Compost Tea Recipe Calculator
Compost tea is an aerated brew of water, finished compost, and a little microbial food — and the recipe scales linearly. This free compost tea recipe calculator takes your batch size in gallons and instantly gives you the exact amounts of compost (or worm castings) and unsulfured molasses to add, plus the 24–36 hour brew-time guidance, from the classic 5-gallon recipe: 2 cups compost, 1 fl oz molasses.
Enter your numbers
Your results
| Batch size | — |
| Molasses in tablespoons | — |
| Base ratio per gallon | 0.4 cups compost · 0.2 fl oz molasses |
| Brew time | 24–36 hours, aerated |
Estimates: ingredient densities and cup measures vary; brew performance depends on continuous aeration, water temperature (ideally 65–80 °F), and compost quality. Use the tea within 4 hours of finishing the brew.
How to use this calculator
Enter the size of your batch in gallons of water — 5 is the classic bucket recipe, but the same math scales to a 1-gallon watering can or a 55-gallon drum. Pick whether you're brewing with finished compost or worm castings (the ratio is identical, only the label changes), and optionally include fish hydrolysate as an extra microbial food. Everything recalculates instantly as you type.
Results give you the compost in cups, the molasses in both fluid ounces and tablespoons (since most home brewers measure with a spoon), and fish hydrolysate in tablespoons if you selected it. Add the solids to your brewing bucket in a mesh bag, add the molasses dissolved in warm water, run an air pump with an air stone for 24–36 hours, then strain and apply the tea within 4 hours while the microbes are still alive.
How it works
The recipe scales linearly from the standard base recipe: per 5 gallons of water — 2 cups finished compost (or worm castings), 1 fluid ounce unsulfured molasses, and optionally 1 tablespoon fish hydrolysate. Dividing by 5 gives the per-gallon ratio: 0.4 cups compost per gallon and 0.2 fluid ounces molasses per gallon. So for any batch, compost (cups) = gallons × 0.4, molasses (fl oz) = gallons × 0.2, and fish hydrolysate (tbsp) = gallons × 0.2. Molasses is also shown in tablespoons using 1 fl oz = 2 tbsp — so the classic 5-gallon batch needs 2 tablespoons of molasses.
This is a brewer's ratio guide, not a lab analysis: actual microbe counts depend on your compost's biology, water temperature, and how well you aerate. Keep the batch between about 65 °F and 80 °F, never let the brew sit unaerated, and smell it before use — finished tea should smell earthy and sweet, never rotten.
Frequently asked questions
What is compost tea?
Compost tea is a liquid made by steeping finished compost or worm castings in aerated water with a microbial food like unsulfured molasses for 24 to 36 hours. The aeration breeds beneficial microbes, and the resulting liquid is sprayed or poured on plants as a mild organic fertilizer and soil biology booster.
How long should compost tea brew?
Brew compost tea for 24 to 36 hours with continuous aeration from an aquarium pump and air stone. Less than 24 hours under-develops the microbial population; beyond 36 hours the microbes can run out of food and die off, making the tea go anaerobic.
Can I use worm castings instead of finished compost?
Yes. Worm castings work the same way as finished compost in a tea and use the same ratio: 2 cups per 5 gallons of water (0.4 cups per gallon). Castings tend to produce a microbe-rich tea with less sediment.
Can you use too much compost tea on plants?
Compost tea is gentle, but a full-strength brew is concentrated — dilute it about 1:4 (one part tea to four parts water) for foliar spraying or watering houseplants. Apply within 4 hours of the end of brewing while the microbes are still active, and avoid using it on plants within a day of chemical fertilizer.
Is this calculator for compost tea or the compost browns-to-greens ratio?
This page scales compost tea brewing recipes (water, compost, molasses). If you are building a compost pile and need to balance carbon and nitrogen, use our Compost Ratio Calculator instead.