What this calculator works out
Enter the internal measurements of a pot, trough, window box or hanging basket and this gives its volume in litres, how much compost it actually needs once you leave a gap for watering, and how many bags that comes to.
Litres is the useful unit here because bagged compost is sold that way — 40, 50 or 60 litres a bag — while pots are described in centimetres, and the two rarely meet in the middle.
The shape matters more than most people expect. Nearly every plant pot is narrower at the base than the top, and treating one as a straight-sided cylinder can overstate the compost needed by around 40%.
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Before you calculate
- Measure the inside of the planter. A thick-walled or double-skinned pot can hold noticeably less than its outside dimensions suggest.
- Measure the depth inside too, from the base up to the rim, not the height of the pot on the outside. Many pots have a recessed foot.
- Note the shape honestly. Most plant pots taper, and that changes the answer more than people expect.
- Leave a gap below the rim. Compost filled flush to the top sheds water over the side instead of soaking in.
A pot 40 cm across the top, 28 cm across the base and 35 cm deep holds about 28 litres of usable compost. Treat the same pot as a straight-sided cylinder 40 cm across and you get 40 litres — about 40% more than you need. The narrower base takes a surprising amount of volume out, and it is the single most common reason people come home with too much compost.
How this calculator works
Every shape is measured in centimetres, worked out in cubic centimetres, then divided by 1,000 to give litres. That last step is the useful one, because compost is sold in litres rather than by weight.
1 litre = 1,000 cm³
Rectangular: length × width × fill depth
Straight round: π × radius² × fill depth
Tapered round: (πh ÷ 3) × (R² + Rr + r²)
Half sphere: πd² × (3R − d) ÷ 3The tapered formula is the volume of a frustum — a cone with its point cut off, which is the shape of an ordinary plant pot. R is the radius at the top of the compost, r the radius at the base and h the depth of compost. Because the pot narrows as it goes down, the calculator works out the radius at whatever level you have chosen to fill to, rather than using the rim measurement throughout.
A hanging basket is treated as a half sphere, and a part-filled one as a spherical cap. This is why filling a basket to within 3 cm of the rim removes a quarter of its capacity rather than a tenth: the widest part of a bowl is at the very top, so the last few centimetres hold far more than the first few.
Watering gap. The calculator subtracts this before working anything out, so the figure it gives is what you actually need to buy, not the brim capacity of the pot.
Worked example: three tapered pots
Three patio pots, each 40 cm across the top, 28 cm across the base and 35 cm deep, filled to within 3 cm of the rim:
- Fill depth: 35 − 3 = 32 cm
- Radius at the compost surface: 14 + (20 − 14) × 32/35 = 19.5 cm
- Volume per pot: (π × 32 ÷ 3) × (14² + 14 × 19.5 + 19.5²) = 28,430 cm³, or 28.4 litres
- Three pots: 85.3 litres
- At 50 litres a bag: 2 bags
Two bags covers it with about 15 litres spare, which is a sensible margin because compost settles. Had the pots been treated as cylinders, the answer would have been 121 litres and three bags — a whole bag more than needed.
Understanding the result
The figure to buy against is the total in litres. Bag counts are rounded up, because you cannot buy most of a bag.
| Container | Typical size | Usable compost |
|---|---|---|
| Window box | 80 × 20 × 18 cm | 24 litres |
| Hanging basket | 35 cm across | 8 litres |
| Small patio pot, tapered | 30 cm top, 22 base, 26 deep | 11 litres |
| Large patio pot, tapered | 40 cm top, 28 base, 35 deep | 28 litres |
| Half barrel | 60 cm across, 45 deep | 119 litres |
| Grow bag trough | 90 × 35 × 25 cm | 69 litres |
Weight is worth a thought as well as volume. Wet compost is heavy, roughly 700 to 900 grams a litre once watered, so a 100-litre planter can weigh the better part of 90 kg when full and freshly watered. On a balcony, a roof terrace or an old timber floor, that matters. Fill large containers in place rather than trying to move them afterwards.
Things that affect the result
- Settling. Fresh compost drops by five to ten per cent after the first few waterings. Buying slightly over is sensible.
- Bag volume is loose volume. A 50-litre bag holds 50 litres of loosely filled compost. Pressed into a pot it occupies a little less.
- Drainage material. Crocks or gravel in the base displace compost. If you use them, subtract that depth from the internal depth.
- Root balls. Planting a well-established shrub can take up a third of a pot's volume before any compost goes in.
- Wall thickness. Frost-proof and double-skinned pots can lose several centimetres of internal diameter.
- Rim shape. A heavily flared rim exaggerates the top diameter. Measure where the compost will actually sit.
Common mistakes
- Treating a tapered pot as a cylinder. The commonest error, and on a typical patio pot it overstates the answer by around 40%.
- Measuring the outside. Use internal dimensions throughout.
- Forgetting the watering gap. Filling to the rim wastes compost and makes watering nearly impossible.
- Mixing up litres and cubic metres. There are 1,000 litres in a cubic metre. Loose topsoil is sold by the cubic metre; bagged compost by the litre.
- Assuming a pot sold as "40 litres" needs 40 litres. That figure is usually brim capacity, and sometimes it is the outside volume.
- Buying to the exact figure. Compost settles. Round up.
Frequently asked questions
How many litres of compost does a pot need?
It depends on the shape as much as the size. A tapered pot 40 cm across the top and 35 cm deep takes about 28 litres, while a straight-sided pot of the same top diameter and depth takes about 44 litres. Measure the top, the base and the internal depth and let the calculator handle it, rather than guessing from the pot's diameter alone.
Why does my pot say 40 litres but the calculator says 28?
Manufacturers usually quote brim capacity, filled level with the rim, and some quote the volume of the pot as an object rather than what it holds. This calculator subtracts the watering gap, so it gives you what to buy rather than what the pot could theoretically contain.
How many pots will a 50-litre bag fill?
Roughly two large tapered patio pots, four to six small ones, two window boxes, or six hanging baskets. Enter your own measurements for a figure you can shop against.
Do I need crocks or gravel in the bottom for drainage?
Modern guidance is that you generally do not. Research indicates a layer of coarse material at the base does not improve drainage and can make it worse, because water is held at the boundary between the two materials rather than passing through. A free-draining compost and an open drainage hole matter far more. If you do add a layer, subtract its depth from the internal depth before calculating.
Is peat-based compost banned in the UK?
No. The government announced in 2022 that it intended to ban retail sales of peat compost, but the legislation has been repeatedly delayed and none is currently in force, so peat-based products remain legal to sell. In practice peat-free is now the dominant option on garden centre shelves, and organisations including the RHS stopped selling peat-based compost years ago. Check the bag if it matters to you — peat-free is stated prominently when it applies.
Can I reuse last year's compost?
For permanent shrubs and trees, topping up and refreshing the top few centimetres each spring is usually enough. For annual bedding and vegetables in containers, old compost is generally depleted of nutrients and structure, so most gardeners replace it and spread the old material on borders rather than throwing it away.
What is the difference between compost and topsoil?
Compost is a growing medium made from organic material, designed to be open, light and free-draining, which is what containers need. Topsoil is soil, heavier and denser, and it compacts in a pot. Use compost in containers and topsoil in the ground or in raised beds, where a mixture of the two is common.
How heavy will a full planter be?
Wet compost weighs roughly 700 to 900 grams a litre, so a 50-litre planter is around 35 to 45 kg once watered, and a 100-litre one can approach 90 kg. This matters on balconies, roof terraces and older timber floors. Position large containers before filling them.
Is what I enter stored?
No. Figures are processed in your browser and never transmitted or retained.
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References
These organisations publish the guidance and figures behind this page.
- Royal Horticultural Society — guidance on container gardening, growing media and drainage
- RHS — statement on peat — the RHS position on peat-free growing and the state of the transition
- GOV.UK — Defra policy on peat in horticulture, including the announced but not yet enacted retail ban
Guidance and regulations change. Check the current position at the source before relying on it.