Calibration settles the rate. This lesson is what follows: how far a tank goes, how many tanks a field takes, and what actually goes into each one.

$A = \dfrac{T}{V}$ — the ground one tank covers. $T$ is the usable tank volume in litres and $V$ the application rate in L/ha, so litres over litres-per-hectare leaves hectares. Its mirror is $N = \dfrac{A\,V}{T}$, the tank loads a field requires, where $A$ is now the field area and $N$ a bare count.

$N$ almost never lands on a whole number, and what you do with the fraction is a real decision, not a rounding convention. A partial load rounds UP: 7.5 loads means eight fills, because the hectares riding on that last part tank do not spray themselves. Round down and a strip goes untreated; the growth stage moves on and the second pass never works as well as the first would have.

Now the trap this lesson exists for. A label states a product rate — litres of formulation per hectare — but the residue limit, the re-entry interval and the maximum seasonal load are all written against the active ingredient. The bridge is the jug's own strength: $R_{ai} = c\,R_v$ — R-a-i equals c R-v — where $c$ is the formulation concentration in g of active per litre of product (480 g/L is a common one), $R_v$ the product rate in L/ha, and $R_{ai}$ the active ingredient rate in g/ha. Two products at the same label rate and different strengths put different chemistry on the field, and only one of those numbers is regulated.

Handguns and knapsacks work differently again, because a spot sprayer treats until the target is wet rather than covering a measured area. Their instruction is a concentration: $P = 100\dfrac{V_p}{T}$, where $V_p$ is the product volume, $T$ the finished tank volume — product and water together — and $P$ the mix strength as a percentage. A 2% solution is 2 L of product in a 100 L tank, and it says nothing at all about how much lands per hectare.