Nutrient dosing calculator
Labels are written for gallons. You are mixing a mason jar. Enter the container and the label dose, and get the exact amount to add.
Add to 0.95 L
Show the maths
Every nutrient bottle gives its dose per gallon or per litre. Almost nobody mixes a whole gallon. You are filling a 32-ounce jar, or a 2-litre bottle, or a bucket you deliberately only filled to four gallons, and now you are doing fractions over the sink with a plant waiting.
This does the conversion, applies the right strength for the growth stage, and tells you the answer in a measurement you can actually make.
Why the dose matters more than the brand
Overfeeding is the single most common way beginners kill a plant, and it is entirely self-inflicted. More nutrient does not mean more growth. Past a certain concentration the solution starts pulling water back out of the roots rather than into them, which burns the tips and edges of leaves and stunts everything.
What strength to use, and when
| Stage | Strength | Why |
|---|---|---|
| Seeds germinating | Plain water | The seed carries its own food until the first true leaves appear. |
| Fresh seedling just transplanted | 50% | Young roots burn easily and are not yet feeding heavily. |
| Young plant, two weeks in | 75% | Growing steadily but still building root mass. |
| Established leafy growth | 100% | The label rate is designed for this stage. |
| Fruiting tomatoes and peppers | 100% bloom formula | Same total strength, different ratio. Switch bottles, not doses. |
Those percentages are of whatever the label says, which is why the calculator asks for the label figure rather than assuming one. Formulations differ enormously between brands, and the bottle in your hand is the authority.
Common containers, already worked out
For the four reservoirs used in our build plans, at a label dose of 2 teaspoons per gallon:
| Container | Volume | At 100% | At 50% (seedlings) |
|---|---|---|---|
| Mason jar, 32 oz | 0.95 L / 0.25 gal | 1/2 tsp | 1/4 tsp |
| Soda bottle, 2 L (usable) | 1.8 L / 0.48 gal | 1 tsp | 1/2 tsp |
| Countertop tote, to fill line | 11 L / 2.91 gal | 5 3/4 tsp | 2 7/8 tsp |
| 5 gal bucket, filled to 4 gal | 15.1 L / 4 gal | 8 tsp (2 tbsp + 2 tsp) | 4 tsp |
If your label says something other than 2 teaspoons per gallon, use the calculator. Half the products on the shelf are dosed in millilitres per litre instead, and a few are in grams.
Grams are a special case
Some nutrients, particularly the budget powders, are dosed by weight. There is a strong temptation to convert grams into teaspoons so you can use a spoon, and you will find plenty of calculators that do exactly that.
Mixing order, which matters more than people expect
Water first, always
Fill the reservoir before any nutrient goes in. Adding water to concentrate rather than the other way round is how you get precipitate.
One part at a time
Add the first part, stir until it has fully dispersed, then add the next. Never pour two concentrates together: calcium from one and phosphate from the other will bind into a white sludge that neither you nor the plant can use.
Micro before bloom, if your label says so
Three-part systems usually specify an order, commonly micro, then grow, then bloom. Follow the bottle rather than any general rule.
Mix, then measure pH
Nutrients change the pH of water, so a reading taken beforehand tells you nothing useful. Adjust to 5.5 to 6.5 after everything is in.
How to measure small amounts accurately
- Above about 1/2 teaspoon: a standard measuring spoon set is fine. Level it off, do not heap it.
- Below 1/2 teaspoon: a cheap oral syringe from a pharmacy is far more accurate than trying to judge an eighth of a spoon, and it costs almost nothing.
- Anything in grams: a 0.1 g kitchen scale. No substitute.
- Never use the bottle cap unless the label tells you its capacity. Cap sizes vary between products and even between bottle sizes of the same product.
A note on what this cannot tell you
This calculator converts a dose. It does not know your water. If you are on hard tap water you may already have a good deal of calcium and magnesium, and on reverse osmosis or distilled water you have essentially none, which is why some growers need a cal-mag supplement and others do not.
It also cannot predict the EC or PPM your mix will land on, because that depends on the specific formulation and on what was already dissolved in your water. If you want that number, measure it. Our pH and PPM guide covers the targets.
Frequently asked questions
How much nutrient do I use for a mason jar?
A 32 oz mason jar holds a quarter of a US gallon, so use a quarter of whatever the label specifies per gallon. If the label says 2 teaspoons per gallon, that is half a teaspoon, or a quarter teaspoon at the half strength a seedling wants.
Should I use half strength for seedlings?
Yes, for roughly the first two weeks after transplanting. Young roots burn easily and are not feeding heavily yet. Move to three-quarter strength as the plant establishes and full strength once it is growing well.
Can I convert grams to teaspoons?
Not reliably. Grams measure weight and teaspoons measure volume, and the conversion depends on the density of that specific powder. Two different nutrient salts can differ by a factor of two in the same spoon. Weigh powders on a scale that reads to 0.1 g.
What happens if I use too much nutrient?
Leaf tips and edges go crisp and brown, growth slows, and in severe cases the plant wilts even though the reservoir is full, because the concentrated solution pulls water back out of the roots. Dilute the reservoir with plain water and mix weaker next time.
Do I add all the parts together before pouring?
No. That is the one mixing mistake that ruins a batch. Add each part to the water separately and stir in between. Combining concentrates makes calcium and phosphate bind into a white precipitate the plant cannot absorb.
Why does my label say millilitres per litre when I measure in gallons?
Because the product is sold internationally. The calculator handles either, so set the unit to match your bottle rather than converting in your head.
Does this work for any nutrient brand?
Yes. It uses whatever dose your label states rather than assuming a formulation, which is why it asks you to enter the label figure. Always follow the bottle in your hand over any general guidance, including ours.
Not sure what to feed in the first place?
Nutrients 101 covers what is actually in the bottle, which format to buy, and how often to change the reservoir.