Potting Mix Ratio Calculator – Ingredient Litres for Any Mix Recipe

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Why mix your own potting soil

Bagged potting mix works, but it is a compromise made for every plant at once. Seedlings want fine, gentle mix. Cacti want sharp drainage. Moisture-loving plants want mix that holds water. Mixing your own lets you match the recipe to the plant, and it is usually cheaper in bulk. This calculator splits any total volume into ingredient amounts for four proven recipes.

Enter the total mix you want to make in litres, and pick a recipe. The calculator gives the litres of each ingredient. Recipes use parts, so the math is simple: a 2:1:1 recipe has four parts in total, and each part is one quarter of your total. The tool does that division for you, including awkward totals that do not divide neatly.

The four recipes

Seed starting mix uses one part peat or coir, one part perlite, and one part vermiculite. It is fine, light, and holds water gently, which suits tiny seeds and fragile roots. It has little food in it, so feed seedlings weakly once they have true leaves. Do not use garden soil for seeds: it crusts, drains badly, and can carry disease.

General potting mix uses two parts peat or coir, one part perlite, and one part compost. This suits most houseplants, vegetables in pots, and flowers. The compost adds food and microbes, the peat holds water, and the perlite keeps air in the mix. It is the recipe to use when you are unsure.

Cactus and succulent mix uses two parts potting soil, two parts perlite, and one part coarse sand. It drains fast and dries fast, which is what desert plants need. Ordinary potting mix stays wet too long for cacti and rots their roots. Use coarse sand, not fine beach sand, which packs tight and blocks drainage.

Moisture-loving mix uses three parts peat or coir, one part compost, and one part perlite. It suits ferns, hostas in pots, and tropical plants that hate drying out. It still needs perlite for air, because even moisture lovers rot in airless, soggy mix.

A worked example

Suppose you want 30 litres of general potting mix. The recipe 2:1:1 has four parts, so each part is 7.5 litres. You need 15 litres of peat or coir, 7.5 litres of perlite, and 7.5 litres of compost. For 30 litres of seed mix at 1:1:1, each part is 10 litres, so you need 10 litres of each ingredient.

Awkward totals work the same way. For 25 litres of cactus mix at 2:2:1, there are five parts of 5 litres each, so you need 10 litres of potting soil, 10 litres of perlite, and 5 litres of coarse sand. The calculator handles these divisions exactly, so you do not need to do fractions with a bucket in each hand.

Mixing technique

Mix dry ingredients in a wheelbarrow or large bucket, and stir well before adding water. Dry peat and coir repel water at first, so add water slowly and keep stirring until the mix is evenly damp like a wrung-out sponge. Fill pots with damp mix, not dry mix, because dry mix shrinks away from pot walls and water runs down the gap instead of soaking in.

Wear a dust mask when handling dry perlite and peat, since the dust irritates lungs. Work outdoors or in a ventilated shed. Wash hands after mixing, and store leftover mix in a closed bag so it stays damp and does not blow away.

Common mistakes to avoid

The biggest mistake is skipping perlite. Without it, peat mixes compact into a soggy brick within weeks, and roots suffocate. Another mistake is using fine sand instead of coarse sand or perlite: fine particles fill the air gaps instead of creating them. Always use coarse materials for drainage.

People also forget to feed. Homemade mixes contain little long-term food, especially seed mix. Start feeding with a weak liquid feed once seedlings have two sets of true leaves, and feed potted plants regularly through the growing season.

Frequently asked questions

Can I substitute coir for peat? Yes. Coir is made from coconut husks, holds water well, and is more renewable than peat. It has almost no nutrients, so it behaves like peat in recipes. Some coir is salty, so rinse it if the bag does not say it is washed.

How long does homemade mix last? Dry ingredients keep for years in closed bags. Mixed, damp mix keeps for months if covered. If stored mix smells sour, spread it out to air for a day before use, or mix in fresh perlite.

Do I need lime? Peat is acidic, and lime balances it for most plants. Many bagged peats already include lime. If you buy raw peat, add a small handful of garden lime per bucket of mix, unless you grow acid-loving plants like blueberries.

Use the calculator above to split any batch into exact ingredient amounts, mix damp, and match the recipe to the plant. Garden results are general guidance estimates only. Your local climate, your soil, and the instructions on your seed packets govern what actually grows, and results vary with weather and conditions.

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