This perlite soil mix calculator helps container gardeners whose pots stay wet too long turn a soil guess into a clear plan. You enter a few numbers, and the tool works out the amounts for you. That matters because soil is the one garden input you cannot fix in a day: too little organic matter and plants starve, too much of one amendment and the mix goes wrong for a whole season. A written plan keeps each bed balanced, so plants grow without stress.
Perlite is a light volcanic rock, puffed up by heat until it looks like white popcorn. It does not break down, it does not feed plants, and that is exactly its job: it opens air pockets in a potting mix so water drains through instead of sitting around roots. Pots without drainage stay wet, roots run out of air, and plants rot from below. A good share of perlite keeps the mix open, so roots can breathe even after a heavy watering.
To use the tool, enter each value in order: Number of pots, Pot volume in litres, Perlite share in percent. Then press Calculate to see the result. Each line shows one part of the plan, so you can spot a number that looks wrong before you carry a bag outside. If you change any value, press Calculate again. The example button fills in a worked case so you can see how the tool behaves before you enter your own numbers.
Here is a worked example. Take 6 pots of 10 litres each with a 30 percent perlite share need 60 litres of mix in total, made of 18 litres of perlite and 42 litres of potting soil, which is 2 bags of perlite. That result is the point of the tool: one clear plan, with each part visible. If a neighbour asks why you mixed the soil that way, you can walk through the same lines. If your bed is bigger or smaller, you change one number and the plan updates. That makes changes fast and keeps the garden honest about what each bed really needs.
Mix perlite in whenever you pot up, and refresh tired pots each spring. Old potting mix compacts over winter, so tip it out, break up the lumps, and add fresh perlite before you reuse it. For Mediterranean herbs and cacti, use a higher share, near 50 percent, because they hate wet feet. For leafy plants that like steady moisture, stay near 20 to 30 percent, so the pot does not dry out too fast in summer.
Three mistakes show up again and again. The first is using perlite alone, with no soil, so plants have nothing to feed on. The second is crushing it while mixing, which destroys the air pockets it was meant to create. The third is forgetting drainage holes: perlite helps water move, but it cannot save a pot with no way out. Mix gently, pair it with real soil, and always check the holes first.
Tips
- Test a small patch first. One pot or one corner tells you more than a whole bed mixed on a guess.
- Add organic matter every season. Soil life eats compost, so a bed fed once slowly goes hungry again.
- Mulch every bed. Bare soil dries fast, while mulched soil holds water for days longer.
How do I know if my soil improved?
Squeeze a handful of damp soil: good soil crumbles softly instead of sticking in a lump or falling to dust. Also watch drainage after rain and how long beds stay moist in a dry week.
Can I mix too much of an amendment?
Yes. Most amendments help in a modest share and hurt in excess, so follow the tool result and mix evenly rather than tipping in a whole bag at once.
How often should I re-test or re-plan?
Once a season is enough for most beds. Re-plan after a wet winter, after adding a large amount of compost, or when a bed starts behaving differently from its neighbours.
Versión en español: Calculadora de Mezcla de Suelo con Perlita