Roundup Mixing Calculator

How to read the mixing rate on your label

Every glyphosate product is different, and the mixing rate is printed on the bottle. There is no single universal ratio, because a product that is 18% glyphosate needs roughly three times as much liquid per gallon as one that is 50.2%. This calculator takes whatever rate you read off your label and does the arithmetic; it does not decide the rate for you.

Three numbers on the label do the work. The rate tells you how much concentrate goes into each gallon of water. The coverage tells you how much ground a gallon of mixed spray covers. The rainfast statement tells you how long the product needs to stay dry. Write the rate down before you start, because the label is hard to read once you have gloves on.

Worked example: a 1-gallon sprayer

The most common setup is a 1-gallon pump sprayer and a concentrate whose label calls for 6 fluid ounces per gallon of water.

• Concentrate: 6 fluid ounces, which is 12 tablespoons or 36 teaspoons
• Water: 1 gallon
• Mixed spray you end up with: about 1.05 gallons
• Ground it covers: about 300 square feet at a normal walking pace

Fill the sprayer about half full with water first, add the concentrate, then top up to the gallon mark. Adding concentrate to a nearly empty tank is how people get a face full of spray, and it also mixes better with water underneath it.

At 6 fluid ounces per gallon, a 32-ounce bottle of concentrate makes about 5.3 gallons of mixed spray. That is roughly 1,600 square feet of driveway, fence line or gravel, which is worth knowing before you buy a bottle bigger than you can use in one season.

Worked example: covering 1,000 square feet

Now a bigger job: 1,000 square feet of gravel driveway, a 1-gallon sprayer, and the same 6 fluid ounces per gallon at 300 square feet per gallon.

• Mixed spray needed: 1,000 divided by 300 = 3.33 gallons
• Tanks to mix: 4 tanks of 1 gallon
• Concentrate for the whole job: 3.33 x 6 = 20 fluid ounces
• Water: about 3.33 gallons

Mixing 4 tanks means either carrying 4 gallons or walking back to the tap. Most people mix two tanks at a time in a bucket and refill, which is fine as long as the bucket is labeled and never used for anything else again.

Cost is worth a line too. If the concentrate costs $20 for 32 fluid ounces, then 20 fluid ounces is about $12.50 of product for the job, plus the water. A ready-to-use bottle costs more per square foot, because you are paying to ship water rather than concentrate.

Mixing rates for common glyphosate products

These are example rates for the main types of glyphosate product sold in the US. They are here to show you the range, not to replace your label.

Notice the pattern: the stronger the concentrate, the less you use per gallon. A 50.2% product at 2.5 fluid ounces and an 18% product at 6 fluid ounces deliver roughly the same amount of active ingredient, which is the whole point of the concentration number on the front of the bottle.

Rates also change with the job. Many labels list a lower rate for annual weeds and a higher one for tough perennials, with a separate rate for brush. Using the annual rate on a mature stand of poison ivy wastes a Saturday and a tank of product.

Safety rules that are not optional

These are the parts of the label people skip, and they are the parts that matter most.

• Wear what the label says. At a minimum that is long sleeves, long pants, closed shoes and socks, chemical-resistant gloves and eye protection. The Personal Protective Equipment section gives the exact list for your product.
• Mix outdoors or in a ventilated space. Never mix in a kitchen, or in a closed garage, or anywhere food is prepared.
• Never put pesticide in a food or drink container. Storing pesticides in soda bottles and milk jugs is a leading cause of serious pesticide poisoning in the US, according to the National Pesticide Information Center.
• Keep people and pets out during mixing and until the spray has dried completely.
• Watch the wind. Stop if the wind is strong enough to carry mist onto cars, windows or plants you want to keep. Fine droplets travel much further than people expect.
• Protect water. Glyphosate products are toxic to aquatic plants and some formulations are toxic to fish. Do not spray over water or anywhere runoff reaches a storm drain, ditch, pond, stream or wetland.
• Wash up. Wash hands and exposed skin with soap and water after you finish, and change out of the clothes you sprayed in.

If anyone is exposed, call Poison Control at 1-800-222-1222. It is free, it runs around the clock, and the staff handle pesticide calls constantly. The Poison Control guidance explains what to have ready when you call, the National Pesticide Information Center publishes a plain-language glyphosate fact sheet, and the EPA pesticide label pages cover the legal side.

Timing, weather and what not to spray

Glyphosate works by being absorbed through the leaves and moved down to the roots, so the conditions around the spray matter as much as the mix.

• Spray when weeds are growing. Actively growing plants take the product in. A drought-stressed or dormant weed absorbs little and shrugs it off.
• Late summer and early fall are the best window for tough perennials such as bindweed, thistle and poison ivy, because the plant is moving sugars down to the roots and carries the product with them.
• Rainfast time. Most labels call for 6 hours of dry weather, and some ask for more. Rain inside that window washes the product off and you will be spraying again.
• Temperature. Uptake slows below 60°F, and plants under heat stress above about 90°F close their pores and absorb less. Mid-morning on a mild day is close to ideal.
• Do not mow first. Give the weeds a few days of leaf growth before you spray, and leave the mower alone for a few days afterward so the product has time to move down.
• Do not spray wet foliage. Water on the leaf dilutes the droplet, and it runs off before the plant can absorb it.

One more limit worth knowing: glyphosate is non-selective. It kills grass, flowers and shrubs as readily as it kills weeds. Overspray and drift are the two ways a homeowner damages a prized plant, and the roots of a nearby tree can take up product that reaches them in the soil.

Spills, skin contact and storage

Most pesticide injuries at home come from a spill or a splash rather than from the spraying itself, so it pays to know the first moves.

• On the skin: take off contaminated clothing, then wash the area with soap and water for 15 to 20 minutes.
• In the eyes: rinse with clean water for 15 to 20 minutes, holding the eyelid open, and get medical attention.
• Swallowed: do not make the person vomit. Call Poison Control at 1-800-222-1222 and have the label in hand.
• Breathed in: move to fresh air. If breathing is affected, call 911.
• Spilled: contain it with absorbent material such as kitty litter, sand or sawdust, sweep it up, and dispose of it the way the label directs. Do not hose it into the street.

Store the bottle in its original container with the label readable, in a locked cabinet or on a shelf a child cannot reach, away from food, pet food and animal feed, and out of freezing weather. Never transfer it to a soda bottle or a milk jug, and never reuse the container for anything else once it is empty.

None of this replaces the label on your own bottle. Rates, restrictions and warnings differ between products and between states, and some states restrict certain uses further. The EPA label guide and the NPIC glyphosate fact sheet are the two places to look if something on your label is unclear.