The fertilizer injector calculator above answers the single question that governs liquid feeding: how much dry water-soluble fertiliser goes into the stock tank so that an injector set to a known ratio delivers the concentration you are aiming for at the hose end. It uses the standard extension formula, works in either ounces per US gallon or grams per litre, and reports both the per-unit rate and the total for whatever tank size you are actually filling.
Arb Digital publishes this as part of a free set of growing and agriculture tools. The arithmetic is not difficult, but it is easy to get backwards, and the consequences of getting it backwards are expensive. A stock solution mixed for a 1:100 injector and then run through a 1:200 injector delivers half the intended nitrogen; the reverse mistake delivers double. Doing the conversion in one place, with the injector ratio explicit, removes the most common source of that error.
What This Fertilizer Injector Calculator Does
A fertiliser injector is a proportioner plumbed into an irrigation line. It draws a small, fixed fraction of concentrated stock solution into the passing water, so the grower mixes one strong batch and the injector dilutes it continuously. The ratio describes that dilution: at 1:200, one part of concentrate ends up in every two hundred parts of finished solution, so fifteen gallons of stock makes three thousand gallons of feed.
What the ratio does not do is set the strength of the feed. That is decided entirely by how concentrated you make the stock. This page takes your target concentration in parts per million of nitrogen, your injector ratio and the percentage of nitrogen in the product, and returns the weight of fertiliser per unit of stock solution, plus the total for the tank.
A boundary worth stating plainly. Our live fertilizer calculator answers a different question: it converts a target nutrient rate per unit area into the weight of dry granular product to spread on that area. This page never touches area. It works in solution concentration, and it exists because liquid feeding through an injector is measured in ppm, not in pounds per thousand square feet.
How to Use It
- Enter your target ppm of nitrogen. Take the figure from your own crop schedule or from a published extension guide for the crop and growth stage. This tool does not supply a target.
- Enter the injector ratio as the larger number: 100 for 1:100, 200 for 1:200. Variable-ratio injectors should be read at their current setting, not at their maximum.
- Enter the nitrogen percentage from the fertiliser label. The same method works for phosphate or potash if you enter that element's percentage instead and read the answer as ppm of that element.
- Choose your units and enter the size of the stock tank you are mixing into.
- Weigh the fertiliser, never scoop it. Bulk density varies between products and between batches, so a volume measure is not a reliable proxy for a weight.
The Formula and How It's Calculated
The published relationship, set out in the Virginia Cooperative Extension publication on the basics of fertilizer calculations for greenhouse crops, is:
Fertiliser per unit of stock = (target ppm × dilution factor) ÷ (% of the element × C)
The dilution factor is the large number in the injector ratio. C is a conversion constant that carries the units: that publication gives 75 for ounces per US gallon, 1,200 for pounds per US gallon and 10 for grams per litre.
Work the default values. Target 150 ppm N, a 1:200 injector, a 17% nitrogen product, working in ounces per gallon. The numerator is 150 × 200 = 30,000. The denominator is 17 × 75 = 1,275. Dividing gives 23.53 ounces of fertiliser per gallon of stock solution. For a 15-gallon tank that is 353 ounces, or 22.1 pounds, and that tank will produce 15 × 200 = 3,000 gallons of finished feed.
The metric route lands in the same place. With C of 10, the same target gives 30,000 ÷ 170 = 176.5 grams per litre. Converting the imperial answer confirms it: 23.53 ounces is 667 grams, and 667 grams in one US gallon of 3.785 litres is 176 grams per litre.
Why the Injector Ratio Is Not the Feed Strength
This is the misconception the University of Massachusetts extension fact sheet on fertilizer injectors for greenhouses spends most of its space correcting. A setting of 1:100 does not mean the injector delivers 100 ppm. It means one gallon of concentrate becomes 100 gallons of finished solution. Two growers with identical crops and identical targets but different injectors will mix stock solutions of quite different strengths, and both will be right.
The relationship is linear and worth internalising: for a fixed target, doubling the dilution factor doubles the required stock concentration. That has a practical ceiling. Every fertiliser has a solubility limit, and a very high ratio combined with a high target can demand more product than will actually dissolve in cold water. When the number this page returns looks implausibly large, the ratio is usually the reason, and the answer is a lower injector ratio or a second stock tank rather than a stronger mix that will not stay in solution.
Two Stock Tanks and the Precipitation Problem
Growers who mix their own fertiliser from individual salts, rather than using a blended water-soluble product, generally run two stock tanks. The reason is chemical: calcium nitrate mixed at concentrate strength with a phosphate or a sulphate source precipitates calcium phosphate or gypsum, which drops out of solution as a sludge, blocks the injector and removes both nutrients from the feed. Kept in separate tanks and injected separately, the two never meet at concentrate strength, and by the time they mix in the irrigation line they are dilute enough to stay in solution.
This page handles one tank at a time, which is the right way to use it in that situation: run the calculation once for each tank, using the element percentage and target that applies to that tank. If you are working with a pre-blended product designed to be mixed in a single tank, one run is enough. Our solution dilution calculator and dilution ratio calculator are useful for the related bench-scale arithmetic when you are preparing trial batches.
Verifying the Feed Rather Than Trusting It
The calculation gives you what the injector should deliver. What it actually delivers depends on the injector working correctly, and injectors drift. Diaphragms wear, suction lines develop leaks, strainers clog and the draw rate falls, and a proportioner running outside its rated flow or pressure range will not hold its stated ratio.
The standard check is an electrical conductivity meter at the emitter. Measure the EC of the incoming water alone, measure the EC of the finished feed, and the difference is the contribution of the dissolved fertiliser. Compare that difference against the value your product's label gives for the target concentration. A large discrepancy points to the injector, the stock tank or the mixing, and finding it with a meter costs minutes; finding it through crop symptoms costs weeks. Extension guidance consistently pairs the ppm calculation with an EC check for exactly this reason.
Water Chemistry the Calculation Cannot See
The formula assumes the only nitrogen in the finished solution is the nitrogen you added. Irrigation water frequently carries dissolved nutrients of its own — nitrate from agricultural runoff, calcium and magnesium as hardness, sodium and chloride in some regions — and carries alkalinity that reacts with the acidifying or basifying tendency of the fertiliser you have chosen. A high-alkalinity source water fed with a basic fertiliser will drift the root zone upward in pH over weeks, and no ppm calculation will show it.
The practical response is a laboratory water test before the feed programme is designed, repeated if the source changes. That test tells you what is already in the water, and the ppm you calculate here is then correctly understood as an addition to it rather than the whole story. Growing environment matters too: transpiration drives how much solution a crop actually takes up, which is why the vapour pressure deficit calculator and the daily light integral calculator belong alongside this one in a greenhouse toolkit.
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Browse All Free Tools Contact Arb DigitalCommon Mistakes to Avoid
- Treating the injector ratio as the ppm delivered. The ratio sets dilution; the stock concentration sets strength.
- Measuring fertiliser by volume. Bulk density varies by product and batch, so scoops are unreliable. Weigh it.
- Mixing calcium and phosphate or sulphate sources in one concentrate tank, which precipitates and blocks the injector.
- Using the total analysis instead of the element percentage. A 17-5-24 product is 17% nitrogen, not 46% of anything.
- Never checking the delivered strength with an EC meter, so a worn or leaking injector goes unnoticed for weeks.
Related Free Tools From Arb Digital
For dry granular products applied by area, use the fertilizer calculator. On the solution side, the solution dilution calculator, the dilution ratio calculator and the concentration converter cover the related bench arithmetic. For the growing environment, see the vapour pressure deficit calculator and the daily light integral calculator. Container and tank sizing is handled by the soil volume calculator and the tank volume calculator. Browse the full free online tools hub for more.
Frequently Asked Questions
Multiply the target ppm by the injector's dilution factor, then divide by the element percentage multiplied by a units constant. Virginia Cooperative Extension gives that constant as 75 for ounces per US gallon, 1,200 for pounds per US gallon and 10 for grams per litre.
No. The ratio describes dilution only: one part of concentrate becomes 100 parts of finished solution. The concentration delivered depends entirely on how much fertiliser you dissolved in the stock tank.
It is the concentrated fertiliser mixture held in a tank that the injector draws from. The injector meters a small fixed fraction of it into the irrigation line, so one tank of concentrate supplies a large volume of finished feed.
Because calcium sources and phosphate or sulphate sources precipitate when mixed at concentrate strength, forming a sludge that blocks the injector and removes nutrients from the feed. Kept in separate tanks, they only meet once diluted in the line.
Yes. Enter the percentage of that element as shown on the label and read the target and the result as ppm of that element. The formula is identical; only the percentage you feed it changes.
Measure the electrical conductivity of the source water and of the finished feed at an emitter, and compare the difference against the value the fertiliser label gives for that concentration. A large gap points to a worn diaphragm, a leaking suction line or a clogged strainer.
Yes, and this calculation does not include it. A laboratory water test tells you what nutrients and alkalinity the source already carries, and the figure calculated here should be understood as an addition to that starting point.
This calculator performs the published extension arithmetic for injector stock solutions and is provided for planning purposes. It does not recommend a feed rate. Take target concentrations from your own crop schedule or a local extension service, and have your irrigation water tested by a laboratory before designing a feed programme.
