Weed Killer Mixing Calculator

Look for wording like "dilute at a rate of X ml per Y litres of water" or "1 part product to Z parts water" in the mixing or directions-for-use section of the label. Enter the ml and litre figures directly into this calculator's first two fields — you do not need to convert anything yourself.

The label tells you the ratio of concentrate to water, but not how much total mixture to make — that depends on how big your watering can or sprayer is. Separating the two means you can reuse the same label rate for a 1 L trigger sprayer, a 5 L watering can, or a 9 L knapsack sprayer without re-doing the maths each time.

It means 1 part concentrate to X parts water by volume — so a 1:83.3 ratio means roughly one measure of concentrate mixed into about 83 of the same measure of water. It's the same figure agronomists and professional spray operators use to compare products, expressed from your own label's rate.

No — the area and coverage-rate fields are optional. Leave them blank to get just the concentrate/water split for a single container. Fill them in if you also want to know how many refills the whole job needs and how much concentrate to buy in total.

It's usually printed on the label as something like "treats up to X m² per litre of diluted solution" or given per pack size (e.g. "treats up to 200 m²" from a specific bottle) — divide the stated area by the litres of ready-to-use solution that bottle makes to get m² per litre.

No — always follow the label's stated dilution rate exactly. UK pesticide product labels are legally binding instructions: using a stronger mix than approved is not more effective in most cases, can damage nearby plants, surfaces, or grass you want to keep, wastes product, and goes against the safe-use guidance published by the HSE and gov.uk.

Under-diluting (a weaker mix) commonly means poorer weed control, so you may need to reapply, using more product overall than mixing correctly the first time. Follow the label's specified rate rather than adjusting it either way.

Most manufacturers recommend using mixed (diluted) solution the same day and not storing it, since diluted product can lose effectiveness and some formulations are less stable once mixed with water. Check your specific product's label for its storage guidance, and only mix as much as you plan to use.

Apply any leftover diluted mixture to the labelled target area rather than tipping it down a drain, gutter, or onto bare soil near water — pouring pesticide down the drain or into a watercourse is both environmentally harmful and against UK guidance. If you consistently have leftovers, mix a smaller batch next time.

Check your product's label for its specific PPE requirements — many concentrates recommend gloves as a minimum when handling and mixing, and some recommend eye protection too. Wash your hands and any exposed skin after use, and always follow the label's PPE instructions rather than general assumptions, since requirements vary by product.

Most weed killer labels explicitly warn against applying near water, drains, or gullies because of the environmental risk to aquatic life, and UK guidance (gov.uk, HSE) reinforces keeping pesticides away from watercourses. Always check the specific buffer distance or restriction on your product's label before spraying near any water feature.

Many weed killers need a rain-free period after application (often several hours, but this varies by product) to be absorbed before it's washed off — check your label for its specific "rainfast" time. Applying just before heavy rain is a common reason weed killer doesn't work as expected.

This calculator is for liquid concentrates that need diluting with water. If your product is a ready-to-use spray (no mixing required) or a granular product measured by weight per area, this tool doesn't apply — check that product's own label for its direct application rate.

The underlying maths — concentrate ml per litre of water, scaled to your container size — applies to any liquid product diluted by a label ratio, not just weed killer. Always use the specific dilution rate printed on that product's own label rather than reusing a weed killer rate for a different chemical.

Small containers (like a 1 L trigger sprayer) at a weak dilution rate can genuinely need less than a whole millilitre of concentrate for accurate mixing. Use a proper measuring syringe or the small-scale markings on a dosing cup rather than estimating by eye, especially for small batches.