Make your plan
Buying potting mix should be simple, but it is surprisingly easy to get the amount wrong.
A large planter can need several bags of mix. A tapered pot may hold much less than it looks like from the top. And if you are filling five or six containers at once, a small mistake in your estimate can quickly turn into an extra bag — or a trip back to the garden center.
The Container Soil & Grow Bag Calculator helps you work out how much potting mix you need before you start filling.
Enter the inside measurements of your container, choose the shape, tell the calculator how many pots you are filling, and add the size of the potting-mix bag you plan to buy.
The calculator above will estimate the total amount of mix you need and how many full bags to buy.
Before you measure your pot
The most important thing is to measure the space that will actually hold potting mix.
Do not automatically use the size written on a product page or plant-pot label. That measurement may describe the outside of the pot.
A container with thick walls can have much less space inside than its outside measurements suggest.
You should also think about how high you really plan to fill the pot. Most containers should have a little empty space below the rim so you can water without soil and water spilling over the sides.
How to measure your container
Measure the space the potting mix will fill, not the outside of the pot.
Measure the inside diameter and the depth you plan to fill.
Measure the top diameter, bottom diameter, and soil depth.
Measure the inside length, width, and fill depth.
Measure it close to its normal filled shape.
Simple rule: measure the inside space the potting mix will use.
Leave some space for watering
You do not normally need to fill a container right to the top.
Leaving a little space between the potting mix and the rim gives water somewhere to sit while it soaks into the soil.
If the pot is filled right to the edge, water can run over the side before it reaches the roots.
This means the full height of the container is not always the correct depth to enter into the calculator.
Use the depth you actually plan to fill.
Leave room for watering
The soil should usually finish a little below the rim.
Use the depth of the potting-mix area when entering your measurement.
Repotting a plant? The root ball already takes up space
If you are moving a plant from a smaller pot into a larger one, you will not need enough new mix to fill the new container from empty.
The old root ball takes up part of that space.
That includes the plant roots and the potting mix already around them.
If your calculator includes a root-ball adjustment, use it when you are repotting an established plant.
You do not need an exact scientific measurement.
A good estimate is normally enough to help you decide how many bags to buy.
For example, if a large root ball fills about one-third of the new pot, the amount of fresh mix needed can be much lower than the full empty-container volume.
Why pot shape matters
A very common mistake is to look at the top of a pot and assume the whole container is that wide.
Many plant pots become narrower toward the bottom.
These are tapered pots.
A 35 cm tapered pot does not hold the same amount of mix as a straight-sided container that is 35 cm wide from top to bottom.
That is why this calculator asks for more measurements when you choose a tapered container.
Same top width. Different soil volume.
The shape below the rim changes how much potting mix fits inside.
How the calculator works
You do not need to understand the maths to use the calculator, but it can help to know what it is doing.
For a straight round container, the calculator treats the pot like a cylinder.
It uses the inside width and depth to estimate the volume.
For a rectangular planter, it uses:
length × width × depth
For a tapered round pot, it uses both the top and bottom widths because the container becomes smaller toward the base.
After that, the calculator can adjust the result for:
- the number of containers;
- an existing root ball;
- extra potting mix you want to keep as a buffer;
- the size of the bags you plan to buy.
The bag result is normally rounded up.
That is useful because you cannot usually buy 2.4 sealed bags of potting mix. You would need three bags.
A simple example
Imagine you have four round patio pots.
Each pot measures about 30 cm across the inside and you plan to fill each one with 26 cm of potting mix.
You could try to guess how many bags you need by looking at the pots.
But that becomes difficult when you have several containers.
Instead, enter:
- round pot;
- 30 cm inside diameter;
- 26 cm soil depth;
- 4 containers.
Then enter the size of the bag you plan to buy.
If your potting mix comes in 25-litre bags, choose or enter 25 litres.
The calculator can now turn the total container volume into a useful shopping number.
That is much easier than standing in a garden center trying to decide whether three bags are enough.
Should you buy a little extra potting mix?
A small amount of extra mix can be useful.
Containers are not always perfect shapes.
Your measurements may be slightly different from the real inside size.
Potting mix can also settle after it has been watered.
Grow bags may spread outward as they fill.
Because of this, adding around 5% to 10% extra can be a sensible starting point when your measurements are approximate.
You do not always need extra.
If you already have an open bag at home, or your measurements are very careful, you can use a smaller buffer.
Why your real pot may use a different amount
It is normal for the calculator result and the real amount of potting mix to be slightly different.
That does not automatically mean the calculation is wrong.
Here are the most common reasons.
The pot is not a perfect shape
Decorative planters can curve inward or outward.
They may also have thick walls or unusual bases.
The calculator uses simple shapes to give you a useful estimate.
Your measurements may be outside measurements
A pot sold as 40 cm wide may be 40 cm across the outside.
The space inside could be smaller.
This difference becomes more noticeable with thick ceramic, stone, or concrete planters.
The mix settles after watering
Fresh potting mix often looks very full when first added.
After watering, it can settle lower.
You may need a little extra mix to top up the container.
A grow bag can spread
Soft grow bags change shape as you fill them.
An empty bag and a full bag may not have exactly the same dimensions.
A root ball can be larger than expected
If you are repotting a plant, a large root ball can take up a surprising amount of the new container.
That means you may need less fresh mix than the empty-container estimate.
Common potting-mix units
Potting mix is not sold in the same unit everywhere.
Some bags use litres.
Others use cubic feet, dry quarts, or gallons.
Here are a few useful approximate conversions.
Quick volume guide
| Unit | Approximate volume |
|---|---|
| 1 cubic foot | 28.3 litres |
| 1 US dry quart | 1.1 litres |
| 1 US liquid gallon | 3.79 litres |
| 1 Imperial gallon | 4.55 litres |
Check the bag carefully. A dry quart and a liquid quart are not the same volume.
What should you use to fill a container?
For most container plants, use a potting mix made for containers.
Ordinary garden soil can become heavy and compacted inside a pot.
Container roots need both water and air around them.
A good potting mix is normally lighter and is designed to hold moisture while still letting extra water drain away.
Different plants may prefer different mixes.
For example, a plant that likes dry conditions may need a more open, fast-draining mix than a plant that prefers steady moisture.
The calculator tells you how much mix you need.
It does not tell you which recipe is best for every plant.
Do you need stones at the bottom of the pot?
You normally do not need a thick layer of rocks or gravel just to improve drainage.
What matters more is having:
- working drainage holes;
- a suitable potting mix;
- a container that does not stay full of water.
Adding a deep layer of stones also removes some of the space that plant roots could use.
If a drainage hole is very large, a small piece of mesh can help keep potting mix from falling through without taking away much root space.
Large containers need more mix, but they have advantages
A large container can cost more to fill.
But extra soil volume can also make container gardening easier.
Small pots can dry very quickly in hot sun or wind.
A larger amount of potting mix usually gives roots more space and holds more water between watering sessions.
This does not mean every plant should go into the biggest pot possible.
The right container still depends on the plant, the space you have, and how easy the pot is to move and water.
Do not forget how heavy the finished pot will be
This is easy to overlook when you are buying containers.
Dry potting mix can feel fairly light.
After watering, the same container can become much heavier.
This matters especially for:
- balconies;
- shelves;
- hanging baskets;
- window boxes;
- rooftops;
- large pots you may need to move.
The calculator estimates volume, not weight or structural safety.
It cannot tell you whether a balcony, shelf, railing, stand, or hook can safely support the finished container.
Think about the full weight of the pot, wet mix, plant, water, saucer, and any support underneath it.
Before you buy your potting mix
5 quick checks before you buy
A few minutes of measuring can save you from buying too much — or running out halfway through planting.
Do not rely only on the outside size.
Leave some room below the rim.
An existing plant already takes up space.
Litres and dry quarts are not the same.
A little extra helps when measurements are approximate.
What this calculator can and cannot tell you
A useful calculator should also be clear about its limits.
This tool can estimate
- container volume
- fresh potting mix needed
- several matching pots
- extra soil allowance
- whole bags to buy
- common volume conversions
This tool cannot tell you
- whether a balcony can hold the weight
- the perfect soil mix for every plant
- the exact volume of an unusual-shaped pot
- how quickly the container will dry
- which pot size is best for a particular crop
Can I calculate several different pot sizes?
Yes.
But if the pots are different sizes, calculate each group separately.
For example, if you have:
- two large patio pots;
- four smaller herb pots;
- one rectangular planter;
run the calculator three times.
Write down the total volume from each result and add them together.
The container-count option works best when all the containers in that calculation are about the same shape and size.
What if my pot already contains some soil?
If you are only topping up a container, you probably do not need to calculate the full pot volume.
Measure or estimate the empty section that still needs filling.
If most of the old potting mix is staying in place, the real amount you need may be much smaller than the full-container result.
Frequently asked questions
How accurate is the Container Soil Calculator?
It gives a useful estimate from the measurements you enter.
Straight-sided containers are easier to estimate than decorative pots, curved planters, or soft grow bags.
The real amount can also change because potting mix settles, roots take up space, and you may not fill the container to exactly the measured depth.
Think of the result as a good shopping estimate rather than an exact measurement.
Should I measure the inside or outside of the pot?
Measure the inside whenever possible.
Outside measurements include the thickness of the container walls.
This can make a big difference with thick ceramic, concrete, stone, and decorative planters.
Do I measure all the way to the top?
Not necessarily.
Measure the depth you actually plan to fill with potting mix.
Leave a little room below the top so you can water without making a mess.
Why does a tapered pot need a top and bottom diameter?
Because it becomes narrower toward the base.
A pot that is 35 cm across at the top but only 24 cm across near the bottom holds much less soil than a straight-sided 35 cm pot.
Can I use this calculator for grow bags?
Yes.
Measure the bag close to its normal filled shape.
Remember that soft containers can spread as you fill them, so the answer is best treated as an estimate.
If the grow-bag maker gives a clear capacity, you can also compare that number with your calculator result.
How much extra potting mix should I buy?
There is no perfect number for every situation.
Around 5% to 10% extra can be a useful starting point when your measurements are approximate.
You may need less if you already have leftover mix at home.
Why did I use less soil than the calculator predicted?
Your pot may be more tapered or curved than the calculator assumes.
You may also have used a shallower fill depth or had a larger root ball than expected.
Why did I need more mix than expected?
The potting mix may have settled after watering.
A grow bag may also have expanded as you filled it.
Small measuring differences can also become more noticeable when you are filling several pots.
Can I use normal garden soil in containers?
For most container plants, potting mix is a better starting choice.
Garden soil can become heavy inside a container.
A container mix is usually lighter and gives roots a better balance of air, water, and drainage.
Do I need rocks at the bottom of a container?
You normally do not need a thick gravel layer for drainage.
A suitable potting mix and working drainage holes are more important.
Does this calculator tell me what size pot my plant needs?
No.
This calculator starts with the container you already have and tells you how much potting mix it may need.
If you are still choosing a pot for a vegetable or herb, use the Vegetable & Herb Pot Size Finder first.
Plan the rest of your container garden
Once you know how much potting mix you need, you can continue planning with these tools:
Vegetable & Herb Pot Size Finder
Choose a practical starting pot size for common herbs and vegetables.
Potting Mix Ratio Calculator
Turn a potting-mix recipe written in parts into the amount you actually need.
Container Garden Budget Calculator
Estimate the cost of containers, soil, plants, supports, and other supplies.
Are my measurements saved?
You do not need an account to use this calculator.
The measurements you enter are calculated in your browser by the tool.
You can also use your browser’s print option if you want to print the result or save it as a PDF.
One last thing
The amount of soil in a container matters, but it is only one part of successful container gardening.
Plants also need enough root space, good drainage, suitable light, a potting mix that suits them, and regular watering.
Use the calculator to make buying potting mix easier.
Then let the plant, the weather, and your growing conditions guide the rest.