Good seedlings start with good mix, and this free seed starting mix calculator works out exactly how much of each ingredient you need. Enter how many trays you have, how many litres each tray holds, and the parts of coco coir, perlite and compost you want, and the tool gives you the total mix and the litres of each ingredient. No more guessing and no more half-empty bags.
To use the tool, start with your tray count and the volume of each tray. A standard seed tray holds about two litres of mix, while a larger tray or a set of pots can hold much more. If you are unsure of the volume, fill one tray with water from a measuring jug and note how many litres it takes. That one measurement makes every later calculation accurate.
Next, choose your parts. A common seed mix is two parts coco coir, one part perlite and one part compost, which holds moisture, drains well and feeds young seedlings gently. You can change the parts freely: more perlite means sharper drainage for seeds that hate wet feet, while more coir means the mix stays moist for longer in a warm room.
The first result is the total litres of mix you need, which is simply trays times litres per tray. This number tells you how much to make in one batch and whether you need to buy more ingredients. Making a little extra is wise, because trays always take slightly more than the maths says once the mix settles, and a spare bucket of mix is useful for topping up.
The other results are the litres of coir, perlite and compost. These are worked out from your parts: each ingredient gets its share of the total. For example, ten trays at two litres each with two parts coir, one part perlite and one part compost need twenty litres in total, split into ten litres of coir, five of perlite and five of compost.
Why each ingredient matters is worth knowing. Coco coir holds water like a sponge and gives roots something soft to grow through, but it holds almost no nutrients on its own. Perlite is light volcanic rock that keeps the mix open and airy, so roots can breathe and water drains instead of pooling. Compost adds gentle nutrients and helpful microbes that young plants need.
Do not use ordinary garden soil for seed starting. Garden soil compacts in small trays, drains poorly, and can carry weed seeds and disease. A purpose-made mix stays light and clean, which is exactly what a tiny seedling needs. If you want to save money, buy coir in compressed bricks, which expand to many litres when soaked, and mix your own in a bucket.
Mixing technique matters as much as the recipe. Soak the coir brick first until it is fully expanded and damp, then mix in the perlite and compost in a large bucket or wheelbarrow. Wear gloves and a dust mask for the dry perlite, and mix until the colour is even. Fill trays loosely and water gently: pressing the mix down hard squeezes out the air that roots need.
A common mistake is making the mix too rich. Seedlings need very little food at first, because the seed itself carries a packed lunch for the first leaves. A mix heavy in compost or fertiliser can burn young roots. Start lean, and feed with a weak liquid feed only once the seedlings have two or three true leaves and are growing strongly.
Storage is simple. Keep leftover dry ingredients in sealed bags or bins, out of rain, and they will last for years. Mixed, damp mix keeps for a few weeks in a covered bucket, but it is best made fresh, because damp mix left standing can grow mould. Label your bucket with the recipe, so next time you can repeat a mix that worked well.
Keep in mind that this calculator gives volumes for a general seed mix, not a prescription for every plant. Cacti and succulents want far more perlite, while moisture lovers want more coir. Use the numbers as a solid starting point, watch how your seedlings grow, and adjust the parts next time until the mix fits your plants and your watering habits.