Potting Mix Recipe Calculator

How to Use the Potting Mix Recipe Calculator

• Set the batch size in cubic feet — the bed or the number of pots you are filling.
• Enter the three component percentages; aim for them to sum to 100.
• Use the base slot for peat or coir, compost for the biology, aeration for drainage.
• Type each material's price per cubic foot to see the blend's real cost.
• Check 'Batch makes' — it flags a mix whose ratios fall short of a full 100%.
• Adjust the recipe against your plants: more aeration for pots, more compost for beds.

The Blend-Volume Formula, Explained

Worked Example: Blending an 8 cu ft Batch

• You want a 50/30/20 rich potting mix and are blending 8 cu ft for a set of patio pots.
• Base: 8 × 50 ÷ 100 = 4 cu ft of coir — the bulk of the pile.
• Compost: 8 × 30 ÷ 100 = 2.4 cu ft for nutrients and biology.
• Aeration: 8 × 20 ÷ 100 = 1.6 cu ft of perlite to keep it draining.
• Batch makes: 4 + 2.4 + 1.6 = 8 cu ft exactly, because the percentages sum to 100.
• Priced at $2.50 coir, $3 compost and $4 perlite per cu ft, cost is 4×2.5 + 2.4×3 + 1.6×4 = $23.20.
• That is under half the price of the same 8 cu ft in bagged mix — the DIY saving a homemade blend is built around.
• Set the same ratios to sum to only 90% and 'Batch makes' reads 7.2 cu ft, quietly warning you the recipe has a hole.

Common Blend Ratios

Key Terms

Base component

The peat or coir bulk of a mix — holds moisture and structure; coir is the renewable, near-neutral stand-in for peat.

Aeration component

Perlite, vermiculite or bark that keeps pore space open so container roots can breathe and drain.

Mel's Mix

The equal-thirds compost, peat/coir and vermiculite recipe that raised square-foot beds are built on.

Soilless mix

A manufactured blend with no actual soil — light, sterile and drainage-dependent, the right product for pots.

Benching / finishing

The nutrients and pH buffer (lime, starter feed) a raw volume ratio omits but a real mix must add.

What This Means at the Garden Center

• Buy the base by the bale and re-bag it: cost per cubic foot collapses from the small-bag aisle to the big compressed brick, which the calculator's per-cu-ft prices let you prove before you load.
• Compost is where a homemade mix lives or dies — three different composts at the same 30% give wildly different fertility, salt and weed pressure, so source it like an ingredient, not a filler.
• Peat wants buffering: it is acidic and initially dry, so a real blend adds garden lime and a wetting agent the volume math never shows; coir needs less pH correction but similar wetting.
• Match drainage to the vessel: bump aeration toward the high end for pots with one hole and beds that hold water, and lean compost-heavy for in-ground beds that can carry a richer, heavier mix.
• Blend slightly more than the bed needs — a batch that runs out at the top spurs a rushed, unratioed top-up of straight compost that throws the whole profile off.

Common Mistakes & Fixes

• Entering percentages that do not add to 100 and wondering why an '8 cu ft' batch only yields 7 — the Batch-makes output exists to catch exactly this.
• Using garden soil as the 'base' in a container blend, which compacts and drowns roots the aeration fraction was meant to save.
• Loading compost far above the recipe for 'more fertility', then trading structure and drainage for a mix that turns into dense, anaerobic sludge in a pot.
• Skipping the nutrients and pH buffer a soilless mix needs — an unamended peat blend grows nothing well once the seed's own food runs out.
• Assuming coir and peat are drop-in equal on cost — coir rehydrates stubbornly dry and peat is acidic, so each carries prep the calculator's clean per-foot price hides.

When to Use This (and When Not To)

Potting Mix Recipe Calculator: Frequently Asked Questions