Buying bags of generic “Garden Soil” from the hardware store is the quickest way to fail at raised bed gardening. It compacts too hard, drains poorly, and lacks nutrients. The gold standard for raised beds is a custom blend known as “Mel’s Mix,” made famous by Square Foot Gardening.
🥣 DIY Soil Mix Calculator
This mix is light, fluffy, and nutrient-dense. But figuring out how many bales of peat moss equal "one third" of your garden volume involves some tricky math involving compression ratios. Stop guessing. Use our DIY Soil Mix Calculator below to generate an instant shopping list for your project.
How to Use This Tool
We designed this calculator to translate "Total Volume" into "bags you actually buy at the store":
- Enter Dimensions: Input the length, width, and depth of your bed. (Note: Standard depth is 6 inches, but 12 inches is better for root crops).
- The Math: The tool calculates the total Cubic Feet and splits it into three equal parts (33% Compost, 33% Peat/Coir, 33% Vermiculite).
- The Shopping List: The result tells you exactly how many compressed bales of peat moss or large bags of vermiculite to buy to achieve that volume.
Why We Built This (The "Secret Sauce")
The hardest part of mixing soil is the expansion factor.
The Peat Moss Problem:
You can't just buy "10 cubic feet" of Peat Moss off the shelf. It is sold in compressed 2.2 cu ft bales that expand to nearly double their size when fluffed. Our calculator accounts for this 2:1 expansion ratio so you don't accidentally buy double the amount you need. (Check our Peat Moss Calculator for more on this).
Educational Guide: The Ingredients
Why do we use this specific 3-part recipe? Each ingredient serves a vital role.
1. Mixed Compost (The Nutrients)
This is the food. Do not buy just one type (like 5 bags of Steer Manure). The secret is to buy a blended mix. Combine mushroom compost, chicken manure, and worm castings. The more diverse your compost, the more trace minerals your plants get.
2. Peat Moss or Coco Coir (The Sponge)
This holds the water. Peat moss can hold 20x its weight in moisture, keeping your plants alive during summer heat. If you want a sustainable alternative, use Coco Coir, which behaves almost identically.
3. Coarse Vermiculite (The Structure)
This keeps the soil fluffy. Vermiculite is a puffed mineral that creates air pockets for roots to breathe. Tip: Buy the "Coarse" grade, not the fine dust. If you can't find it, Perlite is a cheaper substitute, though it floats to the surface over time.
Frequently Asked Questions
Is this mix cheaper than buying bags of soil?
Initially, no. Mixing your own high-quality soil costs more upfront than cheap "Topsoil" bags ($2/bag). However, this mix never compacts and lasts for 7-10 years. You only need to top it off with compost annually, making it cheaper in the long run.
Can I use Perlite instead of Vermiculite?
Yes. Perlite (the white styrofoam-looking rocks) performs the same function: aeration. It is cheaper and easier to find. However, Vermiculite holds moisture better, while Perlite drains faster. For raised beds, Vermiculite is preferred; for containers, Perlite is great.
Do I need to add fertilizer to this mix?
No! That is the beauty of the 1/3 compost ratio. If you use a good blend of composts (manure, mushroom, castings), your soil is packed with nutrients. You shouldn't need to add fertilizer for the first growing season.
