Why does the calculator subtract a watering gap?

Because a pot filled to the rim cannot hold water. Every irrigation sends surplus straight out of the drainage holes before it has wetted the whole root ball. Leaving about 1 in (2.5 cm) of space below the rim gives the water somewhere to go, and the result excludes that gap so you do not buy soil for empty space.

How do I measure a pot for a custom batch?

Measure the inside top diameter across the rim, the inside diameter at the base, and the inside depth from the base to the rim. A standard nursery pot is named by its inside top diameter, so a "10 inch pot" means a 10 in rim, not a 10 in base, which is why tapered pots hold less than the straight-sided calculation.

Why does my 10 inch pot hold less mix than the label says?

Because pot labels describe the rim, and the volume depends on the taper as well as the depth. The calculator treats a tapered pot as a cone frustum, which is usually 5% to 15% smaller than treating the same pot as a full cylinder.

Should potting mix recipes be measured by volume or by weight?

By volume. Potting media is mixed and sold by volume, so percentages in the calculator are volume percentages. Use the same scoop or bucket for each part and fluff bulky perlite before measuring it, because compressed perlite under-reports the volume you are adding.

How much surplus should I add?

Ten percent is a sensible default for hand mixing, and it covers mix left in the bin, on the bench and settled after the first watering. Mixing a large batch in a wheelbarrow or on a clean tarpaulin can justify 15% to 20%, while filling one pot at a time needs almost none.

Can I use this for a window box or a raised trough?

Yes. Choose the rectangular container option and enter the inside length, width and depth. The result is the soil volume of the box, scaled by the number of boxes and split into your recipe.

Does a loam-based compost need more mix than a peat-based one?

No, the volume is identical for the same container, because the recipe percentages are by volume. What differs is the weight: a loam-based compost runs roughly 800 lb per cubic yard against about 450 lb per cubic yard for a peat and perlite mix, so the same batch is far heavier to lift.

Why is my mix settling after potting?

Peat and coir are compressed for shipping and re-expand when they are watered. Filling a pot dry leaves a loose surface that drops by a centimetre or so after the first thorough watering, so wet the mix as you pot, or fill slightly above the final level.

Potting Soil Calculator — Free Online Calculator

What Is the Potting Soil Calculator?

The Potting Soil Calculator is a batching tool for container growing. It works out the volume of soil that fits inside a pot, window box or planter, scales that volume to the number of containers, and divides the result into the parts of a potting recipe so you can mix the right amount by volume.

Container growers cannot rely on a bed depth and an area the way an in-ground gardener can. A pot holds a specific number of cubic inches, that volume is fixed once the plant is potted, and it has to be filled with a medium that holds both air and water. Estimating the quantity by eye is where most mixing goes wrong: too little leaves air pockets in the batch, too much ends up as half-empty bags in the shed.

The calculator takes the container geometry seriously, so tapered pots, straight-sided pots and rectangular troughs are each handled with the right formula, and the watering gap is subtracted before anything is multiplied out.

How Does the Potting Soil Calculator Work?

The first step is the depth of soil. The watering gap you want under the rim is subtracted from the inside depth, giving the height H that the volume formulas use.

A straight-sided round pot is a cylinder: V = pi r squared x H, with the radius being half the inside diameter.

A tapered flowerpot is a truncated cone, so its volume is (pi x H / 3) x (R squared + R r + r squared), where R and r are the top and bottom radii. This is why a 10 in pot holds noticeably less than the cylinder formula for a 10 in diameter, because the base is narrower than the rim.

A rectangular box is simply length x width x H. The same formula covers window boxes, half-whisky-barrels cut lengthwise and rectangular troughs.

The per-container volume is multiplied by the number of containers to give the volume needed to fill them all, and the mixing surplus is then added as a percentage to cover mix left in the bin, on the bench and settled after the first watering.

Finally the batch is split. Each component takes its percentage share of the batch volume, and the weights are estimated from the bulk density of the chosen recipe, which is far lighter for peat mixes than for loam-based composts.

Potting Soil Calculator Formula & Variables

The core mathematical equation utilized by this calculator is expressed as:

\text{Round}\; V = \pi r^{2} H \qquad \text{Flowerpot}\; V = \frac{\pi H}{3}\left(R^{2} + Rr + r^{2}\right) \\ \text{Batch}\; V_{b} = V \cdot n \left(1 + \frac{s}{100}\right) \qquad V_{i} = V_{b} \cdot \frac{p_{i}}{100}, \quad \sum p_{i} = 100

Variable Definitions

Symbol	Variable Meaning & Units
V	Usable soil volume of one container
L, W	Inside length and width of a rectangular container
R, r	Top and bottom inside diameters of a tapered pot, halved for the radii
H	Depth of soil = container depth minus the watering gap
n	Number of containers being filled
s	Mixing surplus, in percent
$p_i$	Volume percentage of component i in the recipe

The volume of soil that fits in a container is ordinary solid geometry, with one adjustment: the height used is the depth of the container minus the watering gap, because that gap is left empty so water can soak in rather than run off the surface. A straight-sided pot is a cylinder, so its volume is pi r squared times the height. A tapered flowerpot is a truncated cone, which is why the three-term term R squared plus R r plus r squared appears. Multiplying by the number of containers gives the volume needed to fill them all, and the surplus percentage covers mix that stays in the mixing bin, sticks to the trowel and settles after the first watering. Each component then takes its volume percentage share of that batch, because potting media is mixed by volume.

How to Use the Potting Soil Calculator

Choose imperial or metric units. Dimensions are entered in inches or centimetres; volumes are shown in both systems.
Pick the container shape: tapered flowerpot, straight-sided round pot or rectangular box.
For flowerpots, choose a standard nursery size. The preset loads the inside top diameter, an approximate base diameter and an approximate height. Switch to custom dimensions if you want to measure your own pots.
Enter the container depth and the watering gap you want to leave below the rim. A gap of about 1 in (2.5 cm) suits most pots.
Enter how many containers you are filling with the same mix.
Choose a recipe, or set your own three-part shares by volume. The shares must total 100%.
Set the mixing surplus. Ten percent is a sensible default; large jobs mixed with a wheelbarrow can justify more.
Pick your retail bag size and, optionally, the price per bag to get a cost estimate.
Read the per-pot volume, the batch volume in cubic feet and litres, the split by component and the number of bags to buy.
Compare the standard pot sizes in the bar chart to see how quickly the soil requirement climbs with pot size.

Step-by-Step Example Calculation

Six 10-inch pots of general soilless mix

Input Values:

unitSystem:imperial

containerShape:flowerpot

potSize:pot_10

depth:6.5

headspace:1

containerCount:6

mixRecipe:general_potting

surplusPct:10

bagSizeCuFt:2

Worked Steps: Usable depth = 6.5 in - 1 in = 5.5 in. Pot volume = (pi x 5.5 / 3) x (5^2 + 5 x 4.25 + 4.25^2) = 370.41 cu in per pot, so six pots need 2222.48 cu in = 36.42 L. Adding a 10% mixing surplus gives a batch of 2444.73 cu in = 1.415 cu ft = 40.06 L, which is one 2 cu ft bag. Split by volume: 1222.36 cu in (20.03 L) peat, 733.42 cu in (12.02 L) perlite and 488.95 cu in (8.01 L) compost. The wet mix weighs about 23.6 lbs (10.7 kg).

Understanding Your Result

Per container: the usable fill volume of one pot in cubic inches, litres and gallons. This is the number to check when a result looks surprising.

Fill depth: container depth minus the watering gap. Under about 3 in suits only seed trays and cuttings.

Volume to fill the pots: the per-container volume times the number of containers, before any surplus.

Soil to mix: the batch including the surplus. This is the quantity to mix or buy.

Recipe split: the volume of each component. Because percentages are by volume, measure with the same scoop or bucket for each part.

Bags needed: the batch rounded up to whole retail bags, which is the number of bags to put in the trolley.

Approximate weight: useful for checking whether a load is manageable, and a reminder that loam-based composts are far heavier than peat mixes.

Depth assessment: a short judgement on whether the soil depth suits seedlings, annuals, vegetables or shrubs and perennials.

Factors That Affect the Result

Pot taper: a tapered pot holds less than a straight-sided pot of the same rim diameter, which is why the frustum formula matters.
Watering gap: a 2 in gap on a shallow pot can remove a third of the soil volume, so this is not a detail to leave out.
Moisture and compaction: potting media settle after watering, and a densely filled pot holds less usable pore space than the same volume measured loosely.
Ingredient size: coarse perlite and pumice occupy more space than fine compost, so measure bulky components loosely rather than tamped down.
Root volume at maturity: a plant that will fill a 12 in pot needs a 12 in pot of soil, and moving up early costs more soil than staying in the same pot.
Recipe density: peat and coir mixes are light and water-retentive; loam-based composts are dense and hold more nutrients but weigh several times as much.

When Should You Use This Calculator?

Potting up: working out how much mix to mix for a tray, propagation bench or batch of cuttings.
Repotting a group of plants: filling ten similar pots with one batch instead of guessing bucket by bucket.
Bulk purchasing: comparing a 2 cu ft bag, a 4 cu ft bag and a bulk cubic yard against the job in front of you.
Recipe scaling: scaling a container recipe up from a five-gallon bucket to a wheelbarrow load without changing the ratio.
Cost planning: checking the cost of pre-bagged potting soil against mixing your own.
Weight checks: deciding whether to carry a load in one trip, or to use a lighter peat-based mix rather than a loam-based compost.

Assumptions & Limitations

Assumes containers are filled evenly to the same depth, with the roots occupying a comparable share of the volume in each pot.
Standard pot presets approximate real nursery pots. If the batch must be exact, measure the inside top diameter, base diameter and depth of your own pots.
Mixing percentages are by volume of loose ingredient, not by weight, and a dry fluffy medium packs down once wetted.
Weight estimates use typical bulk densities for moist media and will differ with how wet the mix is and how coarse the ingredients are.
Assumes a free-draining container. A pot without drainage holes holds water rather than air and needs a different medium.
Does not plan for seasonal changes in root volume, deep watering cycles or the extra medium needed for blocking up cuttings.

Frequently Asked Questions

Calculation Accuracy & Reference Note

The geometry is exact for the dimensions you enter. The presets follow the US nursery trade convention of naming pots by inside top diameter in inches, with heights and tapers taken from the usual trade range.

Standard Reference: University of Missouri Extension potting and planting media for containers; Cornell Controlled Environment Agriculture potting mixes; Royal Horticultural Society growing plants in containers; University of Florida IFAS Extension media facts for the landscape industry.