Product labels state a dose per decare. Your sprayer holds a fixed tank. The number you need at the mixing site is the amount per tank, and that conversion is where most mixing errors happen. Our farm chemical calculator runs one formula and prints its substitution live under the result. This article documents that formula exactly as the code runs it, including the inputs it rejects and the shortcuts it takes.
The formula the tool runs
The component computes two values in sequence, and everything on screen derives from them:
coverage in decares = tank volume / water rate
amount per tank = dose per decare x coverage
The interface states the same thing as Amount to Add = (Tank / Water Rate) x Dose. The division comes first, and the dose multiplies the result. With the shipped defaults of a 500 liter tank, a 50 liter per decare water rate, and a 150 ml dose, the tool computes a coverage of 10 decares and an amount of 1500 ml per tank. The formula card prints the live substitution, so you can verify every run by hand.
Note what the formula does not contain. It has no crop stage term, no adjuvant term, and no concentration correction. If your label states a percentage solution instead of a dose per decare, convert to a per-decare figure first, then enter it.
Coverage decides everything
Coverage answers one question, how many decares one full tank treats. It divides tank volume by water rate, nothing else. Change the water rate and you change the amount per tank, even when the dose stays identical.
The orchard preset demonstrates the effect. It ships a water rate of 80 liters per decare with a 100 ml dose. A 500 liter tank then covers 6.25 decares and takes 625 ml of product. Compare that with the tomato and pepper preset, which uses 50 liters per decare. The same tank covers 10 decares there. Same sprayer, different amount, because the denominator moved.
Water rate is the input you must measure rather than guess. It depends on nozzle size, pressure, and travel speed. If your real consumption is 60 liters per decare and you enter 50, every tank comes out over-concentrated by 20 percent.
The shipped presets, read from the source
The component carries four starter presets. They are input shortcuts that fill the water rate and dose fields when clicked:
1. Tomato and Pepper, 50 liters per decare, 150 ml.
2. Grain and Cereal, 100 liters per decare, 200 ml.
3. Liquid Fertilizer, 50 liters per decare, 500 ml.
4. Orchard, 80 liters per decare, 100 ml.
These are hardcoded convenience values in the source file. They carry no product data, no active ingredient, and no regional calibration. Treat them as shapes for the arithmetic, then replace both fields with your label values before mixing.
Adding field size, tanks and totals
The area field is optional. Leave it blank and the tool stops at the per-tank result. Enter a size in decares and two more values appear:
tanks needed = ceil(field size / coverage)
total chemical = dose x field size
The tool rounds tanks up, because a partial tank still has to be mixed and carried. A 25 decare field at 10 decares per tank returns 3 tanks, with the third one half used. Total chemical multiplies dose by the whole field, 3750 ml in that example, and that is the figure you buy against. The per-tank figure, 1500 ml, is the figure you pour.
Checklist before you mix a tank
- Water rate comes from your own calibration, not from a preset.
- Dose value and unit match the product label exactly.
- The formula card substitution matches your hand calculation.
- Tank count is rounded up and you planned for the partial last tank.
- The unit button shows the unit your label actually uses.
Where the calculator stops
The unit buttons switch the label on the result between ml, g, L, and kg. They do not convert the number. If you compute 1500 ml and press L, the display reads 1500 L. Divide by 1000 yourself before you act on it.
The guard clause hides the result when tank, water rate, or dose is zero or fails to parse. A negative entry survives the parse. With the other two fields positive, the computed amount lands below zero and the card stays hidden. You see nothing instead of a wrong number, which is the safer failure.
The display rounds to one decimal digit, and any value under 0.01 prints as 0. A small tank combined with a large water rate can therefore show a zero amount for a real, tiny dose. One decare is 1000 square meters, and ten decares make one hectare, so convert your field units before entry.
The tool also states its own limit on screen. It is assistance, not guidance. The label on the product decides, and where the two disagree the label wins. In most jurisdictions that is the legal position as well.
Run your own tank and water rate through the
[farm chemical calculator](https://webrecast.com/en/farm-chemical) and check the printed
substitution against your label. If your calibrated water rate differs from any preset, that
difference is exactly the kind of data we want to hear about.
