Herbicide Mixing Calculator

The two label numbers that set your mix

A herbicide label gives you a rate and a carrier volume, and the two only make sense together. The rate is how much product covers a given area, written as fluid ounces per 1,000 square feet or quarts per acre. The carrier volume is how much water the label expects that area to be sprayed with, usually 10 to 30 gallons per acre for a boom sprayer or 2 to 4 gallons per 1,000 square feet for a hand sprayer.

Divide one by the other and you get the strength of the jug:

product per gallon = label rate ÷ carrier volume per area

Take a label that lists 1.5 fluid ounces per 1,000 square feet with a carrier of 2 gallons per 1,000 square feet. The mix is 1.5 ÷ 2 = 0.75 fluid ounces of product for every gallon of water. That is the number the calculator puts at the top of the results, because it is the one you use at the jug.

Two things trip people up. First, the rate is per unit of product, not per unit of active ingredient, so a 41 percent concentrate and a 3.8 pound per gallon amine are not interchangeable even when both say 2,4-D on the front. Second, the carrier volume is a promise about your sprayer. If your nozzle puts out 3 gallons per 1,000 square feet instead of 2, the same 1.5 ounces now lands on less ground and the rate on the ground goes up. The EPA pesticide label requirements put the rate and the application directions on the label for exactly this reason, and the label outranks anything a calculator says.

Multiply a per 1,000 square feet rate by 43.56 to get the per acre figure, because one acre holds 43.56 units of 1,000 square feet. Going the other way, divide the acre rate by 43.56. Getting this wrong by a factor of 43 is the difference between a light spray and a dead lawn.

Worked example: a 4 gallon backpack on 2,000 square feet

You have a 4 gallon backpack sprayer, a fence line of about 2,000 square feet of gravel and weeds, and a label rate of 1.5 fluid ounces per 1,000 square feet in 2 gallons of water per 1,000 square feet.

Start with the area. Two thousand square feet is two units of 1,000, so the product is 1.5 x 2 = 3 fluid ounces and the water is 2 x 2 = 4 gallons. The strength is 3 ÷ 4 = 0.75 fluid ounces per gallon, the same figure as before, which is the check that the mix and the area agree.

Fill in this order: put 2 gallons of water in the tank, add the 3 ounces of product, then top up to the 4 gallon line. Water first keeps the concentrate from sitting against a dry tank floor, and topping up after the product is in gives the mix a second chance to blend as the tank fills.

Now the coverage question. Four gallons at 2 gallons per 1,000 square feet covers 4 ÷ 2 = 2 units, which is 2,000 square feet, so one tank is the whole job. Walk at the pace you will keep for the rest of the job while you spray, because the carrier volume only holds if the walking speed and the nozzle stay the same. A slower pass wets the ground more and quietly raises the rate. If you run out of mix halfway through, stop and recalibrate rather than thinning the rest with water.

One more check before you start: 3 fluid ounces is 6 tablespoons, which is easy to measure with a kitchen measure and hard to eyeball in a jug cap. Pour over a bucket, not over the tank, so a slip does not end up in the mix.

Acres, tank size and how many loads the job takes

Larger jobs switch to acres, and one acre is 43,560 square feet. A pasture label might read 1 quart per acre in 20 gallons of water per acre. Convert the quart first: 1 quart is 32 fluid ounces, so the strength is 32 ÷ 20 = 1.6 fluid ounces per gallon. A 20 gallon tank then holds 20 x 1.6 = 32 ounces, which is exactly one acre of product, so one full tank is one acre of ground.

The table below runs a 1.5 ounce per 1,000 square feet rate at 2 gallons per 1,000 square feet across the areas people spray in a season, and adds the number of 4 gallon tank loads.

The last two rows are the argument for a bigger tank or a boom sprayer. Twenty two backpack loads is a long day, and every refill is a chance to measure wrong or drift off the rate. A 25 gallon tank at the same strength holds 25 x 0.75 = 18.75 ounces of product and covers 25 ÷ 2 = 12.5 units of 1,000 square feet, which is 12,500 square feet in one pass.

Round tank loads up, never down. If the math says 3.2 loads, plan for 4 and mix the last one at the full strength, because a partly filled tank at full strength still sprays the right rate while a weak tank does not.

Why one rate does not carry over to another jug

Herbicide labels look alike and are not alike. A 41 percent glyphosate concentrate and a 3.8 pound per gallon 2,4-D amine can both list a quart per acre, and that number means different amounts of active ingredient on the ground. Mixing by copied rate is how people end up with burnt turf, dead shrubs and a call to the county extension office.

The label also encodes the legal limits. Maximum single application rate, maximum yearly rate, the growth stage of the weed, the temperature range, the buffer distance from water, and whether the product is restricted use all sit in the same document as the mixing directions. The EPA pesticide safety pages spell out why the label is the legal instruction sheet and not a suggestion.

There is a second reason rates differ: formulation. An ester and an amine of the same active ingredient behave differently in hard water and in heat, and one may need a surfactant while the other already has one built in. Adding a surfactant to a product that is already loaded can push the mix past the label rate and burn the leaf before the herbicide does its work.

Practical rule: one product, one label, one mix at a time. If you are spot spraying two different weeds with two different products, use two tanks or two passes. Tank mixing is legal only when the labels allow it, and the label is where that permission is written.

Surfactants, water and getting an even spray

Most post emergent labels call for a nonionic surfactant at 0.25 percent of the finished mix by volume. One gallon is 128 fluid ounces, so 0.25 percent is 128 x 0.0025 = 0.32 fluid ounces, which is about 2 teaspoons or a bit over half a tablespoon. A 4 gallon tank takes 4 x 0.32 = 1.28 fluid ounces.

Water quality changes how well any of it works. Hard water ties up amine formulations, and a water conditioner is often on the label for that reason. Fill from a source you can describe, and if you use a hose, fill the tank before adding product so the hose does not carry concentrate back into the supply.

Coverage matters more than strength for leaf contact products. A coarse spray that runs off the leaf wastes product and puts it in the soil. A fine spray that drifts puts it on your neighbor's roses. The label usually names a nozzle type and a pressure range, and staying inside that range is what keeps the carrier volume where you calculated it.

Spray when the weeds are growing, the air is calm and the temperature is inside the label range. Many products lose effectiveness above 85 degrees and volatilize, which means the vapor moves off target. Early morning after the dew lifts is usually the calmest window, and it also gives the leaf time to take up the mix before the sun hits it.

Mixing order, cleaning and the safety basics

Fill the tank halfway with water, add the product, then any surfactant, then top up. That order keeps dry formulations from clumping and keeps surfactants from foaming before the product is dispersed. Measure product in a dedicated measuring cup, never in the cap of a drink bottle, and keep that cup with the sprayer so it never gets near the kitchen.

Wear what the label lists. Gloves, eye protection and long sleeves are the minimum for most concentrates, and the label's first aid section tells you what to do for skin contact or a splash in the eye. Mix outdoors or in a ventilated space, and never in a garage with the door closed.

Clean the tank the same day. Triple rinse means three separate rinses with a small amount of water each time, pouring the rinse water onto the treated area, not down the driveway. Rinse the nozzle and the screen too, because dried product in a screen changes the spray pattern on the next job and quietly changes the rate.

Store the concentrate in its original container with the label readable, out of reach of children and pets, and away from anything that could freeze. Leftover mix should be sprayed on labeled ground rather than poured out, and if you have more mix than ground, you mixed too much. Mix what the job needs, which is what the calculator is for.

When a label is unclear or two products look interchangeable, the National Pesticide Information Center publishes plain language summaries of active ingredients and their registered products. It is a university run service funded with EPA support, and it is a faster answer than a forum thread.

Tank sizes at a glance

The last table is the one to keep in the truck. It assumes a 0.75 ounce per gallon mix, which comes from a 1.5 ounce per 1,000 square feet rate at 2 gallons per 1,000 square feet.

A 15 gallon tank covers 7,500 square feet, which clears a 5,000 square foot job in one load with mix to spare. That spare is the reason to mix 20 percent less than the table says when the job is smaller than the tank, rather than filling to the top and dumping the rest.