Raised Bed Soil Mix Calculator

Mel's Mix is one-third compost, one-third peat or coir, and one-third coarse vermiculite by volume, and this calculator turns your bed's dimensions into the exact cubic feet and bag counts you need to fill it right the first time.

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What Is Mel\'s Mix?

Mel\'s Mix is the soil recipe at the heart of Square Foot Gardening, created by Mel Bartholomew. It is equal parts, by volume, of three ingredients: compost for nutrients, peat moss or coconut coir to hold moisture, and coarse vermiculite to keep the blend light and well-drained. The classic ratio is one-third, one-third, one-third, which is why a 4 ft x 4 ft x 6 in bed needing 8 cubic feet of mix breaks down to about 2.7 cubic feet of each component. Unlike native garden dirt, this mix starts free of weed seeds and pests, drains beautifully, and is loose enough for roots to push through with almost no resistance.

How We Calculate Your Recipe

The volume of a rectangular bed is simply length times width times depth, with depth converted from inches to feet by dividing by 12. We multiply that by your number of beds, then split the total into three equal parts.

Total cu ft = Length(ft) x Width(ft) x (Depth(in) / 12) x Beds
Each component = Total / 3

Depth Matters More Than You Think

Square Foot Gardening was designed around 6-inch-deep beds, which suit lettuce, herbs, and most shallow-rooted crops. But root crops, tomatoes, and anything you want to grow vigorously prefer 10 to 12 inches. Going from 6 to 12 inches doubles your soil volume and cost, so it pays to match depth to what you actually plan to grow. A 4x8 bed at 6 inches needs 16 cubic feet; the same bed at 12 inches needs 32. Coir is increasingly chosen over peat because peat bogs are a slow-to-renew carbon store, and coir is a renewable byproduct of coconut processing.

Frequently Asked Questions

How much soil do I need for a 4x4 raised bed?
A 4 ft x 4 ft bed at the classic 6-inch depth needs 8 cubic feet of soil total. For Mel's Mix that is about 2.7 cubic feet each of compost, peat or coir, and coarse vermiculite. If you build the bed 12 inches deep instead, you will need 16 cubic feet total, or roughly 5.3 cubic feet of each component.
Can I use coconut coir instead of peat moss?
Yes, and many gardeners prefer it. Coir is a renewable coconut byproduct, has a near-neutral pH compared to acidic peat, and rewets easily after drying out. Use the same one-third proportion. Coir is sold as compressed bricks or bales that expand five to seven times when soaked, so hydrate it fully before mixing and measure the expanded volume, not the dry block.
Why does Mel's Mix use vermiculite instead of perlite?
Coarse vermiculite holds water and nutrients while still keeping the mix light, which is exactly what a shallow raised bed needs. Perlite drains faster and holds almost no water, so it can leave the bed too thirsty in summer. Some gardeners blend the two, but stick to coarse, not fine, vermiculite, since the fine grade compacts and defeats the purpose.
Is bagged or bulk soil cheaper for raised beds?
For one small bed, bagged components are simplest and the price difference is minor. Once you pass roughly 15 to 20 cubic feet, or about a half cubic yard, bulk compost and coir delivered by the yard usually cost far less per cubic foot than bags. Call a local landscape supplier for a delivered quote and compare it against the bag count this calculator gives you.

Practical Guide for Raised Bed Soil Mix Calculator

Buy compost from at least three different sources rather than three bags of the same brand. Mel Bartholomew's original advice was to blend multiple kinds of compost so the finished mix carries a wider range of nutrients and microbial life. A bed filled with one single compost can run hot, salty, or nutritionally lopsided, while a five-way blend self-corrects and feeds plants steadily through the whole season.

Wet the peat or coir before you mix, because dry peat is hydrophobic and will repel water for weeks if you skip this step. Spread it on a tarp, soak it down, and turn it until it is evenly damp and crumbly. Coir bricks expand dramatically when hydrated, roughly five to seven times their dry volume, so a single compressed block can yield a surprising amount of usable material once it is fully wetted.

Plan to top up the bed every season because the volume drops as organic matter breaks down. Compost and peat both decompose and compact, so a bed that started flush will settle an inch or two over a year. Each spring, add a fresh inch or two of compost across the top rather than rebuilding from scratch, which keeps the nutrient supply rolling without re-buying vermiculite, the most expensive component.

Quick Checklist

  • Blend compost from three to five different sources for a balanced nutrient profile
  • Pre-soak peat or coir on a tarp so it actually absorbs water before planting
  • Use coarse, not fine, vermiculite so the mix stays loose and drains freely
  • Mix the three components thoroughly on a tarp before filling, not in layers