Rock Calculator

Estimate rock volume, weight, and cost for an area you plan to fill.

Twenty-one materials, a custom density, metric and imperial units, and live currency conversion.

Updated October 24, 2026

CreatorFrank Zhao

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Introduction

Rock is bought in two different measures: the volume a supplier can load and the weight a truck has to carry. This page connects the two for a rectangular, circular or triangular space, then turns the result into an order weight and a material cost.

The usual job is straightforward. You have a bed, a path, a border or a triangle of ground to cover; you know roughly how deep the layer should be; and you want to know how much stone to order, what it will weigh and what it will cost. The page answers all three from the same set of measurements, with a waste allowance already included in the final weight.

What makes it more useful than a plain volume formula is the reverse direction. Every dimension and every total is an input, so a known area can replace a missing side, a delivered weight can replace a volume, and a budget can replace a price. The triangle mode takes either a base and a height or all three sides, which means a corner bed can be measured as it actually is rather than approximated as a rectangle.

2 triangle setups

Base and height, or all three sides

21 materials

Bulk densities from about 100 to 2,400 kg/m³

20 currencies

Price per kilogram or per ton, with live rates

How to use the calculator

Choose the stone, or enter its density

Select the material that matches the load. The density box fills in with a typical bulk density for that material and is locked, because the number belongs to the material rather than to your project. If your supplier quotes a different figure, or the stone is not listed, choose Other and type the supplier's density in the custom box that appears.

A bulk density includes the air between the pieces. It is not the density of a solid block of the same stone, so do not substitute a geology reference value for the loose material your supplier will tip into the truck.

Pick the shape and measure the space

Choose a rectangle, a circle or a triangle. A rectangle takes a length and a width; a circle takes a radius, which is half the distance across its widest point; a triangle takes either a base and a perpendicular height, or its three sides. The depth is entered separately, in the unit that suits the layer.

Different units are fine: a length in feet, a width in metres and a depth in inches are all converted to one common scale before the arithmetic. Switching shape clears that shape's own measurements, and switching between the two triangle setups clears the other set, so a forgotten side cannot carry over into the next sum.

Enter the depth of the finished layer

Depth is the thickness of the stone once it is in place, not the size of an individual piece. The box opens in centimetres because a decorative layer is usually a few centimetres deep. A 10 cm layer is twice as deep as a 5 cm layer over the same area, and uses twice the volume.

If you already know the volume you want to spread, type it into the volume box instead: with the area in place, the depth is solved. If the area itself is known from a plan, type it into the area box and the missing side, radius or triangle side is worked out.

Set the waste allowance

The allowance is extra material ordered on top of the measurement. It starts at 10% and is applied to the rock weight, not to the area: a 10% allowance multiplies the measured weight by 1.10. It covers settlement, spillage, a slightly uneven subgrade and the unavoidable difference between a measured bed and a real one.

You can hide the row with Adjust the waste factor, but hiding it does not remove the 10% from the calculation. If your supplier or installer already includes a margin in the quoted tonnage, set the allowance to zero so it is not counted twice.

Enter a price and read the order

The unit price is a currency amount per mass unit. Enter what your supplier charges per kilogram or per ton and leave the rest alone; the total cost follows the final weight, including the waste allowance. Without a price the volume and weights still calculate and the cost stays blank. The cost box has its own currency, so set it to the same currency as the price if you want the two figures to read directly in the same units.

Here is the whole path with a 4 m by 5 m rectangle, a 10 cm layer, gravel at 1,800 kg/m³, the default 10% allowance and a price of 0.50 SGD per kilogram:

A = 4 \times 5 = 20\ \mathrm{m^2}

V = 20 \times 0.10 = 2\ \mathrm{m^3}

W = 2 \times 1{,}800 = 3{,}600\ \mathrm{kg}

W_f = 3{,}600 \times 1.10 = 3{,}960\ \mathrm{kg}

C = 3{,}960 \times 0.50 = 1{,}980\ \mathrm{SGD}

The page shows 2 m³, 3,600 kg, 3,960 kg and 1,980 SGD. The first weight is the stone itself; the second is the amount to order once the allowance is added.

How volume, weight and cost are worked out

Every result comes from four relationships: the area of the space, the volume of the layer, the mass of that volume at the chosen density, and the cost of that mass. The shape only changes the first relationship.

Area of the space

A rectangle is the simplest case, and the one most beds approximate:

A_{\mathrm{m^2}} = \left(L \times f_L\right) \times \left(W \times f_W\right)

A circle is measured from its radius rather than its diameter. If you have the distance across the widest point, halve it first:

A_{\mathrm{m^2}} = \pi \times \left(r \times f_r\right)^2

A triangle measured from a base and a perpendicular height:

A_{\mathrm{m^2}} = \frac{1}{2} \times \left(b \times f_b\right) \times \left(h \times f_h\right)

When all three sides are known, the area follows from the semi-perimeter, written $s$ . This is the practical form of Heron's formula, and it is how the page handles a triangle that is not right-angled:

s = \frac{a + b + c}{2}

A_{\mathrm{m^2}} = \sqrt{s\left(s-a\right)\left(s-b\right)\left(s-c\right)}

Three sides only describe a triangle when each one is shorter than the other two added together. If they cannot close a triangle, the area is undefined and the page marks the side that breaks the rule instead of returning a number.

Volume and weight

Once the area is in square metres and the depth is in metres, the volume is a plain product. The depth conversion factor is what makes a layer entered in centimetres or inches work in the same formula:

V_{\mathrm{m^3}} = A_{\mathrm{m^2}} \times \left(d \times f_d\right)

The mass of that volume uses the bulk density of the chosen material:

W_{\mathrm{kg}} = V_{\mathrm{m^3}} \times \rho_{\mathrm{kg/m^3}}

For a material chosen from the list, the density is the tabulated value for that row. For Other, the tabulated row is set aside and the number in the custom density box is used instead. Whichever density row is showing is the one the weight calculation actually uses.

The density matters more than most people expect. A cubic metre of perlite or vermiculite weighs around 100 kg, while the same cubic metre of concrete weighs around 2,400 kg. The volume is identical; the truck load and the price are not. The menu covers 21 materials, and these are representative values from across its range:

Material	Typical bulk density	One cubic metre weighs
Perlite / vermiculite	100 kg/m³	0.10 t
Expanded clay (LECA)	400 kg/m³	0.40 t
Pumice stone	650 kg/m³	0.65 t
Soil	1,400 kg/m³	1.40 t
Crushed granite	1,500 kg/m³	1.50 t
Gravel	1,800 kg/m³	1.80 t
Asphalt	2,300 kg/m³	2.30 t
Concrete	2,400 kg/m³	2.40 t

These are typical loose densities. The same stone can weigh several per cent more after rain or after being compacted, and a different quarry or grading can shift the figure again. When the supplier gives you a number, use it through the Other row.

Waste and cost

The waste factor is a percentage of the measured weight, not a second measurement of the bed. The final weight is the amount to order:

W_f = W_{\mathrm{kg}} \times \left(1 + w\right)

The total cost is the final weight multiplied by the unit price:

C = W_f \times p

Because the price can be quoted per kilogram, per metric ton or per US short ton, the mass unit in the price menu changes the arithmetic. A price of 80 USD per metric ton is the same as 0.08 USD per kilogram; a price per US short ton is divided by 907.18474 kg rather than by 1,000 kg. If the cost currency is different from the price currency, the amount is converted at the reference rate shown on the page.

Mass unit	Equivalent in kilograms
Metric ton (t)	1,000 kg
US short ton (US ton)	907.18474 kg
Imperial ton (long ton)	1,016.0469088 kg
Pound (lb)	0.45359237 kg

A quoted "ton" is not a single thing. A metric ton and a US short ton differ by about 10%, which is enough to change the cost of a large order. Check which one the quote uses before comparing two suppliers.

What can be entered backwards

Every dimension, the area, the volume, the rock weight, the final weight, the unit price and the total cost are all editable. The relationships therefore run in both directions. A known area with one side gives the missing side; in circle mode a known area gives the radius; in the three-side triangle a known area and two sides give the third. A known volume with the area gives the depth. A known weight gives the volume. A known final weight gives the weight before the allowance. A budget gives the final weight when a price is already in place, or the price when the price box is empty.

When a reverse calculation could change more than one of the remaining boxes, the page keeps the value you pinned and adjusts the others. Check the numbers that were filled in after you type a total, especially when both a length and a width were already present.

L,\ W

Length and width of the rectangular space

r

Radius of the circular space

b,\ h

Base and perpendicular height of the triangular space

a,\ b,\ c

The three sides of the triangular space

s

Semi-perimeter, half of the three sides added together

A

Area of the space

d

Finished depth of the rock layer

V

Volume of rock to fill the space

\rho

Bulk density of the chosen material

w

Waste factor, written as a decimal fraction — 10% is 0.10

W

Weight of the measured rock, before the allowance

W_f

Final weight to order, with the allowance included

p

Unit price in the chosen currency per mass unit

C

Estimated total material cost

Mixed units are safe, but mixed measuring methods are not

The page converts every length, depth, area, volume, mass and price to one common scale before it calculates, so feet, inches, metres and centimetres can be mixed in one job. What it cannot fix is measuring one side at the finished grade and another at the excavation depth, or measuring the outside of an edging for one dimension and the inside for the other. Measure all dimensions from the same boundary.

Worked examples

A circular border in asphalt

Situation: A round planting border has a 3 m radius and is to receive a 7.5 cm layer of asphalt. The supplier wants the order in metric tons and quotes 0.08 USD per kilogram.

Inputs

ShapeCircular

Type of rockAsphalt

Radius3 m

Depth / waste7.5 cm / 10%

Unit price0.08 USD/kg

Cost currencyUSD

Result

A = \pi \times 3^2

=

28.2743\ \mathrm{m^2}

V = 28.2743 \times 0.075

=

2.12058\ \mathrm{m^3}

W = 2.12058 \times 2{,}300

=

4{,}877.32\ \mathrm{kg}

W_f = 4{,}877.32 \times 1.10

=

5{,}365.05\ \mathrm{kg}

C = 5{,}365.05 \times 0.08

=

429.2\ \mathrm{USD}

In practice: With the price and cost both set to USD, the order is about 5.37 metric tons. At 80 USD per metric ton, the same calculation gives about 429 USD, which is a useful cross-check when the price is quoted per ton.

A three-sided bed with a custom density

Situation: A corner bed is measured as 6 m, 8 m and 10 m on its three sides. It will take a 12 cm layer of recycled stone whose supplier quotes a bulk density of 1,450 kg/m³. The price is 0.06 USD per kilogram and the allowance is 15%.

Inputs

ShapeTriangular, three sides

Type of rockOther

Sides6 m, 8 m, 10 m

Custom density1,450 kg/m³

Depth / waste12 cm / 15%

Unit price0.06 USD/kg

Cost currencyUSD

Result

s = \frac{6 + 8 + 10}{2} = 12\ \mathrm{m}

A = \sqrt{12 \times 6 \times 4 \times 2}

=

24\ \mathrm{m^2}

V = 24 \times 0.12 = 2.88\ \mathrm{m^3}

W = 2.88 \times 1{,}450 = 4{,}176\ \mathrm{kg}

W_f = 4{,}176 \times 1.15 = 4{,}802.4\ \mathrm{kg}

C = 4{,}802.4 \times 0.06 = 288.14\ \mathrm{USD}

A 6-8-10 triangle is right-angled, so its area is also one half of 6 × 8, or 24 m². Heron gives the same answer and works for triangles that are not right-angled.

In practice: With both currency menus set to USD, the 15% allowance adds 626.4 kg, or 37.58 USD at this price. If the installer already includes that margin in their tonnage, set the waste to zero and the cost drops to 250.56 USD.

How much ground a delivered ton will cover

Situation: Your supplier confirms a 1,000 kg order of gravel, and you have allowed 10% for settlement and edges. You want to know how much of the bed the measured stone will cover at a 5 cm layer, and how much of the load is spare.

Inputs

ShapeRectangular

Type of rockGravel

Depth5 cm

Final weight incl. waste1,000 kg

Waste factor10%

Result

W = \frac{1{,}000}{1.10} = 909.091\ \mathrm{kg}

V = \frac{909.091}{1{,}800} = 0.505051\ \mathrm{m^3}

A = \frac{0.505051}{0.05} = 10.101\ \mathrm{m^2}

The measured stone behind the order covers 10.101 m², and the remaining 90.909 kg is the 10% allowance for settlement and edges. If the 1,000 kg is stone you already have and you will spread every kilogram, set the waste factor to zero and enter 1,000 kg in Weight of the rock: the volume becomes 0.555556 m³, the coverage 11.1111 m², and the final weight 1,100 kg.

In practice: Enter an order that includes the allowance in Final weight incl. waste. If the 1,000 kg is loose stone already in your possession and you intend to spread all of it, enter it in Weight of the rock and set the waste to zero instead; the two boxes differ by the allowance, and using the wrong one changes the coverage by about 10%.

Ordering and measuring tips

Measure the finished surface

Measure the bed at the level the stone will finish, along the middle of the space rather than around the edging. For a circle, measure the full width and halve it; for a triangle, take the height perpendicular to the base, not the length of a sloping side.

Split an irregular bed into simple pieces

An L-shaped bed is two rectangles; a curved bed is a circle plus a rectangle (or two circles). Calculate each piece, add the volumes, and use one density for the whole order. Splitting is more accurate than measuring a bounding rectangle and guessing.

Agree on the ton before comparing quotes

A metric ton is 1,000 kg, a US short ton is 907.18474 kg and an imperial ton is 1,016.0469088 kg. The gap between the metric and US tons is about 10%; on a large order that is a real amount of money and a real difference in what arrives.

Use volume to compare materials, weight to plan transport

A cubic metre of perlite and a cubic metre of concrete fill the same bed, but one weighs about 100 kg and the other about 2,400 kg. Compare coverage by volume, then check the weight against the vehicle, trailer or delivery limit before you order.

Keep the allowance deliberate

A 5–10% allowance is a reasonable starting point for a flat, firm bed with fixed edges; 10–15% suits rough ground, loose edging or an irregular outline. Do not add your own margin on top of a supplier's minimum-order allowance without checking what is already included.

Confirm the density of light or unusual fill

Expanded clay, perlite, pumice, tumbled glass and recycled mixes vary far more than gravel. Use the supplier's quoted bulk density through Other; a generic value can be out by enough to change both the tonnage and the cost.

If the quote is per cubic metre, price it that way

This page prices rock by mass, because that is how most bulk stone is sold. If your supplier quotes by volume instead, the gravel calculator carries both a price per ton and a price per cubic metre, so the two quotes can be compared without converting either one by hand.

Limitations

Bulk density is a typical value, not a guarantee. Moisture, compaction, grading and the source quarry all move it. The same volume can weigh several per cent more after rain, so a weight-based order is always an estimate until the supplier confirms the density of the actual stock.
The geometry is deliberately simple. It assumes a level bed, a uniform depth and a clean rectangle, circle or triangle. Slopes, crowned surfaces, over-dig, compaction, a sub-base layer and irregular outlines are not modeled. Split the space into simple pieces for a closer estimate.
The cost is material only. Delivery, tax, labour, geotextile, edging, sub-base, equipment and minimum-order charges are not included. Currency conversion uses a reference rate rather than a locked supplier quote, and the rate can move before you order.
The final weight is not a transport plan. Check the payload and axle limits of the vehicle, the access width and the surface it has to cross. A load that is within the supplier's delivery limit can still be unsuitable for a narrow drive or a soft lawn.
Use an appropriate professional for safety-critical work. Retaining walls, gabions, load-bearing fill, slope stabilization and drainage design depend on soil conditions, water flow, compaction and local code requirements that this page does not evaluate. For a drainage trench, the French drain calculator sizes the trench, pipe, gravel and filter fabric together.

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