The label says 2 litres of product per hectare. Your knapsack holds 16 litres. So how much goes into the tank?
That question trips up more people than you’d think, because the label talks about land and the sprayer talks about water. This herbicide calculator connects the two. Tell it how big the field is, what the label asks for and how much spray your sprayer puts down, and it tells you what to pour into each tank.
How to use the Herbicide Calculator#
- Enter the area. Type the size of the field and pick hectares, acres or square metres.
- Enter the product rate. Copy the rate from the label for your crop and weed stage, in litres or millilitres per hectare or per acre.
- Enter your spray volume. This is how many litres of finished spray your sprayer applies per hectare or acre. If you don’t know it, use the calibration method further down.
- Enter the tank size. Use the real capacity of your sprayer. A typical knapsack holds 15 or 16 litres.
- Read the result. You get the millilitres of product for one full tank, the total product, the total water and the number of tanks.
The maths, in plain words#
Every tank of spray is a mix of water and product, and the ratio between them stays the same from the first tank to the last. The formula is short:
Here’s a real set of numbers. You have 2 hectares to spray. The label asks for 2 litres of product per hectare and your sprayer applies 200 litres of spray per hectare. You’re using a 16 litre knapsack.
- Total spray mix: 2 ha × 200 L/ha = 400 L
- Total product: 2 ha × 2 L/ha = 4 L
- Product per full tank: 16 × 2 ÷ 200 = 0.16 L, which is 160 mL
- Tanks needed: 400 ÷ 16 = 25
- Water: 400 − 4 = 396 L
One knapsack covers 0.08 hectares at that volume. If that sounds small, it is. A hectare is 12.5 knapsacks of spray, which is why spraying by hand is slow.
How to find your spray volume#
This is the number people skip, and it’s the one that matters most. The label gives you a product rate per hectare. It can’t know how fast you walk, how wide your swath is or how worn your nozzle is. Only you can measure that.
The easiest method is a water test. Mark out a small area, say 10 m by 10 m, which is 100 m² or 0.01 ha. Fill the sprayer with plain water and spray the area exactly the way you would spray the field. Then measure how much water went out.
If you used 2 litres on that 100 m², your spray volume is 2 ÷ 0.01 = 200 L/ha. Do it twice and average it. Repeat it whenever you change the nozzle, the pressure or the person doing the spraying, because each of those changes the answer.
Common rates for a 16 litre knapsack#
If your sprayer is calibrated to 200 L/ha, this is how much product goes into one full 16 litre tank:
| Label rate | Product per tank | Product per hectare |
|---|---|---|
| 1 L/ha | 80 mL | 1 L |
| 1.5 L/ha | 120 mL | 1.5 L |
| 2 L/ha | 160 mL | 2 L |
| 3 L/ha | 240 mL | 3 L |
| 4 L/ha | 320 mL | 4 L |
Change the spray volume and every number moves. At 400 L/ha you use half as much product per tank, because the same hectare now needs twice as many tanks.
Watch the units#
Most mistakes are unit mistakes. A label written in litres per acre isn’t the same as litres per hectare. One acre is 0.4047 hectares, so 1 L/acre works out to about 2.47 L/ha. Millilitres per hectare is a thousand times smaller than litres per hectare. The calculator converts between them for you, but only if you pick the right unit next to each number.
Read the label twice before you type anything in. Look for the words “per hectare”, “per acre” or “per 15 litres”. Some labels give a dose per knapsack, and if yours does, you may not need a calculator at all.
Mistakes that cost money (or a crop)#
- Guessing the spray volume. If you assume 200 L/ha and you really apply 120, you’re underdosing by 40 percent without knowing it.
- Going above the label rate “to be safe”. More product doesn’t kill weeds better. It can damage the crop and speeds up resistance. The label rate is a legal limit as well as a recommendation.
- Mixing in a full tank. Put some water in first, add the product, then top up while agitating. Check your label, because some products have their own mixing order.
- Forgetting the last partial tank. Mix a smaller batch with the same ratio rather than topping up with extra water.
Wear the protective equipment named on the label, keep spray away from water sources and never put leftover mix back into a product bottle.
Frequently asked questions#
It depends on the label rate and your spray volume. With a rate of 2 L/ha and a spray volume of 200 L/ha, a 16 litre tank takes 160 mL. The formula is tank size × rate ÷ spray volume.
Multiply the area by the label rate. For 2 hectares at 2 L/ha you need 4 litres of product in total. Then divide the total spray volume across your tanks.
Divide your spray volume by the tank size. At 200 L/ha with a 16 litre tank that’s 12.5 tanks per hectare.
No. The label rate is the maximum tested and permitted dose. Using more risks crop damage, wasted money and faster herbicide resistance.
No. This herbicide dosage calculator is for liquid products that are mixed with water and sprayed. Dry products have their own application rates.