Weed Killer Mixing Calculator
Work out the exact amount of herbicide concentrate and water to mix for your target area — so every tank is on-label, effective, and free of costly waste.
Overview
Herbicide labels list mixing rates as a ratio — a few ounces of concentrate per gallon of water, or millilitres per litre — but real spraying happens by the area, not by the bottle. This calculator bridges that gap. Enter the size of the space you need to treat, how far a gallon of spray goes, and the mix rate from your product label, and it returns the exact concentrate and water quantities for the whole job, plus a per-tank recipe you can follow at the sprayer.
It's built for homeowners tackling driveways, patios, and lawns; for gardeners spot-treating beds; and for landscapers and turf crews who need repeatable, on-label mixes. Getting the ratio right matters in both directions: under-dosing wastes a trip and lets weeds shrug off the spray, while over-dosing wastes expensive concentrate, risks damage to desirable plants, and can breach the product label.
How it works
- Choose your units. Switch between Imperial (US) and Metric — every field and result updates to match.
- Enter the area to treat in square feet or acres (square metres or hectares in metric).
- Set the coverage rate — how much area one gallon (or litre) of mixed spray covers. A common default of 300 sq ft per gallon is pre-filled; adjust it to your sprayer and walking speed.
- Enter the mix rate from your product label, such as 2.5 fl oz of concentrate per gallon of water.
- Enter your sprayer capacity so the tool can tell you how many full tanks the job takes.
- Press “Calculate mix.” You'll see the total solution, concentrate, and water needed, the number of tank fills, and a clear per-tank recipe. Press “Reset” to start over.
Formula explanation
The calculator works in three steps. First it converts your area to a total volume of finished spray, then it splits that volume into concentrate and water using your label's mix rate.
Total spray = Area ÷ Coverage rate
Concentrate = Mix rate × Total spray
Water = Total spray − Concentrate
Because you fill your sprayer to a set line, the concentrate is counted as part of the tank volume — you add the concentrate first, then top up with water to the mark. That's why water is the total spray minus the concentrate, not on top of it. The number of tank fills is the total spray divided by your sprayer size, rounded up to the next whole tank so you're never short on the last pass.
Practical benefits
- No wasted concentrate. Mix only what the job needs instead of guessing and pouring leftovers down the drain.
- Consistent, on-label results. Every tank lands at the same tested strength, so weeds get an effective dose and your product stays within label limits.
- Fewer refills, less backtracking. Knowing your tank count up front lets you plan the route and load the right amount before you start.
- Safer for the yard and your wallet. Correct dosing reduces the risk of scorching lawns and desirable plants, and stretches an expensive bottle across more jobs.
- Works for any product. Because you enter the label's own mix rate, the tool fits glyphosate, selective broadleaf killers, and pre-emergents alike.
Frequently asked questions
It's printed on the product label under “Directions for Use” or “Mixing.” Rates are usually given as concentrate per gallon (or per litre) of water, and many products list a lighter rate for young or annual weeds and a stronger rate for tough perennials. Use the rate that matches your weeds, and follow any maximum-rate limits stated on the label.
Coverage is how much area one gallon (or litre) of mixed spray treats. To calibrate, fill your tank with plain water, spray a measured strip at your normal pace, and divide the area covered by the gallons used. Spot spraying commonly runs 250–400 sq ft per gallon; broadcast spraying stretches further and thinner. The 300 sq ft per gallon default is a solid starting point.
Add about half your water first, then the measured concentrate, then top up with the rest of the water to the fill line and agitate gently. Adding some water first prevents the concentrate from sitting undiluted at the bottom and helps it mix evenly. Always mix outdoors, wear gloves, and never mix more than you'll use in a session.
The tool rounds the total mix to the amount your area actually needs, but reports whole tank fills so you plan enough passes. For the final partial tank, mix the same ratio at a smaller volume — keep the concentrate-to-water proportion identical. Mix spray fresh for each job; diluted herbicide can lose effectiveness if stored, and storing mixed solution is discouraged on most labels.
Disclaimer: This calculator is provided for general planning and educational purposes only. Herbicide mixing rates, application limits, and safety requirements are set by the manufacturer's label, which is the legally binding instruction — always read and follow it. Results here are estimates based on the values you enter and do not account for product-specific directions, local regulations, weather, or site conditions. This tool does not constitute professional, agronomic, or legal advice. Use pesticides responsibly, wear appropriate protective equipment, and keep products away from children, pets, and water sources.
