Common bed sizes
Uses the settling allowance and bag size selected above. Each ingredient is one third of the total.
| Bed (ft) | Bed (m) | Total (L) | Total (cu ft) | Each ingredient (L) | Bags each |
|---|
What Mel’s Mix is, and why gardeners still use it
Mel’s Mix is the growing medium Mel Bartholomew developed for his Square Foot Gardening method, first published in 1981. Instead of digging a plot and improving whatever soil is underneath, you build a raised bed and fill it with a blend of three ingredients mixed in equal parts by volume: coarse vermiculite, peat moss (or coconut coir), and compost made from several different sources. Bartholomew trained as a civil engineer, and the recipe reflects that. Every ingredient can be measured, so the result is repeatable from one bed to the next.
The reason it works is physical, not magical. A raised bed sits above the ground, so it has no deep subsoil to drain into. Ordinary garden soil in that position tends to pack down, hold too much water and leave roots short of air. Mel’s Mix stays loose. Vermiculite and peat or coir hold water and nutrients while leaving air spaces, and compost supplies the nutrients plants actually take up. Roots can move through it easily, which is why the method allows plants to be spaced closely.
This page gives you a calculator that turns your bed size into litres, cubic feet and bag counts, and the guide below explains how to read those numbers and buy sensibly.
The three ingredients
Coarse vermiculite
Vermiculite is a mineral that is heated until it expands into light, spongy flakes. In the mix it holds water and dissolved nutrients and keeps the structure open. Buy horticultural vermiculite sold as “coarse”. Fine grades are meant for other uses, such as seed starting, and they compact over time. Grade numbers differ between countries and brands (in the United States #1 is the largest particle and #4 the finest), so read the description on the bag instead of trusting a number.
A note on safety and sourcing: vermiculite from one mine in Libby, Montana, which closed in 1990, was contaminated with asbestos. Garden vermiculite sold today comes from other sources, but it is sensible to buy from a reputable brand, keep the material damp while you handle it, and wear a dust mask when pouring.
Peat moss or coconut coir
Peat moss is partly decomposed sphagnum. It holds many times its weight in water and is naturally acidic, roughly pH 3.5 to 4.5, which the compost in the mix helps to balance. Coconut coir is made from coconut husk and is close to neutral in pH. It re-wets more easily than dry peat, and it is the usual choice for gardeners who prefer not to use peat, since peat bogs store a great deal of carbon and regrow very slowly. Coir sometimes carries salts from processing, so rinse it if the supplier suggests so. Whichever you choose, soak it before mixing. Dry peat and dry coir both repel water at first.
Both products are often sold as compressed bales that expand a great deal once loosened and wetted. The expanded volume is what counts for this recipe, so check the expanded volume printed on the package.
Blended compost
Compost is the only ingredient that supplies nutrients in meaningful amounts, so it carries most of the plants’ needs. Bartholomew’s advice was to blend compost from several sources, because different materials contribute different nutrients and microbes. Typical sources include homemade compost, mushroom compost, composted manure, leaf mould and bagged garden compost. Use finished compost that is dark, crumbly and smells earthy. Fresh manure and half-rotted material can burn roots and tie up nitrogen.
How much soil does your bed need?
The volume of a rectangular bed is length × width × depth, as long as all three are in the same unit.
Imperial: length (ft) × width (ft) × depth (in) ÷ 12 gives cubic feet. Divide cubic feet by 27 to get cubic yards.
Metric: length (m) × width (m) × depth (cm) ÷ 100 gives cubic metres. Multiply by 1,000 to get litres.
Then add an allowance for settling, and divide the result by three, because each ingredient is one third of the total. The calculator above does all of this and also converts between the two systems, since a bag in one country may be labelled in litres and in another in cubic feet. One cubic foot is about 28.3 litres, and one cubic metre is about 35.3 cubic feet.
Worked examples
These figures include a 10% settling allowance and use the formulas above.
- 4 ft × 4 ft × 6 in (1.2 m × 1.2 m × 15 cm): 8 cu ft, or 8.8 cu ft with the allowance. That is about 2.9 cu ft (83 litres) of each ingredient.
- 4 ft × 8 ft × 6 in (1.2 m × 2.4 m × 15 cm): 16 cu ft, or 17.6 cu ft with the allowance. That is about 5.9 cu ft (166 litres) of each.
- 4 ft × 8 ft × 12 in (1.2 m × 2.4 m × 30 cm): 32 cu ft, or 35.2 cu ft with the allowance. That is about 11.7 cu ft (332 litres) of each.
- 3 ft × 6 ft × 12 in (0.9 m × 1.8 m × 30 cm): 18 cu ft, or 19.8 cu ft with the allowance. That is about 6.6 cu ft (187 litres) of each.
Square Foot Gardening beds are usually 6 inches (15 cm) deep, because the method relies on shallow, dense planting above a grid. Deeper beds suit root crops such as carrots and parsnips, and they cost proportionally more to fill, which is why the depth you pick matters more to your budget than almost any other choice.
Why you need a settling allowance
A freshly mixed bed is full of air. After the first few waterings, and again after rain, the mix settles, and the surface drops. If you fill to the exact volume you will be topping up within weeks. An allowance of about 10% is a sensible starting point. Raise it toward 15% if your mix is very fluffy or contains a lot of dry, bulky coir, and lower it if you plan to water the layers in as you fill. The calculator lets you set this from 0 to 30%.
Buying: bags, buckets or bulk
Most home gardeners buy the three ingredients in bags. The package states the volume in litres or cubic feet, and that number, not the weight, is what you match to the calculator. Bag sizes vary by country and by brand. Common sizes are 25, 50 and 70 litres in metric markets and 1, 1.5 or 2 cubic feet in the United States and Canada. Pick the size in the calculator that matches what your local shop sells, and the bag count will be correct for your market.
A standard 5-gallon (US) bucket holds about 0.67 cubic feet, which is a handy measuring unit when mixing, as long as you use the same bucket for all three ingredients. Ten-litre and twelve-litre buckets work equally well if that is what you have.
Bulk delivery, priced by the cubic yard or cubic metre, is usually available for compost and sometimes for coir or peat. Vermiculite is rarely sold that way. For a large project it can make sense to buy compost in bulk and the other two ingredients in bags. Compare the cost per litre or per cubic foot, not the price per bag, because bag sizes differ.
Peat moss, coir and vermiculite are usually the most costly part of the mix, and compost is usually the cheapest. If your budget is tight, the section on alternatives below explains what can be changed.
Mixing it step by step
- Prepare a clean surface. Use a tarp or a clean paved area.
- Break up and wet the peat or coir. Work it with water until it is evenly damp rather than soaking. Dry material is the main reason mixes turn patchy.
- Measure with one container. Add equal numbers of buckets of each ingredient. If the calculator says 6 buckets of each, count 6, 6 and 6.
- Combine thoroughly. Turn the pile with a shovel, or pull the tarp corners to roll the material over. Mix until the colour and texture look uniform.
- Fill the bed and water it in. Water gently, let it settle, and top up to the rim.
If the bed sits on bare soil, many gardeners put weed-suppressing cardboard or landscape fabric at the bottom. If gophers or moles are a problem where you live, hardware cloth under the bed is the more reliable protection.
Cheaper alternatives when the full recipe is too costly
The original recipe is the most reliable choice for small beds, but it is expensive at scale. Gardeners fill large beds in several other ways, and each involves a trade-off.
- Replace vermiculite with perlite. Perlite is lighter and usually cheaper. It adds air space but holds less water and nutrients than vermiculite, and it tends to float to the surface when watered heavily.
- Replace peat with coir. This is a straightforward swap for the same volume, and it avoids peat.
- Use a 60/30/10 blend. A common budget recipe is 60% screened topsoil, 30% compost and 10% perlite or coarse sand. It is cheaper and heavier, and it can compact more than Mel’s Mix, so it suits larger and deeper beds better than shallow square-foot beds.
- Buy a ready-made raised bed mix. This saves time and gives a decent result. Check the label for the volume and the ingredients.
- Fill the bottom with coarse organic material. In deep beds, logs, branches and other woody material can fill the lowest part, with the mix on top. This reduces cost, but the wood settles as it rots, so use a bigger settling allowance.
Whichever recipe you choose, you can still use the calculator for the total volume. Divide that figure according to your own ratio instead of thirds.
Common mistakes
- Buying by weight instead of volume. A bag of dry vermiculite and a bag of wet compost weigh very differently. Always compare litres or cubic feet.
- Forgetting that bale volume is compressed. A compressed peat or coir bale can expand to roughly double its size, so read the expanded volume on the package.
- Using a single compost. One source of compost gives a narrower range of nutrients. Blend at least a few.
- Skipping the settling allowance. The bed ends up several centimetres low after the first month.
- Mixing dry. Dry peat and coir resist water and leave dry pockets that roots avoid.
- Adding field soil “to fill the gap”. It defeats the purpose of the mix, because it brings back the compaction you were trying to avoid.
- Using fine vermiculite or unfinished compost. Both lead to a dense, sour bed.
Keeping the bed productive from year to year
One of the advantages of Mel’s Mix is that you do not replace it. After each harvest, remove the old roots, loosen the top layer, and add a layer of fresh compost, around 1 to 2 inches (2.5 to 5 cm), before replanting. Over several seasons the compost fraction breaks down and the volume drops, so topping up with compost and, now and then, a little more of the other ingredients keeps the bed at the right level. Rotate crop families between squares so that one kind of plant does not take the same nutrients every season.
Using the calculator in your own country
This calculator works in 16 languages, including English, German, French, Dutch, Swedish, Norwegian, Danish, Finnish, Italian, Spanish, Portuguese, Japanese, Korean, Hebrew, Tamil and Hindi. Choosing a region switches the language and the unit system together, and you can change the units at any time. The numbers are the same whichever language you use. Only the labels change.
Each region sells soil in different bag sizes, and some supply compost by the litre while others use the cubic foot. Pick the closest bag size in the calculator, then check the volume printed on the bag you intend to buy. When you are ready, use the download button to save a PDF in the language you chose. It lists your bed size, the total volume, the amount of each ingredient and the bag counts, so you can take it to the garden centre or share it with someone helping you build.
Quick summary
- Mel’s Mix is equal parts by volume: coarse vermiculite, peat moss or coir, and blended compost.
- Volume equals length × width × depth. Add roughly 10% for settling, then divide by three.
- Match the calculator to the volume printed on the bags, not to the weight.
- Soak peat or coir first, and blend compost from several sources.
- Top up the bed with compost each season rather than replacing it.
Quantities are estimates. Bag sizes, compression and moisture vary, so allow for a small margin when you buy.
Soil questions answered
Why not use garden topsoil in a raised bed?
Mineral topsoil is heavy. In a raised bed it tends to pack down and hold too much water, which starves roots of air. Mel’s Mix stays loose and drains well while still holding moisture.
Which vermiculite should I buy?
Buy coarse horticultural vermiculite. Grade numbers differ between brands (in the US, #1 is the largest and #4 the finest), so check the bag for “coarse” rather than relying on the number. Fine grades compact over time.
Why blend several composts?
Mel Bartholomew recommends compost from several sources, because each one supplies different nutrients. Avoid fresh manure, which can burn plants. Use finished, dark, crumbly compost.
Peat moss or coconut coir?
Peat is acidic (roughly pH 3.5–4.5). Coir is closer to neutral, but rinse it if it comes from a coastal supplier, as it can hold salts. Soak either one before mixing so it does not repel water.
How much extra should I buy for settling?
Fresh mix settles after the first few waterings. A 10% allowance is a good start; raise it toward 15% for very fluffy mixes.
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