Potting Mix Ratio Calculator

How this is calculated

A PARTS RECIPE IS A RATIO, AND THAT IS WHY IT SCALES. "Four parts base, two parts aeration, two parts compost" says nothing about quantity - it says the mix is half base, a quarter aeration and a quarter compost. Fix any one ingredient and the rest follow. The part can be a scoop, a bucket or a wheelbarrow, as long as it is the SAME container every time and you level it the same way. ⚠️ POTTING MIX IS SOLD BY DRY VOLUME, AND A DRY QUART IS NOT A LIQUID QUART. This catches people out and it is exact rather than approximate: a US liquid quart is 57.75 cubic inches and a US DRY quart is 67.2006 - so the dry quart is about 16.4% larger. Bagged compost, potting soil and perlite are all sold in dry measure, as are cubic feet. If you size a batch in liquid quarts and buy in dry ones you will over-buy by a sixth, which is the harmless direction; the other way round leaves you short. ⚠️ THE POT VOLUME HERE IS AN UPPER BOUND. It treats a pot as a straight-sided cylinder, and almost no pot is. Nursery pots taper, often considerably, so the real volume is lower - which is what the taper allowance is for. Set it to 0 for a genuinely straight-sided container and raise it for a strongly tapered one. If your pots are labelled with a volume, that figure is better than any calculation and you should use it directly. MIX IS COMPRESSIBLE, WHICH IS WHY THE NUMBER IS NEVER EXACT. The same mix loosely tipped and firmly pressed differs substantially in volume, and it settles again after the first watering. Mix a little more than the arithmetic says, expect to top pots up after watering them in, and do not chase the last few percent. THE RATIO IS YOURS, NOT THIS TOOL'S. The defaults are an ordinary starting point, not a recommendation - a mix for seedlings, for cactus, for a shrub that will sit in the same pot for five years and for a hanging basket that dries out daily are all correct and all different. Drainage, water retention and weight all move with the ratio, and the right answer depends on the plant, the pot, the site and how often you water. Adjust from a mix that has worked for you rather than from a number on a page. WET INGREDIENTS BEFORE MIXING, particularly peat and coir, which repel water when bone dry and will then refuse to take it up in the pot. Damp-but-not-soggy is the target, and it is far easier to achieve in the mixing tub than in a planted container. BAGGED MIX AND HOME MIX ARE NOT INTERCHANGEABLE BY VOLUME ALONE. A commercial mix usually carries a starter fertiliser charge that a home mix does not, so an unamended home mix may need feeding sooner. That is a question about the plant, not about the arithmetic here.

Common questions

Why does a parts recipe not say how much to make?

Because it is a ratio rather than a quantity, and that is what makes it useful. "Four parts base, two parts aeration, two parts compost" tells you the mix is half base, a quarter aeration and a quarter compost - and that holds whether you are filling one pot or a hundred. Fix any single ingredient and everything else follows. The only rule is that a part must be the SAME container every time, levelled the same way; a heaped scoop of perlite against a struck scoop of compost quietly changes the recipe.

What is the difference between a dry quart and a liquid quart?

About 16.4%, and it is exact rather than approximate. A US liquid quart is 57.75 cubic inches; a US DRY quart is 67.2006 cubic inches. Potting mix, compost and perlite are all sold in dry measure, as are cubic feet. This matters when you convert: sizing a batch in liquid quarts and buying in dry ones means over-buying by about a sixth, which is harmless, but doing it the other way round leaves you short on the last pot. This tool reports dry quarts throughout because that is what the bag says.

Why is the pot volume an upper bound?

Because it treats the pot as a straight-sided cylinder and almost no pot is one. Nursery pots taper, sometimes sharply, so the actual volume is lower than the cylinder suggests - which is what the taper allowance is for. Set it to zero for a genuinely straight-sided container and raise it for a strongly tapered one. If your pots carry a stated volume on the label, use that figure directly instead: it is a measurement of the actual pot, and it beats any calculation from two dimensions.

Why does my mix never come out at exactly the calculated volume?

Because mix is compressible, and substantially so. The same batch loosely tipped and firmly pressed occupies noticeably different volumes, and it settles again after the first watering - which is why pots need topping up a week later. Peat and coir also change volume as they take up water. The practical answer is to make a little more than the arithmetic says, fill pots without ramming them, water them in and top up. Chasing the last few percent of accuracy is not worth the effort on something this variable.

Is 4:2:2 the right ratio?

It is an ordinary starting point and nothing more, which is why every part is an input. A mix for seedlings, for cactus, for a shrub that will live in the same pot for five years and for a hanging basket that dries out daily are all correct and all different from each other. Drainage, water retention and weight all move with the ratio, and the right answer depends on the plant, the pot, the position and how often you actually water. The most reliable approach is to adjust from a mix that has worked for you rather than from a number on any page.

Should I wet the ingredients first?

Yes, especially peat and coir. Both repel water when they are bone dry, and once that mix is in a pot it can sit refusing to absorb anything while water runs down the sides and out of the drainage holes. Damp-but-not-soggy is the target, and it is far easier to achieve in a mixing tub where you can work it than in a planted container. Coir bricks in particular need soaking and breaking up before they go anywhere near a recipe, and they expand a great deal in the process - measure the parts AFTER hydrating.

Can I just buy bagged mix instead?

Often, and for a few pots it is usually the cheaper answer once you count your time. Mixing your own starts to pay at volume, and it pays most when you need something the shops do not sell - a very free-draining cactus mix, or a heavier mix for a windy site. One difference worth knowing: commercial mixes usually carry a starter fertiliser charge that a home mix does not, so an unamended home mix may need feeding sooner than you expect. That is a question about the plant rather than about the volumes here.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Potting Mix Ratio Calculator at https://www.bfcbrilliance.com/tools/potting-mix-ratio-calculator.

What I entered:
- Pot diameter (in): ___
- Pot depth (in): ___
- Taper allowance (%): ___
- How many pots (pots): ___
- Parts base (peat, coir, bark) (parts): ___
- Parts aeration (perlite, grit, pumice) (parts): ___
- Parts compost (parts): ___
- Parts amendment (worm castings, sand) (parts): ___

What it calculated:
- Total mix to make: ___
- Mix per pot: ___
- Base (peat, coir, bark): ___
- Aeration (perlite, grit, pumice): ___
- Compost: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

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