product per gallon so the full batch is injected exactly within your irrigation run time. The rough EC (electrical conductivity) increase estimate assumes about 700 ppm of dissolved salts raises conductivity by 1 mS/cm — a commonly used rule of thumb, not a lab-measured constant, so treat it as a sanity check rather than a precise reading. As always, injector ratio, product density, and nutrient percentage should come from your actual equipment and label, since small errors compound directly into over- or under-application.
Inputs
Results
Product needed (lbs)
217.4
Target concentration (ppm)
1,997.1
How to Use This Calculator
- Enter your irrigation system flow rate in GPM and field area in acres.
- Set the target nutrient application rate in lbs per acre (e.g., lbs of nitrogen).
- Enter fertilizer nutrient content percentage (e.g., urea = 46% N) and product density in lb/gal.
- Set the injector ratio (%) and total irrigation run time in minutes.
- Read Product needed (gallons), Target concentration (ppm), and Injection rate (GPH) to configure your injector.
How the result changes with Nutrient content (%)
| Nutrient content (%) | Product needed (lbs) | Target concentration (ppm) |
|---|---|---|
| 23 | 434.8 | 1,997.1 |
| 35 | 285.7 | 1,997.1 |
| 69 | 144.9 | 1,997.1 |
| 100 | 100 | 1,997.1 |
What each input means
- System flow (GPM)
- Irrigation system flow rate in gallons per minute.
- Target rate (lb N/acre)
- Desired nutrient application rate per acre (e.g., lbs of nitrogen).
- Field area (acres)
- Total irrigated area receiving fertigation.
- Nutrient content (%)
- Percent nutrient in fertilizer product. Urea=46% N, UAN-32=32% N.
- Injection ratio (%)
- Injector ratio (stock solution ÷ system flow). Typical: 0.5-2%.
- Irrigation run time (min)
- Total irrigation set duration in minutes.
- Product density (lb/gal)
- Liquid fertilizer density. UAN-32=11.06, UAN-28=10.65, water=8.34.
What each result means
- Product needed (lbs)
- Total liquid fertilizer product needed.
- Product needed (gal)
- Total product volume in gallons.
- Target concentration (ppm)
- Nutrient concentration in irrigation water.
- Stock injection rate (GPH)
- Gallons per hour of stock solution injected.
- Product rate (GPH)
- Gallons per hour of undiluted product to inject.
- Stock concentration (lb/gal)
- Lbs of product per gallon of stock solution.
- Est. EC increase (mS/cm)
- Rough estimate of electrical conductivity increase.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersSystem flow (GPM) = 50, Target rate (lb N/acre) = 20, Field area (acres) = 5, Nutrient content (%) = 46 = 7 input(s) provided
- Calculate Product neededProduct needed = totalNutrientLbs / nutrientFraction217.4 = 217.4
- Calculate Target concentrationTarget concentration = (totalNutrientLbs * 453592) / (totalWaterGallons * 3.7854)1997.1 = 1997.1
- Calculate Product neededProduct needed = totalProductLbs / productDensityLbPerGal20.02 = 20.02
- Calculate Stock injection rateStock injection rate = systemFlowGPM * 60 * injectorRatio30 = 30
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What's the difference between the target ppm and the injection rate?
Target concentration (ppm) is how much nutrient should end up dissolved in the irrigation water flowing to the crop — it's computed from total nutrient mass divided by total water volume during the run. Injection rate (GPH) is how fast the injector pump physically pulls stock solution into that water stream, based on your injector ratio and system flow. Both have to be right together: the ppm tells you the target, the injection rate and stock concentration tell you how to hit it.
How does the injector ratio affect the required stock solution concentration?
Injector ratio sets the injection rate directly (system flow × injector ratio), which fixes how many gallons of stock solution get injected over the run time. Since the total pounds of product needed is fixed by your target rate and field size, a smaller injector ratio means less stock volume is injected, so that stock has to be mixed more concentrated (more lbs of product per gallon) to deliver the same total nutrient load.
Why does the calculator call its EC increase estimate 'rough'?
It converts target ppm to electrical conductivity using a single rule-of-thumb ratio of about 700 ppm of dissolved salts per 1 mS/cm — but the actual EC contribution of a fertilizer depends on the specific salts in that product, not just its total ppm. Two fertilizers at the same ppm can raise EC by different amounts, so this output is meant as a sanity check, not a substitute for a real EC meter reading.
Why do I need to enter both field acreage and system flow rate?
Field acreage combines with your target application rate to determine total pounds of nutrient needed, which drives the stock concentration calculation. System flow rate combines with irrigation run time to determine total water volume, which drives the target ppm calculation. They answer separate questions — 'how much fertilizer total' versus 'how concentrated should it be in the water' — and the calculator needs both to solve for injection settings.
Related Calculators
The questions that sit next to this one — chosen by subject, including calculators filed under a different category.
Fertigation Injection Rate Calculator
Injector ratio and PPM from stock solution and flow.
Precision IrrigationChemigation Calculator
Calculate fertigation and chemigation injection pump rates, product volumes, and costs for applying chemicals through irrigation systems.
Unit Conversion ExpandedConcentration Converter
ppm, ppb, mg/L, and mass percent for dilute aqueous solutions (ρ ≈ 1 kg/L).
IrrigationDrip Irrigation Design Calculator
Calculate emitter count, system flow rate, zone sizing, and daily run time for drip irrigation systems.
IrrigationSprinkler System Design Calculator
Calculate precipitation rate, head count, zone sizing, and run times for sprinkler irrigation systems.
More in Agriculture & Farming.