Free potting mix calculator. Enter how many pots you are filling to find the potting mix in litres and the 20 litre bags you need.
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Enter how many pots you are filling. This potting mix calculator plans 5 litres of potting mix per pot, so 10 pots need 50 litres, which is 2.5 bags of 20 litres. Pots vary in size, so use the pot size litres table below to check your pot against the 5 litre planning figure before you buy.
Mix litres equal pots times 5. Ten pots need 50 litres, and 50 divided by 20 gives 2.5 bags of 20 litres. Four pots need 20 litres, which is exactly 1 bag. This 5 litre figure suits a medium pot of about 25 centimetres across. Small herb pots need far less and large patio pots need far more, so check the table below and adjust the pot count up or down for your real pot size.
Standard pots are measured by width across the top. A 10 centimetre pot holds about 0.5 litres, a 15 centimetre pot about 1.9 litres, a 20 centimetre pot about 4.5 litres, a 25 centimetre pot about 8 litres, a 30 centimetre pot about 13.5 litres, a 33 centimetre pot about 18 litres, a 42 centimetre pot about 28 litres, and a 50 centimetre pot about 52 litres. A 25 litre bag therefore fills about 13 pots of 15 centimetres, or about 2 pots of 30 centimetres.
Most pots are close enough to cylinders for planning math: volume equals pi times radius squared times height. A 12 inch diameter pot that is 10 inches tall holds about 1131 cubic inches, which converts to about 18.5 litres, 0.65 cubic feet, or 20 dry quarts. Measure the inside diameter at the top opening, since that is the volume you actually fill, and leave headroom for watering as described below.
Pots taper: the top is wider than the base. Treating a tapered pot as a straight cylinder overestimates the volume by roughly 19 percent. A pot 10 inches across the top, 8 inches across the bottom and 9 inches tall holds about 575 cubic inches with the frustum formula, against 707 cubic inches as a cylinder. The frustum result is about 9.4 litres. If your pots taper sharply, plan with the lower frustum figure and keep one spare bag.
Competitor tables list volume by container type. A standard clay pot of 12 inches holds about 10.7 dry quarts, a 14 inch pot about 15.3 dry quarts, and a hanging basket of 12 inches about 7.9 dry quarts. A window box of 24 inches holds about 11.7 dry quarts and a 36 inch box about 19.7 dry quarts. A 20 quart bag fills two 12 inch clay pots, or one 14 inch basket plus half of another.
Grow bags are sold by nominal gallons, but the crop decides the size. Lettuce, spinach and most herbs suit a 5 gallon bag at about 16 litres filled. Strawberries and bush beans suit a 7 gallon bag at about 22 litres. Peppers and chillies suit a 10 gallon bag at about 32 litres. Tomatoes and courgettes suit a 15 to 20 gallon bag at about 48 to 64 litres. Filled volume is about 85 percent of nominal size, because bags are measured to the rim and nobody fills to the rim.
Fill to about an inch below the rim. That headroom, or watering gap, stops water running straight off the surface. Skip the old gravel in the bottom trick: research consistently shows a gravel layer raises the perched water table inside the pot, leaving roots wetter, not drier. One drainage hole plus a freely draining mix beats gravel every time.
Fresh mix settles 5 to 10 percent after the first waterings, and taller containers compress more, needing 15 to 20 percent extra in some cases. Buy a little extra and round up to whole bags. For conversions, 1 cubic foot equals about 28.3 litres and about 26 dry quarts, 1 litre equals about 1.06 quarts, and 1 US gallon equals about 3.79 litres. Note that nursery trade gallons are nominal sizes, not liquid gallons, so measure the real pot for an exact figure.
Old potting soil can be reused. Tip it out, pick out roots and grubs, mix one part fresh compost to three parts old mix, and add a handful of perlite per bucket plus a balanced slow release feed. Do not reuse mix from a plant that died of root or wilt disease, and do not reuse mix for the same crop family two years running. Old mix that is too tired for pots still works as bulk filler in the bottom third of a deep raised bed.
This calculator plans 5 litres per pot. Ten pots need 50 litres, which is 2.5 bags of 20 litres. Check the pot size table for your real pot size.
About 13.5 litres. A 25 centimetre pot holds about 8 litres, and a 20 centimetre pot holds about 4.5 litres.
Volume equals pi times radius squared times height. A 12 inch diameter pot 10 inches tall holds about 1131 cubic inches, or about 18.5 litres.
A tapered pot is a frustum, a cone with its point cut off, so it holds less than a cylinder drawn around its rim, roughly 19 percent less in the worked example.
Divide total litres by the bag size and round up. Fifty litres is 2.5 bags of 20 litres, so buy 3 bags if you want no shortfall.
No. Gravel raises the perched water table and leaves roots wetter. Use one drainage hole and a freely draining mix instead.
No. Leave about an inch of headroom so water soaks in instead of running off. Fresh mix also settles after watering.
Yes. Mix one part fresh compost to three parts old mix, add perlite and feed, and skip mix from diseased plants or the same crop family two years running.
Balance your compost pile - hit the ideal 30:1 carbon-to-nitrogen ratio.
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