Planter Pot Soil Calculator
Buying too little compost means a second trip to the garden centre, while buying too much leaves you with bags going to waste in the shed. Enter your pot shape and dimensions below to work out exactly how much compost to buy.
Work out how much compost you need
Enter your pot shape and dimensions to calculate compost volume.
For 1 pot, you'll need approximately:
21.2 litres of compost
That's around 1 standard 50L bag of compost.
Recommended products
A few well-reviewed planters and pots to consider.
- Terracotta
- 4 Inch & 6 Inch (Set of 2)
£34.28
Pros
- Etched leaf-vein pattern adds decorative detail
- Two complementary sizes for grouped displays
- Drainage hole in each pot protects roots from waterlogging
Cons
- Terracotta is porous and dries out faster in summer
- Plastic
- 40cm (Set of 4)
£34.99
Pros
- Weather-resistant plastic won't crack like real terracotta
- Set of 4 gives consistent styling across a patio or border
- Drainage holes included in every planter
Cons
- Lighter weight than genuine terracotta, so less wind-stable
- Plastic
- 43L
£22.99
Pros
- 43-litre capacity suits small trees and large shrubs
- Galvanised-look ribbed finish adds a modern edge
- Lightweight plastic is easier to reposition than metal or stone
Cons
- Large footprint needs a dedicated spot on the patio
Frequently asked questions
How much compost do I need for a large planter?
It depends on the pot's volume, which is why it's worth calculating rather than guessing — a 40cm diameter, 40cm tall round pot holds around 50 litres, which is a full standard bag on its own.
Should I fill the whole pot with fresh compost?
Not always. For very deep pots, many gardeners fill the bottom third with broken polystyrene or empty plant pots to save on compost and improve drainage, then top up with compost for the root zone above.
Do I need different compost for pots than for borders?
Yes — container or multi-purpose compost is formulated to retain moisture and stay open in a confined pot, whereas garden soil compacts in containers and drains poorly, so it's worth buying compost rated for pots and containers.
How often will I need to top up compost in a planter?
Compost in pots settles and breaks down over time, so most planters need topping up by a few centimetres each spring, and a full refresh every two to three years keeps plants well fed.
How this Calculator Works
This calculator works by first prompting you for the dimensions of your planter or pot – either its radius (for round planters) or length and width (for square pots). Then, it asks for the desired depth of soil fill in centimetres. The calculator then applies a standard volume formula (πr² for circular shapes and l x w for rectangular ones) to calculate the total volume required. This calculation provides you with the volume in cubic metres (m³), which is the unit used to measure compost.
Compost Volume Guide – UK Sizes & Typical Uses
- **Small Window Boxes (30cm Diameter):** Approximately 0.045m³ of compost is needed, suitable for petunias and geraniums.
- **Medium Hanging Baskets (45cm Diameter):** Roughly 0.082m³ – ideal for fuchsias and trailing lobelias.
- **Standard Patio Planter (90cm x 45cm):** Expect to use around 1.83m³ of compost, perfect for roses and larger bedding plants.
- **Large Raised Beds (1.8m x 2.4m):** A significant volume of approximately 4.32m³ is generally required; this would be suitable for vegetable plots or large herbaceous borders.
- **Square Planters (60cm x 60cm, 15cm depth):** Around 0.36m³ – a good size for herbs and strawberries.
- **Round Hanging Baskets (75cm Diameter):** Approximately 0.707m³ - suited for trailing plants like ivy or clematis.
- **Important Note:** These are estimates only. The actual volume needed will vary depending on the plant’s root system and growth habits. Consider adding a small buffer (10-20%) to ensure sufficient coverage.
Worked Example: Filling a Square Planter
Let's say you are filling a square planter that measures 60cm x 60cm with soil to a depth of 15cm. First, we need the dimensions in metres: 0.6m x 0.6m and 0.15m. Using the formula (l x w), the volume is calculated as (0.6m x 0.6m) x 0.15m = 0.054m³ . This equates to 54 litres of compost – approximately £35-£50 depending on the brand you choose from Amazon UK (e.g., Miracle-Gro Compost, Westland Vegetable Compost).
To ensure a good root environment, it’s recommended to add a layer of horticultural grit at the bottom of the planter before adding the compost. This improves drainage and aeration. The calculator provides the core volume; adjusting for this extra layer would increase the total material needed slightly.
More questions answered
What units does the calculator use?
The calculator outputs volume in cubic metres (m³), which is the standard unit used for measuring soil and compost. You can convert this to litres using a simple online converter – approximately 1 m³ equals 1000 litres.
How accurate is this calculator?
This calculator provides an estimate based on your input dimensions. The actual volume needed may vary slightly depending on the plant's root system and how densely you pack the compost. It’s always best to err on the side of a little extra.
What if I need to fill a planter that isn't a perfect shape?
The calculator is designed for round and square planters. For irregular shapes, measure the length, width, and depth carefully and enter these values into the calculator – it will provide a reasonable estimate based on your input.
Does the calculator account for compaction?
No, this calculator doesn't factor in compaction. Soil compacts as you fill it, reducing the usable volume. It’s recommended to add a small buffer (around 10-20%) to allow for compression.
What compost should I use?
The best type of compost depends on what you're growing. For flowers and bedding plants, a general purpose compost is suitable. For vegetables, you’ll want a vegetable-specific compost enriched with nutrients.
Can I use this calculator for raised beds?
Yes, you can! Simply input the length, width, and desired depth of your raised bed to calculate the volume of compost needed. Remember that large raised beds require significant amounts of soil.
What is a cubic metre?
A cubic metre (m³) is a unit of volume equal to 1000 litres. It's a convenient measure for bulk materials like compost, allowing you to accurately calculate the amount needed for your gardening projects.
My planter has sloping sides – how does this affect the calculation?
The calculator assumes rectangular shapes. For planters with significant slopes, measuring the average depth and using that value will provide a more accurate approximation of the soil volume required.
I've used the calculator but still don’t know how much compost to buy – what else should I consider?
Consider the plant's mature size at the time you are adding the compost. Larger plants require more soil and nutrients than seedlings, so factor that into your calculations.
I'm buying compost from Amazon - can I return it if it’s not what I needed?
Please refer to Amazon’s standard returns policy for any items purchased through their website. Amazon offers a 30-day guarantee on most products, but the seller ultimately determines whether a return is accepted.