Potting Mix DIY Recipe Calculator: US Quarts and Quality Scores
Enter volumes of peat, coco coir, perlite, compost, and more in US quarts to build your custom potting recipe. Get drainage, water retention, and aeration scores, estimated pH range, a donut chart breakdown, and a printable PDF recipe card. Six preset recipes included. Free, no signup.
Load a preset or enter ingredient volumes in quarts. The calculator shows total volume, quality scores, pH estimate, and a donut chart breakdown of your recipe.
Why Making Your Own Potting Mix Beats Buying Bags from Big Box Stores
Walk through the garden department of any Home Depot, Lowe’s, or Walmart in spring and you will find dozens of bags of potting soil competing for shelf space. Some cost two dollars a bag, some cost twenty. Most look nearly identical. And most experienced container gardeners have learned, through disappointment, that bagged potting mixes are wildly inconsistent between brands, between batches of the same brand, and between years. A bag that worked beautifully last season may arrive this season compacted, hydrophobic (water-repellent), or packed with filler ingredients that have no business being in a premium mix.
Making your own potting mix takes about twenty minutes and lets you control exactly what goes into the medium your plants will live in for an entire growing season. You choose the base (peat or coco coir), the drainage amendment (perlite), the fertility source (compost, worm castings), and any specialized ingredients for your specific crops. You measure by quarts in US standard measurements, the same unit used on every bag at the farm supply store. And you know exactly what you made, so you can reproduce it next season, adjust the ratio based on what worked, and scale up or down as needed.
Understanding What Each Ingredient Does
Peat moss is the traditional workhorse base medium for potting mixes across North America. It holds moisture well, has a loose, fibrous structure that roots penetrate easily, and is lightweight. Its main drawbacks are a strongly acidic pH (3.5 to 4.5) that requires limestone addition for most vegetables, and the environmental concern of slow peat bog regeneration. Most commercial potting mixes are 60 to 70 percent peat moss.
Coco coir is the fibrous material extracted from coconut husks, a byproduct of coconut processing that would otherwise go to waste. It behaves similarly to peat in a mix but holds slightly less moisture, drains slightly better, and starts at a near-neutral pH (5.8 to 6.5) that requires no lime correction for most crops. Coco coir has become the preferred peat substitute for growers who prioritize sustainability and pH simplicity.
Perlite is volcanic glass that has been rapidly expanded by heat, producing lightweight white spheres full of tiny air pockets. Perlite adds no nutrients, holds almost no water, and drains instantly, making it the primary tool for improving drainage and aeration in any potting mix. Add more perlite for drought-tolerant plants, less for moisture-loving ones. The potting mix calculator’s drainage and aeration scores rise directly with perlite percentage.
Vermiculite is expanded mica that acts as a sponge, absorbing and slowly releasing moisture and nutrients. Unlike perlite, vermiculite holds water and is used when consistent moisture around roots is important. Seed starting mixes commonly combine perlite and vermiculite to get both drainage and moisture retention. Vermiculite compresses over time and is not a good long-term structural amendment.
Compost provides slow-release nutrients, beneficial microorganisms, and humus that improves the cation exchange capacity of the mix (its ability to hold onto nutrients rather than losing them with each watering). Good compost contains a balance of carbon and nitrogen and has a near-neutral pH. Avoid adding more than 20 to 30 percent compost to container mixes, as it can become compacted and wet when overused.
Worm castings are concentrated compost processed through earthworm digestive systems. They contain plant-available nutrients, beneficial microbes, and plant growth hormones in a form that is immediately accessible to plant roots. Worm castings improve any potting mix and are not possible to over-apply at the 10 to 20 percent range. They are more expensive than bulk compost but require much less volume for the same fertility effect.
Bark fines (also called orchid bark or composted bark) add large air pockets to a mix and are the primary ingredient in orchid growing media. Bark breaks down slowly, maintains mix structure over a full growing season, and creates excellent drainage channels for moisture-sensitive plants like orchids, bromeliads, and some cacti.
Coarse sand adds drainage and weight to potting mixes that need to stay stable in the wind (outdoor patio pots, for example). Use horticultural coarse sand or chicken grit: never use fine beach or playground sand, which can actually worsen drainage by filling air pockets between other particles.
How the Potting Mix Calculator Scores Drainage, Retention, and Aeration
The calculator’s quality scores (drainage, water retention, and aeration, all on a 1 to 10 scale) are weighted averages of each ingredient’s individual properties, proportional to its percentage in the total mix. Each ingredient in the database has pre-assigned scores derived from published horticultural research on growing media properties. As you adjust ingredient volumes, the scores update to reflect the new mix composition.
Reading the Quality Scores
A drainage score of 7.0 or higher means water flows through the mix readily and is unlikely to pool or become waterlogged. This is ideal for succulents, cacti, lavender, and root vegetables. A drainage score below 4.0 means the mix holds significant water and will stay moist for days after watering: ideal for moisture-loving plants like ferns and lettuce, but risky for plants prone to root rot. A water retention score above 7.0 means the mix stays moist between waterings, which reduces how often you need to water but increases root rot risk if drainage is also high. The aeration score reflects the volume of air pockets available to roots. Most plants need an aeration score of at least 5.0 for healthy root respiration.
The pH Estimate
The estimated pH range is a weighted average of the pH ranges of your chosen ingredients. It is an approximation, not a laboratory measurement. Always test your finished mix with a pH meter or pH test kit before planting, especially if you are using peat moss (strongly acidic) or biochar (strongly alkaline) in significant quantities. Add dolomite lime to raise pH (about 1 tablespoon per gallon of mix per 0.5 pH unit correction). Add elemental sulfur to lower pH (about 1 teaspoon per gallon for 0.5 unit correction).
The Donut Chart
The donut chart shows each ingredient’s percentage of the total mix volume as a colored slice. Hover over each slice to see the ingredient name, volume in quarts, and percentage. This gives you a quick visual check that your intended ratios match the ingredient proportions you actually entered. The PDF recipe card includes the ingredient table with both quart volumes and percentages for reference at the mixing bench.
Three Real US Home Garden Potting Mix Recipes from Different Growing Situations
These three recipes cover the most common home garden scenarios in the United States, using ingredients available at farm supply stores, garden centers, and online suppliers across the country.
Six Expert Tips for Mixing Your Own Potting Media at Home
Combine all ingredients in a large tarp spread on the ground or in a wheelbarrow before filling pots. Attempting to mix in a pot or in the bags themselves leads to uneven distribution, with dry perlite floating to the top and heavy compost settling to the bottom. Spread all ingredients in layers on the tarp, then fold the tarp edges over and roll the pile to mix, similar to kneading bread dough. For large batches, a cement mixer rented from a US hardware store for an afternoon makes the job fast and thorough.
Both peat moss and coco coir can become hydrophobic (water-repellent) when dry. If you mix them dry into a pot and then water, the water runs around the dry clumps rather than through them, leaving dry pockets in the root zone. Pre-wet your peat or coco coir the day before mixing: pour it into a large bin, add warm water gradually while turning, and let it absorb overnight. The material should feel evenly moist but not wet to the touch before you incorporate it with perlite and other ingredients. Pre-wetted coco and peat also blend more evenly with other dry ingredients.
Peat moss has a pH of 3.5 to 4.5, which is too acidic for most vegetables and herbs that prefer pH 6.0 to 6.8. Before planting into a peat-based mix, add dolomite lime (a calcium and magnesium carbonate) at a rate of approximately 1 tablespoon per gallon of mix (or about 5 tablespoons per 5-gallon batch). Mix the lime evenly throughout and wait 48 hours before planting, since lime takes time to fully react. The calculator’s pH estimate for peat-heavy mixes will read below 5.5: the lime correction brings it up to the vegetable-friendly range. Coco coir mixes do not need lime correction for most crops.
The quart is the standard US measurement for potting media, and it is the unit on the front of most perlite, vermiculite, and coco coir bags sold in the US. Keep a clean 1-quart plastic container (an old yogurt quart tub works perfectly) dedicated to measuring potting mix ingredients. Scoop each ingredient into the tub, level off the top, and dump into your mixing area. This takes the guesswork out of ratios and makes your recipes exactly reproducible season after season. One US gallon equals 4 quarts: a 5-gallon mixing batch equals 20 quarts total, which is a practical working batch for most home container gardens.
After mixing, do two quick tests before filling pots. First, the drainage test: fill a small pot with your mix, water it thoroughly, and time how long water flows from the drain holes. It should drain freely within 30 to 60 seconds for most crops. If standing water sits on top for more than a minute, add more perlite. Second, the pH test: take a handful of pre-wetted mix and use a soil pH meter (available at most US garden centers for under $20) to check the reading. Adjust before planting rather than after plants are growing, since pH correction is harder once plants are established. Write down your final recipe and test results so you can reproduce or refine it next season.
Farm supply chains like Tractor Supply, Rural King, and regional co-ops sell perlite, peat moss, vermiculite, and compost in much larger bags at significantly lower per-quart cost than hardware store or garden center retail bags. A 4-cubic-foot bale of peat moss from a farm supply typically costs about the same as a 2-cubic-foot bag at a garden center, cutting your peat cost nearly in half. Perlite in 4-cubic-foot bags (about 100 quarts) is a fraction of the cost of quart-sized retail bags. For a gardener mixing 50 to 100 gallons of potting mix per season, buying in bulk reduces ingredient cost by 40 to 60 percent compared to small retail bags.
Quick Reference: Potting Mix Ingredient Properties Comparison
Use this table to select ingredients and adjust ratios for your specific crop needs. Drainage, retention, and aeration scores correspond to the calculator’s scoring system (1 to 10). pH ranges are natural pre-amendment values.
| Ingredient | Drainage | Water Retention | Aeration | Natural pH | Best Use |
|---|---|---|---|---|---|
| Peat Moss | 2/10 | 9/10 | 5/10 | 3.5 to 4.5 | General base, add lime for most crops |
| Coco Coir | 5/10 | 8/10 | 6/10 | 5.8 to 6.5 | Sustainable peat alternative, most crops |
| Perlite | 9/10 | 1/10 | 9/10 | 7.0 to 7.5 | Drainage and aeration for all mixes |
| Vermiculite | 3/10 | 8/10 | 5/10 | 7.0 to 7.5 | Moisture retention for seeds and seedlings |
| Compost | 5/10 | 6/10 | 4/10 | 6.0 to 7.0 | Fertility and microbiology for all crops |
| Worm Castings | 5/10 | 7/10 | 4/10 | 6.5 to 7.0 | Concentrated fertility, 10-20% max |
| Bark Fines | 8/10 | 3/10 | 8/10 | 5.0 to 6.0 | Orchids, bromeliads, moisture-sensitive crops |
| Coarse Sand | 9/10 | 1/10 | 7/10 | 7.0 to 7.5 | Succulents, cacti, heavy pots for wind resistance |
| Biochar | 6/10 | 5/10 | 6/10 | 7.5 to 9.0 | Microbe habitat, CEC improvement, 5-15% max |
| Topsoil | 3/10 | 7/10 | 3/10 | 6.0 to 7.0 | Raised beds only, avoid in containers |
| Reference | More perlite | More coco/peat | More perlite | Lime raises pH | Sulfur lowers pH |
Source: Container media properties from Penn State Extension greenhouse and nursery management guides and University of Maryland Extension container gardening publications. Scores are approximate and reflect typical product behavior; actual values vary by brand, particle size, and processing method.