Calculate how much fertilizer to add to your injector stock tank from a target ppm, fertilizer analysis, and 1:X ratio, or verify your injector’s true ratio.
Fertilizer Injector Ratio Formula
OZ = (PPM * R) / (N * 74.891517...)
GL = (PPM * R) / (N * 10)
AR = VD / VS
- OZ is mass ounces of dry soluble fertilizer per US gallon of final stock solution; dissolve and then fill to the volume mark
- GL is the grams of fertilizer to dissolve per liter of stock solution
- PPM is the target nutrient concentration at the hose end after dilution (mg/L)
- R is the X in a 1:X injector ratio (a 1:100 injector has R = 100)
- N is the nutrient percentage from the fertilizer label (20 for a 20-20-20 fertilizer)
- 74.891517… uses exact mass-ounce and US-gallon definitions; 75 is the customary rounded greenhouse coefficient. One mass oz per US gal is 7.4891517… g/L.
- 10 is the metric constant relating 1 g/L to 1,000 ppm
- AR is the actual injector ratio measured during a calibration test
- VD is the volume of diluted output collected, and VS is the volume of stock solution drawn, both in the same units
The calculator uses these formulas in three modes. The stock mix mode solves the first formula (or its metric version) for the amount of fertilizer to dissolve in the stock tank, and multiplies by tank size to get the total batch weight. The delivered ppm mode rearranges the same formula to check what concentration an existing mix sends to your plants. The calibration mode divides the diluted output volume by the stock volume the injector actually drew, converts both to milliliters internally, and compares the measured ratio against the rated 1:X setting to report ratio deviation and the separate reciprocal concentration error. A user-entered manufacturer tolerance can be applied; there is no universal 5 percent service threshold.
Common Injector Ratios and Stock Strengths
This table uses the stock-to-finished-volume convention and exact US unit conversions. It converts example injector settings between ratio and percentage form and shows how much 20 percent nitrogen fertilizer each setting requires in the stock tank to deliver 100 ppm N at the hose end. Higher ratios need a stronger stock, which is why high-dilution injectors can push low-analysis fertilizers past their solubility limit.
| Injector ratio | Injection % | Stock is stronger by | oz/gal stock (100 ppm N, 20% N) | g/L stock (100 ppm N, 20% N) |
|---|---|---|---|---|
| 1:16 | 6.25% | 16x | 1.07 | 8.0 |
| 1:32 | 3.13% | 32x | 2.14 | 16.0 |
| 1:50 | 2% | 50x | 3.34 | 25.0 |
| 1:100 | 1% | 100x | 6.68 | 50.0 |
| 1:128 | 0.78% | 128x | 8.55 | 64.0 |
| 1:200 | 0.5% | 200x | 13.35 | 100.0 |
Interpreting a Calibration Test
Compare the measured ratio against the manufacturer’s specified conditions and tolerance. A lower actual X means more stock per finished volume; a higher X means less. Ratio deviation = (actual X / rated X – 1) * 100, while concentration error for unchanged stock = (rated X / actual X – 1) * 100. The two percentages are not equal. The table gives example checks, not fault diagnoses or universal acceptance limits.
| Deviation from rated ratio | What it means | Action |
|---|---|---|
| Within 2% | Small measured ratio deviation; acceptance depends on the product specification | Follow the manufacturer’s tolerance and test interval |
| 2% to 5% | Measured ratio drift; cause is not established | Compare with the specified tolerance and repeat the test |
| More than 5% overfeeding (negative) | More concentrate drawn per finished volume than the rating implies | Confirm repeated measurements and follow manufacturer service guidance |
| More than 5% underfeeding (positive) | Less concentrate drawn per finished volume than the rating implies | Check operating conditions and product guidance, then retest |
| Erratic between runs | Variable measured ratio; pressure, measurement and equipment effects need checking | Test at stable pressure; service if it persists |
Example Problems
Example 1: How much 20-20-20 for 150 ppm N through a 1:100 injector?
You want a fertilizer contribution of 150 mg/L nitrogen, use a 20 percent N dry soluble fertilizer, a 1:100 stock-to-finished-volume ratio, and prepare 30 US gallons of final stock. Exact unit conversion gives OZ = (150 * 100) / (20 * 74.891517…) = 10.014485 oz per US gallon. Total dry mass is 300.434560 oz, or 18.777160 lb. Dissolve, then fill the complete solution to 30 US gallons. The stock nutrient concentration is 15,000 mg/L and the injection fraction is 1 percent. Using the customary rounded constant 75 gives the nearby planning approximation 10 oz/gal and 18.75 lb; the calculator uses exact definitions. Check the product’s solubility, compatibility and crop requirements.
Example 2: Is the injector still drawing at its rated ratio?
You collect 10 US gallons of finished output while 13.5 US fluid ounces of stock are drawn. The measured ratio is exactly 1280 / 13.5 = 1:94.814815 after consistent unit conversion. Against a rated 1:100, ratio deviation is -5.185185 percent. The same stock would deliver 5.46875 percent more nutrient than expected at 1:100, excluding background nutrients. This is outside an example user-selected 5 percent ratio tolerance, but the appropriate acceptance and service criteria come from the manufacturer.
FAQ
What does a 1:100 injector ratio mean?
A 1:100 ratio means the injector delivers 1 part of concentrated stock solution for every 100 parts of finished water leaving the hose. Put differently, 1 gallon of stock produces 100 gallons of diluted feed water. Because of that dilution, the stock tank must be mixed 100 times stronger than the concentration you want the plants to receive. A 1:100 setting is the same as a 1 percent injection rate.
How do I convert an injector percentage to a ratio?
Divide 100 by the percentage. A 2 percent injector is 100 / 2 = 1:50, a 1 percent injector is 1:100, and a 0.5 percent injector is 1:200. Going the other way, divide 100 by the X in 1:X to get the percentage. Some injectors, such as many Dosatron models, display percentage on the dial while feed charts list ratios, so this conversion comes up constantly when matching a chart to your equipment.
Why does my EC meter reading not match the calculated ppm?
An EC meter measures conductivity from all dissolved ions, not nutrient-specific nitrogen mg/L. Compare the fertilizer contribution after subtracting the source-water EC against a product-specific EC chart. Generic TDS conversions, including 500 or 700 scales, do not turn conductivity into nitrogen ppm. Differences can also arise from mixture preparation, water chemistry, incomplete dissolution and actual injector ratio. A volume calibration checks the dilution ratio; it does not identify all causes of an EC mismatch.
