How to use

Choose what you want to find: fertilizer for a known amount of water, or water for the fertilizer you have.
Choose how your label states the rate: ml per litre, a ratio 1:N, or grams per litre for soluble powder.
Type in the rate from the label.
Pick a strength. Half or quarter strength is gentler for seedlings and for feeding in very hot weather.
Pick your container (watering can, sprayer, bucket) or type any volume in litres.
Read the amount to add, also shown in teaspoons, tablespoons and bottle caps.

How it's calculated

First the label rate is turned into "amount per litre of water":

ml/L or g/L: used as is
Ratio 1:N: 1 part concentrate to N parts water, so per litre = 1000 ÷ N ml

Then:

Concentrate = rate per litre × litres of water × strength
Reverse mode: Water (L) = concentrate you have ÷ (rate per litre × strength)
Teaspoons = ml ÷ 5, tablespoons = ml ÷ 15, caps = ml ÷ your cap size

Worked example

Your liquid fertilizer label says 2 ml per litre, and you have a 4.5 L watering can:

2 ml/L × 4.5 L = 9 ml of fertilizer
That is about 1.8 teaspoons, 0.6 tablespoon, or 0.9 of a 10 ml cap
For seedlings at half strength: 2 × 4.5 × 0.5 = 4.5 ml

A label that says 1:500 into a 5 L can: 1000 ÷ 500 = 2 ml per litre, so 2 × 5 = 10 ml.

Reverse: you have 10 ml left in the bottle and the rate is 2 ml/L → 10 ÷ 2 = 5 L of water.

Tips

Always follow the label of your own product. The calculator only does the arithmetic.
Measure the cap once: fill it with water and pour it into a measuring cup or a syringe. Caps of different brands hold different amounts.
Weigh soluble powders on a kitchen scale; a spoonful of powder can vary a lot depending on how fine it is and how full the spoon is.
Water the soil first if it is bone dry, then feed. Feeding a dry pot can burn the roots.
In the hot afternoon, leaves can scorch. Feed in the early morning or late afternoon.
More is not better. If plants show brown leaf tips, cut back to half strength.
Rinse your sprayer with clean water after use so it does not clog.

Mixing for hydroponics? Use the EC calculator instead, because AB nutrients are mixed by EC, not just by ml.

CalculatorsHydroponic AB Nutrient EC / TDS CalculatorHydroponic AB nutrient calculator: how many ml of stock A and B to reach your target EC, with TDS in ppm on the 500 and 700 scales, plus a top-up mode.

CalculatorsNPK Converter (P₂O₅ to P, K₂O to K)NPK converter: change P₂O₅ to P and K₂O to K, see the nutrients in a weight of 15-15-15, and find how much fertilizer gives a target amount.

FAQ

What does a ratio like 1:200 mean? One part of fertilizer to 200 parts of water. For 1 litre of water that is 1000 ÷ 200 = 5 ml of fertilizer.

Can I mix a big batch and keep it? It is best to mix what you will use the same day. Diluted organic fertilizers in particular can smell and spoil in our heat.

Is a teaspoon accurate enough? For liquid fertilizer in a watering can, yes — 1 teaspoon is about 5 ml and 1 tablespoon about 15 ml. For small sprayers and seedlings, a syringe or measuring cup is more accurate.

Should I use half strength for foliar spraying? Many labels give a separate, lower rate for spraying leaves. Use that rate if your label has one; if not, a weaker mix is the safer choice.