Liquid Fertilizer Dilution Calculator
How much liquid fertilizer to mix into your watering can.
Liquid fertilizer is only useful at the right dilution. Too weak and plants get nothing; too strong and it burns roots and leaves. The label gives a ratio. 1:100 means one part concentrate to one hundred parts water. And this calculator turns that ratio into millilitres for the exact amount of water in your can.
It also works backwards: enter the concentrate you have and see how much solution it makes. For dry fertilizer, use the Fertilizer Calculator instead, and for the beds you are feeding, the Raised Bed Soil Calculator.
How this is worked out
Concentrate (ml) = water (ml) ÷ ratio. Water in millilitres = litres × 1,000, or US gallons × 3,785. Millilitres per litre = 1,000 ÷ ratio. Solution made (L) = concentrate (ml) × ratio ÷ 1,000.
Assumptions:
- The ratio is parts of water to one part of concentrate. 1:100 means 100 ml of water per 1 ml of concentrate.
- Gallons are US gallons (3,785 ml). If your jug is an imperial gallon (4,546 ml), convert to litres first.
- The result is the amount to mix for one watering. It is not a feeding recommendation. The label decides how often to feed.
Three worked examples
1. A 10-litre watering can at 1:100. 10,000 ml ÷ 100 = 100 ml of concentrate. 10 ml per litre of water.
2. A 2-gallon can at 1:200. 2 × 3,785 = 7,570 ml of water. 7,570 ÷ 200 = 38 ml of concentrate, or about 1.3 fl oz.
3. A 500 ml bottle at 1:50. It makes 500 × 50 ÷ 1,000 = 25 litres of solution. Two and a half full 10-litre cans.
Millilitres per litre at common ratios
| Ratio | ml per litre of water | ml per 10-litre can |
|---|---|---|
| 1:50 | 20 ml | 200 ml |
| 1:100 | 10 ml | 100 ml |
| 1:200 | 5 ml | 50 ml |
| 1:500 | 2 ml | 20 ml |
Common dilution mistakes
- Making it stronger for faster growth. Exceeding the label rate burns roots and leaves. If plants need more feed, feed more often at the label rate. Never stronger.
- Confusing ml per litre with the ratio. A label that says 10 ml per litre is 1:100. Enter 100 as the ratio, not 10.
- Mixing US and imperial gallons. A US gallon is 3,785 ml and an imperial gallon is 4,546 ml. A 20% difference that matters at strong ratios. Check which gallon your jug uses, or work in litres.
- Eyeballing the dose. A capful is not a measurement. Caps differ by bottle. Use a measuring jug or a syringe for small volumes.
- Feeding dry soil. Water the soil first with plain water, then feed. Concentrate hitting dry roots is far more likely to burn them.
What to do next
Mix only what you will use today. Diluted fertilizer does not keep well, and the nutrients settle out. Water first with plain water, then apply the feed, and note the ratio on the bottle so next time is a one-minute job. For dry granular fertilizer, the Fertilizer Calculator works out the product and bag count.
Putting the result to work
Make the habit boring and repeatable. Write the ratio on the bottle with a permanent marker, and keep one measuring jug and one syringe with the fertilizer. Not in the kitchen, not in the shed, but with the bottle, so measuring is a one-minute job instead of a search. Mix only what you will use today: diluted fertilizer does not keep, the nutrients settle, and a leftover can is a guessing game next week.
Feed in the morning or evening, not in the midday sun, and water the soil with plain water first so the feed spreads evenly instead of pooling on dry roots. For container plants, feed little and often. A small pot has little soil to buffer a strong dose, so half-strength twice as often is safer than full-strength once. Keep a note of what you fed and when, on the bottle or in a notebook: if plants look overfed, the note tells you what to change.
How often to feed
The ratio is only half the job. The other half is frequency, and that comes from the label, not from this calculator. As a rough guide for typical liquid feeds: fast-growing vegetables every 1 to 2 weeks while they are actively growing; container plants weekly at half strength, because a small pot has little soil to buffer a strong dose; houseplants monthly in the growing season and not at all in winter; lawns monthly at most, and never on dormant grass. These are starting points, not rules. If the label says something different, the label wins.
| Where | Typical frequency | Strength |
|---|---|---|
| Vegetables in beds | Every 1 to 2 weeks in active growth | Label rate |
| Containers and pots | Weekly | Half strength |
| Houseplants | Monthly in the growing season | Label rate |
| Lawns | Monthly at most | Label rate |
| Newly planted seedlings | Wait until established | Half strength |
Stop feeding when growth stops. Plants that are dormant, stressed by drought, or freshly transplanted cannot use extra nutrients, and the excess sits in the soil or washes away. If leaves yellow after feeding, the cause is usually too much, too strong, or too often. Flush the soil with plain water and skip the next feed.
How dilution works
- Read the ratio on the label: 1:100 means 1 part concentrate to 100 parts water.
- Convert your water to millilitres: litres x 1,000, or US gallons x 3,785.
- Concentrate (ml) = water (ml) / ratio. Example: 10 L of water at 1:100 needs 10,000 / 100 = 100 ml.
- That is also 10 ml per litre of water. Mix well before watering.
- To work backwards: solution made (L) = concentrate (ml) x ratio / 1,000.
What if my label gives ml per litre instead of a ratio?
It is the same thing. Ten millilitres per litre is 1 to 100, because one litre holds 1,000 millilitres and 1,000 divided by 10 is 100. Enter 100 as the ratio in the calculator, and it will give you the millilitres for any volume of water.
Can I make it stronger for faster growth?
No. Exceeding the label rate can burn roots and leaves, and it does not make plants grow faster. Plants can only take up so much at once. If plants need more feed, feed more often at the label rate, not stronger, and check whether something else is limiting growth, such as light or water.
US gallon vs imperial gallon?
This tool uses the US gallon, which is 3,785 millilitres. An imperial gallon is 4,546 millilitres, about 20 percent larger. If your jug is an imperial gallon, convert to litres first and enter litres. That removes the ambiguity entirely.
How much solution does my bottle make?
Multiply the concentrate in millilitres by the ratio and divide by 1,000. A 500-millilitre bottle at 1 to 100 makes 50 litres of solution. Enter the amount you have in the calculator and it shows the total for you.
Should I water before feeding?
Yes. Water the soil first with plain water, then apply the diluted feed. Concentrate reaching dry roots is far more likely to burn them, and wet soil spreads the feed evenly through the root zone instead of pooling in one spot.
Conversions: 1 US gallon = 3,785 ml; 1 US fluid ounce = 29.57 ml. Always follow the product label. Rates differ by product, and this tool is a mixing aid, not a feeding recommendation.
