A soil calculator for raised beds and potted plants can help you avoid frustrating mistakes when setting up your beds or containers. Have you ever gone to the garden center and guessed at how much potting soil or raised bed soil to buy? If so, you likely brought home too little or too much.
The good news is that you don’t need to be a math whiz to use a soil calculator formula and ensure that you get the right amount. We’ve answered some common soil volume questions here and provided the tools necessary for you to do your own calculations. We’ve also found a great online calculator you can use if you prefer.
Perlite is a versatile soil amendment used by gardeners to improve drainage, aeration, and moisture retention in potting mixes and garden beds. But how much perlite should you add per gallon of soil? The ideal perlite to soil ratio depends on the type of plants you’re growing and their specific needs
When added to soil, perlite’s porous structure creates air pockets that allow oxygen to reach plant roots while also providing room for water drainage. This prevents soil compaction and improves root growth. Perlite also helps retain moisture and nutrients. But too much perlite can lead to excessive drainage, while too little may compact the soil. Finding the right balance is key.
Recommended Perlite to Soil Ratios
As a general guideline, a good starting ratio is:
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1 part perlite to 2 parts potting soil – This ratio works well for most outdoor container plants and indoor houseplants.
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1 part perlite to 1 part potting soil – Seedlings and cuttings benefit from more perlite to prevent damping off.
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Up to 1 part perlite to 3 parts soil – Use less perlite for moisture-loving plants like orchids.
But recommended perlite ratios ultimately depend on the plant. Here are more specific guidelines:
Seed Starting Mixes
Young seedlings need loose, well-aerated soil. Aim for:
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1 part perlite to 1 part vermiculite and peat or coco coir
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Or a mix of 1 part perlite and 1 part peat or coco coir
Potting Mixes for Mature Plants
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Cacti & succulents – 1 part perlite to 2 parts soil. They need fast drainage.
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Orchids – 1 part perlite to 4 parts bark or moss. Orchids prefer moisture retention.
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Vegetables – 1 part perlite to 2-3 parts soil. Veggies need moisture and nutrients.
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Trees, shrubs & perennials – 1 part perlite to 3-4 parts soil. Less perlite for larger plants.
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Annuals & herbs – 1 part perlite to 2 parts soil. More perlite for frequent watering needs.
Amending Garden Beds
When amending in-ground beds:
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Clay soil – Add up to 1 part perlite per 2 parts existing soil
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Sandy soil – Add 1 part perlite per 4-5 parts existing soil
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Loam soil – Add 1 part perlite per 4-6 parts existing soil
Perlite Volume Per Gallon of Soil
To find out how many cups or quarts of perlite you need per gallon of soil, refer to these approximate volumes:
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1 gallon of soil = 4 quarts or 16 cups
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1 quart of perlite = ~2 cups
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1 cup of perlite = 8 oz or 240 ml
With this in mind, here are some perlite-to-soil volume ratios:
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2 parts soil to 1 part perlite:
- 1 gallon of soil = 4 quarts
- 1 quart of perlite for every 2 quarts of soil
- So 2 quarts perlite per 1 gallon soil
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3 parts soil to 1 part perlite:
- 1 gallon of soil = 4 quarts
- 1 quart perlite for every 3 quarts soil
- So 1 quart perlite per 1 gallon soil
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4 parts soil to 1 part perlite:
- 1 gallon soil = 4 quarts
- 1 quart perlite for every 4 quarts soil
- So 1 cup perlite per 1 gallon soil
These volumes serve as a good starting point. Tweak the ratios as needed based on your plants and soil conditions. Too much perlite causes excess drainage while too little leads to compaction. Test different ratios to find the right balance.
Perlite Particle Size
Perlite comes in different grades from extra fine to coarse. Particle size affects drainage and aeration:
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Fine perlite (grades 1-2): Use for seed starting mixes and cuttings. Provides aeration while retaining moisture.
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Medium perlite (grade 3): The most common size used for general purpose potting soil and container plants.
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Coarse perlite (grade 4): Provides maximum drainage. Best for amending garden beds and very dense or compacted potting soils.
For seed starting mixes, use fine grades. Medium grades work for most mature potted plants. Coarse perlite can be too porous for containers, leading to excessive drainage. Reserve it for heavy in-ground soils.
Factors Affecting Ideal Perlite Ratio
Consider these factors when deciding how much perlite to use:
Plant type – Adjust ratios based on water and drainage needs. For example, orchids need more moisture retention while cacti require fast drainage.
Soil texture – Sandy soils need less added perlite than clay soils. Loams fall in between. Test drainage and amend as needed.
Container size – Use more perlite in smaller containers, which are prone to quick moisture loss. Larger planters need relatively less.
Environment – Indoor plants and greenhouses require more perlite than outdoor plants. Balance moisture retention and drainage.
Watering frequency – Frequent watering needs more perlite for drainage. Drought tolerant plants need less.
Plant age – Seedlings need more perlite. As plants mature, reduce perlite ratio to retain moisture and nutrients.
Time of year – Use slightly more perlite in hot summer months to prevent moisture loss. Reduce ratio in cool winters.
Signs Your Soil Needs More Perlite
Watch for these signs your potting mix or garden soil needs more perlite:
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Water pools on the surface or drainage is slow
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Soil remains soggy between waterings
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Roots appear stunted, brown or mushy
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Leaves wilt, yellow or drop even with regular watering
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Surface crusting or compaction develops
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Seedlings damp off due to poor drainage
If your plants show these symptoms, try amending with more perlite, up to 1 part perlite to 2 parts soil. This will improve drainage and aeration.
Is There Such a Thing as Too Much Perlite?
Yes, it is possible to add too much perlite. Using excessive amounts can lead to:
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Soil drying out too quickly between waterings
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Difficulty retaining moisture and nutrients
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Tip burn or browning along leaf margins
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Wilting or shriveling, even right after watering
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Stunted growth and smaller leaves
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Plant instability or tipping over due to lightweight mix
If this occurs, reduce the perlite ratio and amend the soil with organic matter like compost to help retain moisture and nutrients.
The ideal perlite to soil ratio depends on the plant and its needs. But a good general starting point is 1 part perlite to 2 parts potting mix. Adjust up or down based on soil texture, container size, watering frequency, and environmental factors. Too much perlite causes excess drainage while too little leads to compaction. Observe plants and amend ratios as needed until the right balance is struck. With the proper perlite content, plants will thrive with improved drainage, aeration and moisture retention.
Soil Calculator – How much soil do I need?
The calculations provided here will allow you to buy the right amounts to fill your containers or raised beds, but how much it takes to fill them initially is not the only factor to consider. High-quality potting and raised bed mixes are loamy, meaning they are aerated and will pack down to some degree with watering.
As you make your determinations on how much soil you’ll need using the information we’ve provided, err just a bit on the high side so that you’ll have a little left over to add after your soil has settled. Adding about 15 percent to your final calculations should provide you with an adequate buffer.
How much potting soil do I need?
If you’re filling round pots the calculations necessary to answer this question can be a bit involved, so we’ll avoid them and give you direct answers. Based on their top diameters, here are the soil amounts required to fill the most common sizes of round pots:
- 8 inch (20 centimeter) – 1 gallon (4 quarts or 3.8 liters) – 0.15 cubic foot
- 10 inch (25 centimeter) – 3 gallons (12 quarts or 11 liters) – 0.46 cubic foot
- 12 inch (30 centimeter) – 5 gallons (20 quarts or 19 liters) – 0.77 cubic foot
- 14 inch (36 centimeter) – 7 gallons (28 quarts or 26.5 liters) – 1 cubic foot
- 16 inch (41 centimeter) – 10 gallons (40 quarts or 38 liters) – 1.5 cubic feet
- 20 inch (51 centimeter) – 17 gallons (68 quarts or 64 liters) – 2.3 cubic feet
- 24 inch (61 centimeter) – 25 gallons (100 quarts or 95 liters) – 3.3 cubic feet
5 quick tips Perlite
FAQ
What is the ratio of perlite to soil?
The optimal soil to perlite ratio depends on the specific plant and its needs, but a common starting point for general potting mixes is a 2:1 or 3:1 ratio of potting mix to perlite. For example, you could use 2 parts potting mix and 1 part perlite.
Can you add too much perlite to soil?
How much perlite to amend soil?
Perlite improves drainage and keeps the soil light, which helps plant roots grow more easily. Here’s a simple guide for how much perlite to use: General rule: Add about 10-20% perlite by volume to your soil. For example, if your raised bed needs 100 liters of soil, mix in about 10-20 liters of perlite.
How much perlite to put in potting mix?
To add to sieved compost or potting mix to create a lightweight homemade mix for plants, use at a rate of around one part perlite to four parts potting mix or …
How much perlite to add to soil?
The ideal amount of perlite to add to soil depends on several factors, including the type of plant being grown, the size of the container, and the composition of the soil mix. Gardeners must carefully consider these factors when deciding how much perlite to add to their soil.
How much perlite should I put in my garden?
However, for plants that require excellent drainage, such as cacti and succulents, a higher ratio of perlite to soil (up to 50%) may be necessary. For seed starting and rooting cuttings, a ratio of 1:1 perlite to soil is recommended to ensure proper moisture retention and soil aeration.
What is a good perlite ratio for clay soil?
For clay soil, adding 25-50% perlite is recommended. This mixture significantly improves drainage and aeration, making it easier for plant roots to access water and nutrients. Adjusting the ratio based on ongoing plant health is also advisable. What is the ideal perlite ratio for sandy soil?
How much perlite do you need for succulents?
Optimal Ratios for Soil Types: Clay soil benefits from 25-50% perlite, sandy soil requires only 10-20%, and loamy soil does well with approximately 10% perlite. Plant-Specific Needs: Adjust perlite ratios based on plant type; succulents may need 50-70%, while leafy greens thrive with only 10-20%.
Is perlite a good soil mix?
Perlite is suitable for many types of plants, particularly those that prefer well-draining soil. However, some moisture-loving plants may require a different mix that includes more organic material. Incorporating perlite into soil can significantly improve its aeration, drainage, and overall structure.
What happens if you add too much perlite to soil?
Excessive amounts of perlite can lead to overly dry soil, especially in hot climates, which might require more frequent watering. Monitor your plants for signs of stress and adjust your perlite ratios as needed. How Much Perlite Should I Add to Soil?