How the sprayer mix is calculated
Herbicide, insecticide and fungicide labels give the rate in one of three ways. The calculator uses the matching formula:
- Per acre:
product per tank = rate per acre × tank size ÷ gallons per acre. - Per 100 gallons:
product per tank = rate × tank size ÷ 100. - Per gallon:
product per tank = rate × tank size.
Example
A herbicide labeled at 32 fl oz per acre, sprayed at 20 gallons per acre with a 4-gallon backpack sprayer: 32 × 4 ÷ 20 = 6.4 fl oz per tank. One acre takes 20 ÷ 4 = 5 tank loads.
Calibrate first: the 1/128 acre method
The number most people get wrong is gallons per acre. A quick way to measure it with a backpack sprayer: mark an area of 340 square feet (for example 18.5 × 18.5 ft), which is 1/128 of an acre. Spray it with clean water at your normal pace and pressure, then measure how many fluid ounces you used. Because there are 128 fluid ounces in a gallon, the fluid ounces you used equal your gallons per acre. Recalibrate whenever you change nozzles, pressure or the person spraying.
Important: the label is the law. Follow its rates, personal protective equipment, re-entry and pre-harvest intervals, and any use restrictions.
Frequently asked questions
How much herbicide goes in a 4-gallon backpack sprayer?
It depends on the label rate and how many gallons per acre you spray. At 32 fl oz per acre and 20 gallons of spray per acre, a 4-gallon tank takes 6.4 fl oz (32 × 4 ÷ 20).
What is spray volume or carrier volume?
It is the gallons of total mix (water plus product) your sprayer puts out per acre. You get it by calibrating: measure how much water you use over a known area and scale it to one acre.
How do I convert pints or quarts per acre to fluid ounces?
1 pint = 16 fl oz and 1 quart = 32 fl oz. A rate of 1.5 pints per acre is 24 fl oz per acre.
What if the label gives the rate per 100 gallons?
Choose "per 100 gallons of water". For a 4-gallon tank, divide the rate by 25: 2 quarts (64 fl oz) per 100 gallons is about 2.6 fl oz per tank.