Chemigation Calculator
Calculate fertigation and chemigation injection pump rates, product volumes, and costs for applying chemicals through irrigation systems.
About this calculator
This calculator works backward from a target nutrient rate to the exact pump setting needed to inject the right amount of fertilizer or chemical during an irrigation event. It starts by multiplying your target rate (lb of active nutrient per acre) by field acres to get total nutrient needed, then divides by the product's active-ingredient concentration to get total product weight, and converts that to gallons using product density. The injection window isn't the full irrigation run time — the engine subtracts a start delay (time for the system to fill and reach the field before injection begins) and an end flush (time to clear chemical out of the lines before shutdown) from total run time, so the pump only has to deliver product during the effective injection hours. Dividing product volume by that effective window gives the injection rate in gallons per hour, which is also converted to mL/min for calibrating smaller peristaltic pumps.
Separately, the calculator estimates the resulting concentration in the irrigation water in parts per million, using total water volume (system flow × 60 × run time) and the standard 8.34 lb/gallon water-weight conversion — useful for checking against label-rate limits or crop tolerance thresholds. A key thing to watch: the ppm calculation is a simplified mass-balance approximation, not an exact chemistry model, and it assumes product mixes uniformly and instantly with the water stream. It also assumes constant system flow throughout the run — real center-pivot or block irrigation flow can vary as the system starts up or as zones change, which would shift the actual delivered rate from what's calculated here.
Inputs
Results
Product volume (gal)
1,059.6
Injection rate (GPH)
110.37
How to Use This Calculator
- Enter System flow rate (GPM), Field size (acres), and Target rate (lb nutrient/acre).
- Set Product concentration (%), Product density (lb/gal), and Irrigation run time (hr).
- Adjust Start delay (%), End flush (%) as needed.
- Review Product volume (gal) and Injection rate (GPH).
- Use Total nutrient needed (lb) and Total product needed (lb) to inform your decision.
How the result changes with Product concentration (%)
| Product concentration (%) | Product volume (gal) | Injection rate (GPH) |
|---|---|---|
| 16 | 2,119.1 | 220.74 |
| 24 | 1,412.7 | 147.16 |
| 48 | 706.4 | 73.58 |
| 80 | 423.8 | 44.15 |
What each input means
- System flow rate (GPM)
- Total irrigation system flow rate in gallons per minute.
- Field size (acres)
- Area being treated in this irrigation event.
- Target rate (lb nutrient/acre)
- Desired application rate of active nutrient per acre.
- Product concentration (%)
- Active ingredient concentration in the product. UAN-32 = 32%, 10-34-0 = 34% P2O5.
- Product density (lb/gal)
- Weight of product per gallon. UAN-32 = 11.06, 10-34-0 = 11.65.
- Irrigation run time (hr)
- Total duration of the irrigation event.
- Start delay (%)
- Percentage of run time before starting injection (to fill system).
- End flush (%)
- Percentage of run time after injection stops (to flush lines).
- Product cost ($/gal)
- Cost per gallon of fertilizer or chemical product.
What each result means
- Total nutrient needed (lb)
- Total pounds of active nutrient for the field.
- Total product needed (lb)
- Pounds of product to supply the required nutrient.
- Product volume (gal)
- Gallons of product to inject.
- Injection window (hr)
- Effective injection duration after start delay and end flush.
- Injection rate (GPH)
- Required injection pump setting in gallons per hour.
- Injection rate (mL/min)
- Injection rate for small pump calibration.
- Dilution ratio (water:product)
- Ratio of irrigation water to injected product.
- Concentration in water (ppm)
- Final nutrient concentration in irrigation water.
- Product per acre (gal/acre)
- Gallons of product applied per acre.
- Total product cost ($)
- Total cost of chemical/fertilizer product.
- Cost per acre ($)
- Product cost per treated acre.
- Water applied (in)
- Total irrigation depth applied in inches.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersSystem flow rate (GPM) = 800, Field size (acres) = 125, Target rate (lb nutrient/acre) = 30, Product concentration (%) = 32 = 9 input(s) provided
- Calculate Product volumeProduct volume = totalProductLbs / productDensityLbPerGal1059.6 = 1059.6
- Calculate Injection rateInjection rate = productVolumeGal / effectiveInjectionHr110.37 = 110.37
- Calculate Total nutrient neededTotal nutrient needed = targetRateLbPerAcre * fieldAcres3750 = 3750
- Calculate Total product neededTotal product needed = totalNutrientLbs / concentrationFraction11718.8 = 11718.8
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
Why does the injection window exclude the start delay and end flush time?
The pump can't inject product the instant irrigation starts or right up until it stops — the start delay accounts for the time it takes the system to fill and reach the field, and the end flush accounts for the time needed to clear chemical out of the lines before shutdown. The calculator subtracts both from total run time to get the effective injection hours, then divides total product volume by that shorter window — which is why the injection rate is always somewhat higher than product volume divided by full run time.
How is the concentration in irrigation water (ppm) calculated?
It's a mass-balance approximation: total product weight times its active-ingredient fraction times one million, divided by total water volume (system flow × 60 × run time) times 8.34 lb per gallon of water. This gives a useful estimate for checking against label-rate or crop-tolerance limits, but it assumes the product mixes uniformly and instantly with the water stream rather than modeling real-world mixing dynamics.
Why does the calculator report both GPH and mL/min for injection rate?
Gallons per hour (GPH) is the standard sizing unit for larger agricultural injection pumps, while mL/min is how smaller peristaltic or metering pumps are typically calibrated. The calculator converts the same GPH figure to mL/min using the fixed 1 GPH = 63.09 mL/min relationship, so you can read off whichever unit matches your actual equipment.
What happens to the results if I change the product concentration?
Product concentration sets how much of the product's weight is active nutrient, so a lower concentration means more total product (and more gallons) are needed to deliver the same target nutrient rate per acre — total product weight is total nutrient needed divided directly by the concentration fraction. That higher volume flows through to a higher injection rate, higher product cost, and a higher resulting ppm concentration in the water, all else held equal.
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