Container Mix Compost Calculator

How Much Compost in a Container Mix?

Container mixes behave differently from in-ground soils. Because containers drain only from the bottom, they need a balance between moisture retention (compost, coir, or peat) and aeration (perlite or vermiculite). Too much fine material — including well-decomposed compost — creates a dense, waterlogged medium. Too much perlite drains too fast and requires constant watering.

University of Maryland Extension recommends including compost in container media and gives workable examples ranging from 50:50 compost blends to 100% compost or leaf mold for some vegetables and herbs. ^[1] For most home gardeners, a more conservative blend is easier to irrigate and less likely to accumulate salts, so compost.tools defaults to 25–30% compost for general-purpose container mixes.

A reliable general-purpose container mix: 25–30% mature compost + 40% coir or peat + 25–30% perlite. For heavy feeders like tomatoes or squash, lean toward 30% compost. For drought-tolerant or low-nutrient plants like succulents, drop compost to 5–10% and increase coarse grit or perlite to 50%+.

Why Compost Percentage Matters in Containers

The primary risk with high compost ratios in containers is soluble salt accumulation. As compost continues decomposing in a pot, nitrogen and other nutrients release — which is good — but if drainage is poor, salt can build up in the medium and burn roots. The threshold varies by compost type: manure-based composts tend to have higher electrical conductivity (EC) than wood-chip or food-scrap-based composts. If you're using a high compost ratio (above 40%), measure the compost's EC with an inexpensive soil EC meter. Target below 3.5 dS/m for seed starting, below 5 dS/m for established transplants.

For vermicompost specifically — the product of worm bins — much higher ratios are workable because worm castings have naturally low EC and high plant-available nutrients. See our Worm Bin Calculator to estimate how much vermicompost your bin can produce over time.

Container Pot Volume Reference

Standard pot sizes are often listed by diameter but actual volume varies by pot height and shape. As a reference: a standard 1-gallon nursery pot holds about 6 quarts; 5-gallon nursery containers hold about 18–20 quarts; 15-gallon containers hold 55–60 quarts. The calculator uses the cylinder formula (π × r² × h) for round pots and length × width × height for rectangular planters — enter the interior dimensions for accurate results.