Knapsack Spray Mix Calculator NZ
This works out how much product goes into a knapsack sprayer, how many tank loads a job takes and how much product you need in total, using the rate printed on your own label rather than any rate supplied here. It exists because spray labels state rates in two quite different ways and mixing them up is the most common spraying mistake there is. A per hectare rate, such as 2 litres a hectare, is an amount of product spread over an area, and turning it into an amount per tank depends on knowing how much ground a tank covers. A concentration, such as 100 millilitres per 100 litres of water, is an amount per volume of water and does not depend on area at all. Read one as the other and the mix can be out by a factor of several, in either direction. This page asks you which kind of rate your label gives and does the right arithmetic for that kind. Underneath it is a calibration helper, because a per hectare rate is only as good as your water rate: nozzle wear, pressure and walking pace all move it, so measuring what you actually apply over a known area beats assuming a figure. Nothing here prescribes a product or a rate. That comes from the label, and the label is the legal instruction.
The last tank is a part load of about 10 L, so it takes proportionally less product rather than a full tank worth. Mix only what you will use. Measure the product into a part-filled tank, top up with water and agitate, unless the label says otherwise.
Fill the knapsack with plain water, spray a measured area at your normal pace and pressure, then measure how much you used. That gives you the water rate you are actually applying, which is often nothing like the one assumed.
Type this figure into the water rate above if it differs from what you assumed.
How it works
Total water is your area in hectares multiplied by your water rate, and the number of tank loads is that divided by the tank volume, rounded up. What happens next depends on which kind of rate your label gives.
A rate per hectare
One tank covers the tank volume divided by the water rate, in hectares. Multiply that area by the hectare rate and you have the product for a full tank. Total product is the whole area multiplied by the hectare rate. The last tank is usually a part load, so it takes proportionally less product, and pouring a full tank's worth into it is a real way to overdose a corner of a paddock.
A concentration per 100 litres
This one ignores area entirely. Product for a tank is the concentration multiplied by the tank volume and divided by 100 litres, and total product is the same scaled to the total water. Concentrations are common on labels aimed at handheld and knapsack use, and on spot spraying instructions, where you spray to wet rather than to a measured area.
Worked example
A 2,000 square metre paddock corner is 0.2 hectares. At a water rate of 200 litres a hectare that is 40 litres of water, which is three loads of a 15 litre knapsack: two full and one part load of 10 litres. A full 15 litre tank covers 750 square metres, or 0.075 of a hectare, so at a label rate of 2 litres a hectare each full tank takes 150 millilitres of product. The whole job takes 400 millilitres. Had the same 2 litres been read as a concentration instead, you would have put 300 millilitres in the first tank and used well over the whole job's product before finishing it.
Related calculators
- Dilution Ratio: mixing to a ratio rather than a rate.
- Acres to Hectares: getting the area into the right unit.
- Stock Units: what the paddock you are spraying will carry.
- Lifestyle Block Stocking: sizing stock to a small block.