Enter your product label's rate, your tank size, and your area — get exactly what goes in the tank. The label's numbers, your math done.
On your bottle, e.g. "1.5 fl oz per gallon of water."
Usually "1 gallon treats 1,000 sq ft."
Spot-spraying? Estimate just the weedy patches.
Mix this
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Total spray solution needed
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Tank loads
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Concentrate total (buy check)
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THE LABEL IS THE LAW — its rates, its safety gear, its re-entry times. This page only does the label's arithmetic. Never mix products unless the label allows it; never reuse herbicide sprayers for anything else.
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The mix math
gallons of spray = area ÷ label coverage concentrate per tank = label rate × gallons in the tank
Every spray label reduces to those two lines, and every mixing mistake is one of them skipped: right concentration but sprayed too thick over the area (overdose by stealth), or a perfectly measured tank that covers half the lawn. Measure the concentrate with a real measuring cup — "a glug" is not a unit.
A worked example
A label reading 1.5 oz/gallon, 1,000 sq ft per gallon, treating 2,000 sq ft with a 2-gallon sprayer: 2 gallons of solution — one full tank with 3 oz of concentrate, walked evenly over the area. A 4,000 sq ft job is two of those tanks, 6 oz total — useful to know before discovering the bottle only holds 4.
Making it actually work
Spray when weeds are growing and temps are mild, skip the mow for two days either side, and give it 2–3 weeks — systemic herbicides kill roots slowly, and respraying at day 5 "because nothing happened" is how lawns get overdosed. Thick grass is the real long-term weed control: thefeeding andmowing height pages are the prevention half of this page.
Frequently asked questions
How much weed killer concentrate goes in my sprayer?
Two numbers from YOUR product's label do it: the mix rate (ounces per gallon) and the coverage (gallons of spray per 1,000 sq ft, typically 1). This calculator multiplies them out for your tank and area — it deliberately doesn't guess rates, because they vary product to product and getting them wrong goes badly in both directions.
What happens if I mix weed killer too strong?
Counterintuitively, selective herbicides often work WORSE overdosed — they burn the weed's leaves so fast the plant stops moving the chemical to its roots, and the weed regrows. Too strong also risks damaging the grass the product was selected to spare. The label rate is the tested sweet spot, not a lawyer's minimum.
When should I spray weeds?
Actively growing weeds, mild temps (most labels say under 85°F), no rain for 24 hours, no wind, and don't mow for 2 days before or after — the weed needs leaf area to absorb through. Fall is secretly the best broadleaf window: perennial weeds are pulling reserves down to their roots and carry the herbicide with them.
Why do sprayers need calibrating?
Because 'a gallon covers 1,000 sq ft' is only true at one walking pace and pressure — yours. The quick check: put a measured quart of plain water in the sprayer and spray a marked 250 sq ft evenly. Ran out early = you spray heavy (use a bit less area per tank); water left = you spray light. One practice pass makes every mix after it accurate.