Fertilizer Dilution Calculator - Millilitres of Concentrate per Litre

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Mixing liquid fertilizer without burning plants

Liquid fertilizer is strong stuff. A bottle that feeds a whole garden from a few capfuls can also scorch leaves and kill roots when mixed too strong, and a mix that is too weak simply wastes your time. The label usually gives a dilution ratio like 1:100, which means one part concentrate to 100 parts water, but turning that ratio into millilitres for your watering can is where most people guess, and guessing is how plants get burned.

This calculator does that conversion for you. Enter the ratio from the label and the amount of water you plan to mix, and it gives you the millilitres of concentrate to add and the millilitres per litre of water. One number fills the can, the other lets you scale the mix up or down without recalculating from scratch.

How the dilution math works

The math is a single division. A ratio of 1:X means each litre of water needs 1,000 divided by X millilitres of concentrate, because a litre holds 1,000 millilitres. At 1:100 that is 10 millilitres per litre, so a 10-litre watering can needs 100 millilitres. At 1:200 it is 5 millilitres per litre, and at 1:50 it is 20 millilitres per litre.

The ratio counts water parts, not total parts, which is the convention most garden labels use. If your label instead says one part in a total of 100 parts of mix, the difference is tiny at weak ratios and the label’s own measuring cap usually settles it. When a label gives a dose per gallon or per square metre instead of a ratio, use that dose directly rather than converting it through this tool.

A worked example

Suppose your tomato feed says 1:100 and you want to fill a 10-litre watering can. Divide 1,000 by 100 to get 10 millilitres per litre, then multiply by 10 litres to get 100 millilitres of concentrate. Pour the water first, add the 100 millilitres, and stir well so the feed is even through the can.

Now suppose you only need 2 litres for a few pots at 1:50. The per-litre dose is 20 millilitres, so 2 litres need 40 millilitres. Notice how the per-litre figure makes scaling easy: once you know it, any can size is a single multiplication. Keep a small measuring syringe or cup just for fertilizer, because a kitchen spoon is not accurate enough at these small volumes.

Tips for safe feeding

Always add concentrate to water, not water to concentrate, and mix in the can you will water from so nothing is lost in transfer. Feed in the cool of the morning or evening, never onto dry soil in hot sun, because strong sun plus fertilizer salts burn leaves fast. Water the soil first if it is dry, then apply the feed, so roots meet a diluted solution rather than a salty shock.

Feed little and often during active growth rather than one heavy dose. Most plants use a weak feed every week or two far better than a strong monthly flood, and weak feeding leaves a margin for error. Write the ratio and the dose on the bottle with a marker, because a label washed off by a wet shed leaves you guessing next season.

Common mistakes

The worst mistake is reading 1:100 as one capful per can without checking the cap size. Caps differ wildly between brands, so a capful from one bottle can be double the dose of another. The second mistake is mixing by eye in a coloured can where the level is hard to see, which leads to over-strong mixes. Measure the water and the concentrate separately, every time.

People also store mixed feed for weeks, during which it can ferment, grow algae, or lose strength. Mix only what you will use that day, rinse the can afterwards, and never pour leftover feed onto one unlucky plant as a way to empty the can.

Safety and limits

Fertilizer dilution results are general guidance estimates only. Always follow the product label for fertilizer rates, never exceed label rates, and local climate and soil conditions govern how plants respond. Keep concentrate away from children and pets, wear gloves, wash hands after handling, and never use food containers for measuring or storing fertilizer. If leaves yellow or brown after feeding, flush the soil with plain water and skip the next feed.

Frequently asked questions

Can I mix two fertilizers in one can? Only if both labels say they can be mixed. Some feeds react and form solids that block watering cans, and some combinations double up a nutrient past safe levels.

Is a stronger mix better for hungry plants? No. Roots can only take up so much at once, and the excess salts stay in the soil and damage roots. A label-rate feed given regularly beats a strong feed every time.

Use the calculator above with the ratio printed on your bottle, measure both the water and the concentrate, and feed at label strength. Precise mixing is a small habit that prevents the most common feeding damage in the garden.

Fertilizer Dilution Calculator – Millilitres of Concentrate per Litre