Landscape Rock Calculator
Estimate the rock volume for a rectangular or circular bed.
Six length units, six area units and five volume units, with an adjustable waste allowance.
Updated October 22, 2026
Introduction
A rock order is a volume, but a garden bed is measured in two dimensions. This calculator bridges the two: give it the bed and the finished depth of the layer, and it returns the surface the dimensions cover and the volume of rock to buy, already increased by the allowance you set.
It handles both common bed shapes. A rectangular bed takes a length and a width; a circular bed takes a radius, which is half the distance across its widest point. Every box is an input, not just a display: if you already know the covered area, type it in and the missing side or the radius follows; if you already know how much rock you need, type the volume and the depth — or, with a depth already there, the allowance — is worked out. The default waste allowance is 10%, a starting point for settlement, spillage and ground that is not perfectly even.
The calculator deliberately stops at volume. Weight and cost depend on the rock itself, its moisture and your supplier, so the page leaves those to a density-based tool. What it does well is keep the bed, the layer and the allowance consistent no matter which of them you start from.
20 m²
A 4 m × 5 m bed covers 20 m²
2 bed shapes
Rectangle from sides, circle from the radius
10%
The starting waste allowance, editable
How to use the calculator
Pick the shape that matches the bed
Choose Rectangular for a bed with four straight sides and Circular for a round bed, a tree ring or any bed measured across a centre. The inputs change with the shape: length and width for the rectangle, a single radius for the circle. Switching the shape clears the other shape's measurements, so a hidden number cannot slip into the next calculation.
Measure the bed itself
For the rectangle, measure the length and the width along the middle of the bed rather than around the edging. For the circle, measure the full distance across the widest part and halve it for the radius. Mixed units are fine: a length in feet and a width in metres are converted before they are multiplied.
If the area is already known from a plan, type it into the total area box instead: with a length in place the width is filled in, and in circular mode the radius comes back from the area. A bed that is L-shaped, curved or made of two joined areas is not a rectangle; split it into pieces, run each piece separately, and add the volumes at the end.
Enter the depth of the finished layer
Depth is the thickness of the rock once it is in place, not the size of the stones. The box opens in centimetres because a rock layer is usually a few centimetres thick, and its menu covers the same length units as the bed measurements. A deeper layer uses proportionally more rock: twice the depth is twice the volume.
If you are ordering a fixed amount of rock instead, type that total into the volume box: with the bed area in place, the depth settles at the layer that amount will cover.
Set the waste allowance
The default is 10%, which multiplies the measured volume by 1.10. Keep a smaller allowance on a flat, firm bed and raise it where the ground is rough, the edging is loose or the bed slopes. The allowance adds to the order, not to the area: the surface stays based on your measurements.
The allowance can also be the value the calculator fills in. With a bed area, a depth and a total volume all present, the volume is the one you pinned, so the allowance is what adjusts to make the sum work.
Read the area and the volume
A 4 m by 5 m rectangular bed with a 10 cm layer and the default 10% allowance:
Covered area
Rock before the allowance
Rock to order
The same order reads 2.88 cubic yards when the volume unit is switched, and 77.69 cubic feet. The unit menu changes how the order is written, not how much rock it represents.
Any box can be your starting point
The area and the volume are calculated from what you enter, and they can also be typed in themselves. That cuts both ways: if you type an area that conflicts with the sides already in place, one of those sides is recalculated to match, and if you type a volume, the depth — or, when the area and a depth are already there, the allowance — is the value that moves. Glance at the boxes after entering a total so you know which one the calculator settled.
Used the other way round, the area is a useful check on your measuring, and the volume is the purchase quantity: it already includes the waste allowance.
How the volume is calculated
Three statements produce everything on the page: one for each shape, and one for the rock layer. Each is solved in both directions, so any box can be the one you fill in first.
Rectangular bed: length times width
A 4 m by 5 m bed covers before the depth is considered. Give the area and one side instead, and the other side is worked out.
Circular bed: π times the radius squared
A bed 4 m across has a radius of 2 m, so its area is . Give the area instead, and the radius comes back from .
Rock volume: area, depth and allowance
The depth is converted to the same base unit as the bed dimensions before it multiplies, and is the allowance as a decimal: . Give a total volume instead, and the depth — or, with the area and a depth in place, the allowance — is worked out.
Units are converted before they multiply
Each measurement is converted to a common base before the multiplication, then the result is projected into the area or volume unit you selected. That is what allows a length in feet, a width in metres and a depth in centimetres in one calculation. The factors come from the definitions of the units rather than from rounded approximations:
The allowance is a multiplier, not a second measurement
A 10% allowance does not add 10 cm to the depth. It multiplies the finished volume, which is why a 2.00 m³ bed becomes 2.20 m³ and not 2.10 m³. The same rule applies at any percentage: 15% gives 1.15 times the measured volume, 25% gives 1.25 times. Filled forward, the volume field is the quantity to order, so it is the number to write on the order, not the pre-allowance figure.
Every box can be a starting point
The area and the volume are not only results. Type a known area with one side in place and the other side follows; type a known volume with the area and a depth in place and the allowance follows. The calculator keeps the two statements consistent, so a total you type can recalculate one of the values already in the boxes — glance at the area, the side and the depth after entering a total, and you will see which value moved.
One limit is worth knowing. A known area returns a missing side only when the other side is already in place, and it fixes a circle completely because the radius follows from . A known volume can return a depth when the area is there, or the area when a depth is there, but it cannot invent the bed's shape: in rectangular mode one side still has to be measured before the other can follow.
When the volume and the bed are already known, the depth that the volume implies is:
For example, 2.20 m³ spread over the same 20 m² bed at a 10% allowance works out to a depth of 0.10 m, or 10 cm — the volume and depth used throughout this guide. That is the same relationship the depth box follows when you type a total volume.
Two input warnings worth knowing
A depth above 1 m and a waste allowance above 100% are both accepted, but the field is flagged. Neither value is impossible, yet both are far more often a unit or formatting mistake: a depth of 1.5 m where 1.5 cm was meant, or an allowance that orders more than double the measured volume. The number remains visible so you can see what would be ordered and correct the input.
Worked examples
One round bed with a non-default allowance, and one order converted into the units a supplier actually quotes.
A round island bed with a generous allowance
Situation: A circular bed is 4 m across and needs a 5 cm layer of rock. The ground around the edges is loose, so a 15% allowance feels safer than the default 10%.
Inputs
Result
The surface reads 12.5664 m² and the order reads 0.72 m³, or 0.95 cubic yards.
In practice: The five extra points over the default raise the order from 0.69 m³ to 0.72 m³ — about 4.5% more rock, or roughly 0.03 m³. That is a small change on a single island bed, but on a long border the same percentage is a much larger pile, so the allowance is worth deciding before you order.
From cubic metres to cubic yards and bags
Situation: The 4 m by 5 m bed from the quick start needs 2.20 m³. Your supplier quotes bulk rock by the cubic yard, while the garden centre sells 0.5 cubic foot bags. How many of each is the same order?
Inputs
Result
One cubic yard is 27 cubic feet, so 54 half-cubic-foot bags fill a cubic yard: 2.88 cubic yards comes to about 156 bags either way.
In practice: If 2.20 m³ is the number you already have from a supplier, type it into the volume box and the depth settles at 10 cm for this bed. Otherwise, bulk and bagged rock are the same volume in different packaging, so neither is more accurate: what changes is the price and the number of trips. 156 bags is a lot of lifting, while 2.88 cubic yards is a single bulk delivery with a minimum load that may be larger than the bed needs, so compare the two totals before choosing.
Measuring and ordering tips
The arithmetic is only as good as the measurements going into it. These are the decisions that change the order most.
Measure the bed, not the border
Run the tape along the middle of the planting area, from edge to edge. If the bed tapers, take three widths — one at each end and one in the middle — and use the average, or split the taper into two rectangles. A curved edge can be approximated by a rectangle plus a half-circle, calculated as two separate runs.
Decide the finished depth before you measure
The depth box asks how thick the finished layer should be, so settle that first. Pick a depth that covers the largest stones and leaves the surface you want; a decorative dressing is thinner than a layer that must suppress weeds or carry foot traffic. Then measure the bed once, and keep the depth in the unit that feels natural — the box opens in centimetres, which is usually the right scale for a rock layer.
Match the allowance to the ground
The allowance is where you absorb everything the flat formula does not see: loose edges, a rough subgrade, slight over-digging and settlement. For a 2.00 m³ measured volume, the effect of the common settings is easy to see:
10%
2.20 m³
Default
15%
2.30 m³
Rough edges
20%
2.40 m³
Uneven ground
25%
2.50 m³
Loose subgrade
Each five-point step adds 5% of the measured volume, not 5% of the final order. On a small bed that is a handful of shovels; on a border of several cubic metres it is another trailer load, so decide the allowance before the total is used for a price.
Check the units once, then leave them
Length, area and volume each carry their own unit menu, and choosing a different unit never changes the measurement behind the number. If you switch the volume from cubic metres to cubic yards, expect the digits to change while the order stays the same. The one mistake worth guarding against is entering a depth in metres when centimetres were meant (1.5 m instead of 1.5 cm): the depth box warns above 1 m for exactly that reason.
Limitations
This is a quantity estimate for planning an order, not a delivered quantity or a quote.
- •The bed is treated as flat and level. Slope, crowned ground, edging depth and compaction are not modelled. The waste allowance is your buffer for all of them, so raise it when the site is not flat.
- •One shape per calculation. An irregular bed needs to be split into rectangles and circles and calculated in parts, then added up. The area covers only the shape currently selected; a bed made of more than one shape needs a separate run for each.
- •Volume only — no weight or cost. A cubic metre of rock weighs different amounts depending on the stone, its size, how wet it is and how it was loaded. For weight and a price, the gravel calculator takes a bed and a depth and turns the volume into weight and cost once you pick a bulk density.
- •The volume preview is rounded for reading. It shows two decimals of the display unit while the full value is kept, so a number ending in .00 is not proof that the bed measured exactly. Use it as the order figure and confirm the final quantity with the supplier.
- •Supplier minimums and bag sizes decide the practical order. Bulk rock is often sold with a minimum load, and bags come in fixed volumes, so a calculated 2.88 cubic yards may become a larger delivery. Bag labels may also quote weight rather than volume; check before buying.
For a large decorative project or any bed tied into drainage or retaining work, confirm the quantity and the specification with the designer or supplier before the rock is delivered.
Related Calculators
Plant Spacing Calculator
Optimize your garden layout with our plant spacing calculator. Determine how many plants you need for an area, the planting grid and spacing, and estimate total plants cost.
Board Foot Calculator
Calculate board feet from lumber dimensions. Convert timber volume measurements across different unit systems.
Square Yards Calculator
Multiply a length and a width into square yards and price the material. Seven length units, nine area units from square centimeters to acres, and twenty currencies converted at live exchange rates.
Glass Weight Calculator
Weigh a glass pane or rod from its shape, its dimensions and the density of the glass itself. Eight shapes, nineteen glass densities, a custom density for anything not listed, and the total for any number of identical pieces.
Log Weight Calculator
Estimate the green weight of a log or a board from its species and the dimensions you can measure, then total any number of pieces or estimate how many fit in a stack. Sixty-five species, metric and imperial length, volume, density and mass units, and reverse solves for the volume, the length or a diameter.
Natural Gas Converter
Convert natural gas between volume units (ft³, Mcf, MMcf, m³, Mcm, MMcm) and energy units (therms, BTU, MBTU, MmBTU, joules, MJ, BOE, kBOE, MmBOE). Instant bidirectional conversion.