A compost tea calculator takes the guesswork out of brewing AACT (actively aerated compost tea). Enter your batch size, tea type, and brew time, and get exact amounts of compost, water, molasses or other food source, required air pump output, application rate per square foot, and foliar dilution ratio — everything you need to brew a biologically active tea every time.
Brewing Parameters
Used to calculate whether batch covers your area
Full Recipe
Application Tips
How to Use the Compost Tea Calculator
Compost tea is one of the most cost-effective ways to inoculate your soil with billions of beneficial microorganisms. The key to success is getting the ratios right — too much molasses makes the brew acidic and bacteria-dominant; too little leaves the organisms without energy to multiply. This compost tea calculator scales every ingredient to your batch size automatically.
Step 1: Set Batch Size and Brew Duration
Enter how many gallons (or liters in metric) you want to brew. Common home sizes are 5 gallons for a small garden or 25+ gallons for larger plots. A 24-hour brew is standard — it provides enough time for bacteria and fungi to multiply without the tea going anaerobic. Extend to 36 hours in cooler temperatures (below 55°F/13°C).
Step 2: Choose the Tea Type
Select bacterial-dominant for vegetables, annuals, and lawn grass — these plants thrive in the bacterial-dominated top inch of soil. Choose fungal-dominant for trees, woody perennials, and shrubs that rely on fungal networks for nutrient access. Balanced tea works for most mixed gardens and is the best starting point if you are unsure of your soil biology.
Step 3: Enter Your Garden Area
Enter the square footage (or square meters) of the area you want to treat. The calculator compares your batch size to the standard soil drench rate (1 gallon per 100 sq ft undiluted) and tells you whether one batch covers your space or how many batches you need.
Step 4: Apply Within 4 Hours
Use the tea immediately after brewing completes — within 4 hours of turning off the pump. Strain the tea through a burlap or weed-fabric bag before applying to prevent clogging sprayers. For foliar feeding, dilute 1:10 with dechlorinated water and spray both sides of leaves early morning or evening. For soil drench, apply undiluted around the root zone.
Frequently Asked Questions
Is this compost tea calculator free?
Yes, the compost tea calculator is completely free with no usage limits. All calculations run locally in your browser — no signup, no account, and no data ever leaves your device.
Is my data private?
Yes, everything is calculated in your browser. Your inputs are never sent to any server or stored remotely.
What is compost tea?
Compost tea (also called AACT — actively aerated compost tea) is a water-based extract of finished compost that is brewed with aeration and a food source (like molasses) to multiply beneficial microorganisms. When applied to soil or sprayed on foliage, it introduces bacteria, fungi, protozoa, and nematodes that improve plant health and soil structure.
What is the difference between bacterial-dominant and fungal-dominant tea?
Bacterial-dominant tea uses a higher compost-to-water ratio and more simple sugars (molasses) as food. It suits vegetables, annuals, and lawns that thrive in bacterial-dominated soil. Fungal-dominant tea uses less molasses, adds a fungal food source (like oat flour or fish hydrolysate), and is better for trees, shrubs, and perennials that prefer fungal-rich soil.
How much compost do I use per gallon of water?
A common ratio is 1 cup (about 4 oz dry) of finished compost per gallon of water for a balanced tea. Bacterial-dominant recipes often use up to 2 cups per gallon; fungal-dominant recipes use 1/2 to 1 cup. The goal is to extract and multiply biology, not to make a thick slurry.
How strong does my air pump need to be for compost tea?
You need at least 1 GPH (gallon per hour) of airflow per gallon of brew. A 5-gallon batch needs a pump rated at least 5 GPH (or roughly 0.35 LPM), though higher is better. Most aquarium pumps are rated in LPM — convert by multiplying GPH by 0.063. Dissolved oxygen should stay above 6 ppm to keep the brew aerobic.
How do I apply compost tea?
Apply within 4 hours of finishing the brew before oxygen levels drop and beneficial organisms die. For soil drench, apply 1 gallon per 100 sq ft undiluted, or 1:10 (tea:water) for foliar spray. Apply early morning or evening to avoid UV degradation on foliage. Do not apply to edibles within 24–48 hours of harvest.
Can I store compost tea?
Compost tea should be used within 4–6 hours of completing the brew. Without continuous aeration, oxygen levels drop rapidly, the brew turns anaerobic, and beneficial aerobic organisms die. Storing it longer risks breeding harmful anaerobic bacteria. Brew only what you can use immediately.