Planter Soil Volume: How Much Compost You Need

Planter Soil Volume: How Much Compost You Need

Posted by Amy Hall on 28th Oct 2026

Planter Soil Volume: How Much Compost You Need

Decorative planter and measuring tools title card

To find your planter’s soil volume, measure the inside of the planter, multiply length × width × fill depth (or use the round-pot formula below), convert the answer to litres, then add about 10% for settling. Divide by the bag size and round up. That is the whole method, and the rest of this guide shows you how to do it for square troughs, round pots and tapered planters, with figures you can check against your own containers.

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TL;DR

  • Measure the inside of the planter, not the outside, and measure to the fill line, which is usually about 5 cm below the rim.
  • Rectangular planter: length × width × fill depth. Round planter: π × radius² × fill depth. Tapered planter: use the frustum formula, not the rim size.
  • 1,000 cm³ is 1 litre, and 1 m³ is 1,000 litres, so you can move between centimetres, metres and bag sizes easily.
  • Add roughly 10% for settling, subtract anything that takes up room (drainage layer, old compost, rootballs), then round up to whole bags.
  • For most containers, a peat-free multipurpose compost is the usual choice rather than garden soil.

Table of Contents

Why getting the volume right matters

Buying too little compost means a half-finished planting day and a second trip. Buying far too much means bags left in the shed and money wasted. A few minutes with a tape measure and a calculator avoids both.

The good news is that planters are simple shapes. If you can find the volume of a box or a tin can, you can find the volume of nearly every planter you own. Take it one step at a time and you will have a reliable figure in litres, which is the unit most compost bags use.

1. Measure the inside, not the outside

This is the mistake that causes most over-buying. Pot walls can be thick, especially on glazed ceramic, stone and some wooden planters with a liner. Using external dimensions overestimates the soil you need, so calculations should use the internal measurements.

Grab a rigid ruler or tape and note down three things:

  • Internal length and width (or internal diameter for a round pot).
  • Internal depth, from the base of the planter to the top of the planter.
  • Your fill depth, which is the depth minus the gap you will leave at the top.

Work in centimetres, as it is easier to measure a pot that way. You can convert to litres at the end.

2. Decide where the soil level will sit

Compost should not reach the rim. Watering needs a small reservoir, otherwise water and soil spill over the edge. The RHS advises filling to a level where the rootballs of your plants sit about 5 cm below the rim.

So, as a working rule, your fill depth is the internal depth minus 5 cm. In a small 15 cm deep pot you will be filling to roughly 10 cm. In a 40 cm deep trough you will fill to about 35 cm, although see the section on drainage layers and bulky fillers below, as these reduce how much compost you actually need.

3. Rectangular and square planters

This is the easiest calculation:

Volume (cm³) = internal length × internal width × fill depth

Then divide by 1,000 to turn cubic centimetres into litres.

Worked example: a window-box style trough

  • Internal length 80 cm, internal width 20 cm, internal depth 25 cm.
  • Fill depth: 25 − 5 = 20 cm.
  • 80 × 20 × 20 = 32,000 cm³ = 32 litres.

Add 10% for settling (32 × 1.1 = 35.2 litres). If your compost comes in 20-litre bags, you need two. If it comes in 40-litre bags, one bag covers it with a little to spare.

If you have several identical planters, calculate one and multiply by the number you own, then divide by the bag size and round up once at the end. Rounding each planter separately will make you overbuy.

4. Round planters with straight sides

For a plain cylinder, the formula is:

Volume = π × radius² × fill depth

The radius is half the internal diameter, and π is about 3.14. Again, divide cubic centimetres by 1,000 for litres, as 1,000 cm³ equals one litre.

Worked example: a large round tub

  • Internal diameter 40 cm, so radius 20 cm.
  • Internal depth 35 cm, fill depth 30 cm.
  • 3.14 × 20 × 20 × 30 = about 37,700 cm³ = 37.7 litres.
  • With 10% for settling: about 41.5 litres.

5. Tapered pots and flared planters

Most pots you see in garden centres are wider at the top than the bottom. Treat these as cylinders using the rim diameter and you will overestimate. A tapered round pot is a truncated cone, and it needs the frustum formula: V = πh(R² + Rr + r²) ÷ 3, where R is the top radius, r is the bottom radius and h is the fill height.

Worked example: a tapered pot

  • Top (fill-line) diameter 40 cm, so R = 20 cm.
  • Base diameter 25 cm, so r = 12.5 cm.
  • Fill height 35 cm.
  • R² = 400, R × r = 250, r² = 156.25. These add up to 806.25.
  • 3.14 × 35 × 806.25 ÷ 3 = about 29,500 cm³ = 29.5 litres.

Treat the same pot as a straight cylinder at the rim width and you get about 44 litres, nearly 15 litres too many. That is a whole extra bag in some cases.

If the formula feels like a lot, here is a shortcut for flared pots: measure the diameter at the fill line and at the base, average the two, and use that average in the cylinder formula. It is a close estimate for gently tapering pots, and the frustum formula is the more exact route for steeper tapers.

For a tapered rectangular planter, such as a square tub that narrows towards the base, the same logic applies: average the top and bottom dimensions, or break the planter into simpler parts and add up the pieces.

6. Irregular shapes and troughs with curved ends

Not every planter is a neat box. For awkward designs, the practical approach is to split the shape into simple blocks such as rectangles and triangles, work out each volume separately, and add the results together.

A trough with rounded ends, for instance, is a rectangle in the middle with two half-cylinders at either end. Work out the rectangle, then treat the two ends together as one full cylinder and add them.

7. Quick reference table

These figures use the straight-sided formulas and already assume you are filling to about 5 cm below the rim. Use them as a starting point, then refine with your own measurements.

Planter (internal size) Fill depth Volume With 10% settling
Round, 20 cm wide 20 cm 6.3 L 6.9 L
Round, 30 cm wide 25 cm 17.7 L 19.5 L
Round, 50 cm wide 40 cm 78.5 L 86.4 L
Rectangular, 60 × 20 cm 20 cm 24 L 26.4 L
Rectangular, 100 × 25 cm 25 cm 62.5 L 68.8 L
Rectangular, 120 × 40 cm 40 cm 192 L 211.2 L

If you are choosing a planter and want to see how common sizes translate into litres, our UK planter size guide walks through sizes from 1 to 100+ litres and what they suit.

8. Convert litres, cubic metres and bags

Here is the conversion in plain terms:

Example in metres: a long trough measuring 1.2 m × 0.4 m with a fill depth of 0.4 m gives 1.2 × 0.4 × 0.4 = 0.192 m³, which is 192 litres.

To get from litres to bags, divide your total by the bag size and always round up to the nearest whole bag. Bags are commonly sold in litres, so this works with whatever size you buy. A planter needing 41.5 litres is two 25-litre bags, not 1.66 bags.

For very large jobs, such as lots of big tubs, bulk bags are cheaper per litre, but they are sold in cubic metres or hundreds of litres, so the same conversion applies.

9. Allow for settling

Compost straight from the bag is light and airy. After planting and the first few waterings it sinks. Allowing about 10% for compaction is a sensible default for pots and troughs. For tall, deep containers, where there is more material to compress, you may want to go a little higher.

In practice, the simplest way is to fill in stages. Fill most of the way, water gently so the compost drops, then top up to your fill line. You will see exactly how much it settles and avoid an over-full pot that overflows on the first watering.

Gardener watering compost in a deep trough

10. Subtract what takes up space

The empty-pot volume is an upper limit. Several things reduce the compost you actually need:

  • Drainage layers. If you put a layer of crocks or gravel in the base, measure its depth and deduct it from your fill depth. Many pots do not need one if the drainage holes are adequate. If you are unsure, our guide to drilling planter drainage holes safely explains how to get water flowing without relying on stones.
  • Rootballs. When you plant a shrub or a large perennial, the rootball itself takes up room. Existing rootballs, old compost and drainage material displace volume, so back-fill around them rather than counting the whole container as empty.
  • Old compost. If you are refreshing a planter rather than starting from scratch, measure how much of the old compost you are keeping, and deduct it.
  • Bulky fillers. Some gardeners put upturned pots or lightweight filler in the bottom of very deep planters to save compost and weight. Deduct the space they take.

As a rule of thumb, buy the bags for the net volume plus 10%, and if you have a little left over, keep the bag sealed and use it for topping up in spring.

11. Choosing what goes in the planter

Volume is only half the question. What you fill with matters too.

For most containers, the RHS notes that peat-free multipurpose compost is often used, though some plants have different needs. Garden soil on its own tends to compact in pots and drain poorly, so it is better mixed with compost than used alone. The RHS also says you can make your own mix by combining soil with garden compost.

If you make your own garden compost, remember it is rich. A council guide to home composting suggests diluting it for seedlings, for example equal parts compost, loam and leaf mould or coir, rather than using it neat. If you are weighing up a bin for your own compost, we stock composters in a range of sizes.

When you compare bagged options, a natural and organic potting soil mix is one example of the kind of blend that suits container planting, where you want a light, open structure for roots.

Plants that like sharp drainage need a different recipe. The RHS suggests, for example, half peat-free multipurpose compost and half grit or sand for aloes. When you mix grit into compost, count it as part of your total volume: for a 50/50 mix in a 40-litre planter, you need roughly 20 litres of each.

If you accidentally filled a pot with ordinary garden soil, it is not a disaster for hardy, undemanding plants. But for anything long-term, tip it out and blend in compost, or replant with a better mix before the roots struggle.

11. Choosing what goes in the planter — overview diagram

12. Does the material of the planter change the amount?

The volume calculation does not change, but the pot does affect how fast the compost dries. Terracotta loses moisture through its walls, while plastic holds it. That may influence how tightly you fill, how much you water and whether you choose a moisture-retentive mix. Our comparison of terracotta and plastic planters covers the practical differences for busy gardeners.

Shady spots dry out more slowly, too, so check our notes on plants that cope in shady pots before you decide on the mix.

A fast method if you do not want to calculate

Sometimes a rough method is enough. Here is a three-step shortcut:

  1. Fill the empty planter with water from a jug of known size (say 2 litres) up to your fill line, counting jugs as you go.
  2. The number of litres of water is the number of litres of compost, roughly.
  3. Add 10% and round up to whole bags.

It is a little messy, but it is accurate for odd shapes and avoids any formulas at all. Tip the water out onto thirsty plants afterwards.

Planters or raised beds?

If your calculation is producing numbers in the hundreds of litres, it is worth asking whether a raised bed would suit your space better. The volume rules are the same, but the cost per litre and the options for planting change. Our guide to planters versus raised beds compares them honestly.

Our perspective

We see the same pattern again and again with garden containers: people choose the pot they like the look of, then guess at the compost. A little measuring first changes the whole job. You stop guessing, you buy the right number of bags in one trip, and you plant a container that is full enough to look generous without spilling onto the patio.

Our advice is to think about volume at the moment you choose the planter, not afterwards. A bigger planter holds more compost, stays moist for longer and gives roots more room, but it also costs more to fill and is heavy to move once planted. Choose the size you can realistically fill, water and lift into place, and you will enjoy it for longer.

If in doubt, round up to the next bag, keep the offcuts of compost for topping up, and write the litre figure down for next year. You will be surprised how quickly it becomes second nature.

— Peter

Ready to choose your planters?

Once you know how many litres you need, choosing the container is the enjoyable part. Browse our range of outdoor pots and planters, from small pots to large troughs, with free delivery on orders over £70, next-day delivery options and easy returns within 14 days. If you are starting seedlings or herbs, our 9cm black square pots come in packs of 12.

FAQ

How much soil do I need for a planter?

Measure the internal length, width and fill depth in centimetres, multiply them together, and divide by 1,000 to get litres. For a round planter, use 3.14 × radius² × fill depth instead. Add about 10% for settling and round up to whole bags.

How far below the rim should I fill a planter?

About 5 cm is the usual target, as the RHS advises that rootballs should sit around 5 cm below the rim. This leaves room to water without the compost washing over the edge. Very small pots can manage with a smaller gap.

How many litres is a cubic metre of soil?

One cubic metre is 1,000 litres. If your planter works out at 0.192 m³, for example, that is 192 litres. Divide by the bag size to find the number of bags.

Should I fill a planter with topsoil or compost?

For most containers, peat-free multipurpose compost is the usual choice, as garden soil alone can pack down and drain badly in a pot. You can blend soil with garden compost if you want to make your own mix. Specialist plants such as succulents may need extra grit for drainage.

Do I need a layer of gravel at the bottom?

Not always. If your planter has proper drainage holes, compost alone will usually drain well. If you do add a layer, measure its depth and deduct it from your fill depth so you do not buy more compost than you need.

Sources

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