Here, compost means the potting compost or potting mix used to grow plants in containers; garden compost is a soil improver and is not automatically a complete container mix

Measure where the compost will actually go

1. Measure between the inside walls

   Measure the growing space, excluding the timber thickness, cap boards and decorative overhangs

   If the interior contains substantial dividers or a raised floor, measure each usable compartment or the space above that floor

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2. Choose the finished fill line

   Measure vertically from the inside base to the intended compost surface, leaving room for watering below the rim

   The RHS container guide shows positioning rootballs within the container and leaving a gap above the compost so watering does not wash it over the edge

   If a separate surface dressing will occupy part of the depth, decide where the growing mix itself will finish before entering the measurement

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3. Account for the shape

   Use rectangular for a box with straight sides, straight round for a cylinder, and tapered round for a pot narrowing towards its base

   For the tapered option, take the upper diameter at the intended fill line, which can be smaller than the opening at the rim

For example, a 100 × 40 cm outer box with walls 2 cm thick would have an internal length of 96 cm and width of 36 cm if that simple wall construction were the only deduction

At 30 cm fill depth, the outside measurements imply 120 L, while those inside measurements give 103.68 L, a difference of 16.32 L before allowing for plants

Real corner posts, liners, battens and floors can change the usable space further, so this example explains the effect rather than supplying dimensions for a particular planter

Rootballs and existing mix

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The empty-container capacity includes space that a rootball or retained growing medium may already occupy; only the remaining space needs fresh mix

Enter occupied volume per container, then let the calculator multiply it by the quantity of identical containers

How to estimate a rootball

Use a known nursery-pot capacity as a rough starting point if the rootball fills it, allowing for taper, watering space and missing soil

Alternatively, a cylindrical estimate 20 cm across and 18 cm deep is about 5.65 L; an irregular or tapered rootball may occupy less

If the likely range is 4–6 L, calculate both ends of that range; if the bag count stays the same, a more precise estimate may not change the purchase

Subtract retained mix only once

Either measure the full growing space and subtract occupied volume, or measure only the unfilled space; do not deduct the same rootball or retained mix twice

For a raised internal floor, measure fill depth above it and exclude the void underneath

Formulas for three common shapes

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RectangularStraight roundTapered round

The tapered formula describes a truncated cone with straight sides between circular sections; it is an approximation for curved, bulging or irregular containers

Use a stated usable capacity where available for an unusual shape, or divide a complex box into simpler non-overlapping compartments and add their volumes

One litre is 1,000 cm³ and one cubic foot is approximately 28.3168 L; if you calculate a rectangular volume in inches, divide by 1,728 to obtain cubic feet

If your building plan uses millimetres, divide each dimension by 10 before entering centimetres; entering millimetres as centimetres multiplies the resulting volume by 1,000

Quick capacity reference

These calculated examples use internal dimensions and the intended fill depth, with no rootballs, retained mix or extra allowance deducted

Volumes are displayed to one decimal place; bag counts use the underlying unrounded values and assume 40 L bags

These are reference sizes, not capacity claims for Firesided products or retail planters with matching outside dimensions

Worked example: buying bags for one planter

Rectangular planter with a rootball

Use 90 cm internal length, 30 cm internal width and 25 cm fill depth, with an estimated 7.5 L already occupied

The subtraction belongs before the bag calculation; rounding the empty capacity into bags first hides how much fresh mix you actually need

Straight round pot

For a 40 cm internal diameter and 30 cm fill depth, the capacity is approximately 37.70 L

Measure the diameter across the pot through its centre; the distance from the centre to the wall is the radius and must not be entered as the diameter

Tapered round pot

For a 40 cm diameter at the fill line, 30 cm internal base diameter and 30 cm fill depth, the straight-sided tapered volume is approximately 29.06 L

Treating this pot as a 40 cm-wide cylinder would overestimate its capacity by about 8.64 L because the lower section is narrower

Imperial example

An empty rectangular growing space 24 × 12 × 10 inches holds 2,880 in³, or approximately 1.67 ft³ / 47.19 L

With bags labelled 1 ft³, buy two; select cubic feet as the bag unit in the calculator, even if you measured the container in centimetres

Combine different containers before rounding into bags

The quantity field is for identical containers with the same dimensions and occupied volume; calculate different sizes separately

For example, three empty rectangular boxes requiring 45 L each, the round pot needing 33.70 L and the tapered pot needing 26.06 L together require about 194.76 L

Rounding each container separately would suggest eight 40 L bags; combining the volumes allows the remainder from one bag to be used in another container

Only combine orders this way when the containers need the same growing medium; plants requiring different mixes should have separate totals

Avoid the common calculation mistakes

Should I automatically add 10%?

The calculator adds no fixed allowance; examine uncertain dimensions, rootball estimates and settling instead, then decide whether extra stock is useful for your project

A displayed estimate of 39 L with a 40 L bag leaves little margin, while 60 L purchased as two 40 L bags already leaves 20 L; the same extra percentage is not equally useful in both cases

Will the bag volume exactly match the final fill?

Treat the label as the purchase quantity and the result as a planning estimate; handling, firming and settling can change the volume actually used

Can I use garden compost instead of potting mix?

The RHS distinguishes decomposed garden material used to improve soil from prepared growing media for containers; choose the mix for the plant and container, since a volume calculation is not a planting recipe

This calculator estimates volume and bag count; it does not calculate the loaded weight or assess the planter’s construction

For other material quantities, explore Firesided’s tools and calculators or browse DIY garden projects