How Much Does a Boulder Weigh? Size Chart and Formula

How much does a boulder weigh? More than most people guess. A round granite boulder about 2 feet across weighs roughly 700 pounds, and one about 3 feet across is already over a ton. Size matters far more than anything else, because weight climbs with the cube of the width: double the size of a boulder and it weighs about eight times as much. This guide gives a size chart, a simple formula you can use with a tape measure, typical weights for common rock types and tips for buying, hauling and placing landscape boulders.

The short answer

Solid granite weighs about 165 to 170 pounds per cubic foot. Basalt is a little heavier, and sandstone and many limestones are a little lighter.
A 2-foot round boulder is about 700 lb. A 3-foot one is about 2,300 lb (just over 1 ton). A 4-foot one is about 5,500 lb (close to 3 tons).
Formula: length × width × height in feet × 0.52 (rounded) or about 0.67 (blocky) × the rock’s weight per cubic foot.
Real boulders vary. Flat, cracked or porous stones weigh less than the chart; dense, blocky stones weigh more. For a firm number, weigh it on a truck scale.

Boulder weight chart by size

These figures assume a roughly round granite boulder at 165 pounds per cubic foot, with the same measurement in all three directions. Most landscape boulders are flatter than that, so treat each line as an upper-middle estimate for a stone of that width.

1 ft (12 in): about 85 lb
1.5 ft (18 in): about 290 lb
2 ft (24 in): about 690 lb (0.35 ton)
2.5 ft (30 in): about 1,350 lb (0.67 ton)
3 ft (36 in): about 2,330 lb (1.2 tons)
4 ft (48 in): about 5,530 lb (2.8 tons)
5 ft (60 in): about 10,800 lb (5.4 tons)
6 ft (72 in): about 18,700 lb (9.3 tons)

Notice the jumps. Going from 3 feet to 4 feet more than doubles the weight, and a 6-foot boulder weighs about eight times as much as a 3-foot one.

How to estimate the weight of your boulder

Measure three directions: the longest length, the width at a right angle to it and the height. Use the widest points, in feet.
Multiply them to get the volume of a box that would just hold the stone.
Apply a shape factor. A boulder fills only part of that box. Use about 0.52 for a rounded, egg-shaped stone and about 0.6 to 0.7 for a blocky, square-edged one.
Multiply by the weight per cubic foot for the rock type (see below). Divide by 2,000 for US tons.

Worked examples:

3 × 3 × 2 ft rounded granite: 18 cu ft × 0.52 = about 9.4 cu ft × 165 = about 1,550 lb.
3 × 3 × 2 ft blocky granite: 18 cu ft × 0.67 = about 12 cu ft × 165 = about 2,000 lb, or 1 ton.
4 × 3 × 2.5 ft boulder: 30 cu ft × 0.52 to 0.67 = about 15.7 to 20 cu ft, or roughly 2,600 to 3,300 lb in granite.
Metric: a round granite boulder 1 meter across is about 0.52 cubic meters × about 2,650 kg per cubic meter = roughly 1,390 kg.

Weight by rock type

Solid rock weight follows its density, which is set by the minerals in it and how much open pore space it has. Typical ranges for solid stone, in pounds per cubic foot:

Granite: about 165 to 172. The usual baseline for landscape boulders.
Basalt and trap rock: about 175 to 187. Dark, dense and noticeably heavier for its size.
Limestone: about 145 to 168. Dense limestone is close to granite; softer, more porous stone is lighter.
Sandstone: about 137 to 156. Often sold as flat or stacked “ledge” boulders.
Fieldstone: a mix of whatever the glaciers or the farm field left behind, usually close to granite.
Lava rock: full of gas bubbles, so a lava boulder can weigh far less than a granite one the same size. Ask the supplier for its weight.

These are solid-rock figures. A pile of boulders in a truck bed or on a slope has gaps between the stones, so a cubic yard of loose boulders weighs less than a cubic yard of solid rock. Our page on riprap sizes and weight covers that loose-placed tonnage for armor stone.

Buying landscape boulders by the ton

Many stone yards price boulders by the ton, while some sell larger feature stones by the piece. Ask how they sell before you pick stones out.
Count boulders per ton by dividing 2,000 by the weight of one stone. Three 2-foot boulders come to about a ton; a single 3-foot boulder is already a little more.
Pick the face you want to see. Yards usually let you choose stones by color and shape. Note which side should face up and out.
Plan for burial. Boulders look natural when the bottom portion, often around a third, sits below grade. That means buying a larger stone than the part you will see.
Get the scale ticket. If you pay by weight, the yard weighs the loaded truck and subtracts the empty weight. Keep the ticket.

Moving and placing a boulder safely

Check every capacity. Compare the estimated weight with the rated operating capacity of the loader or excavator, the payload of the truck and the rating of the trailer, and leave a margin because your estimate can be off.
Hand moves stop early. A pry bar, rollers and a hand truck can shift a stone of a few hundred pounds. Anything near half a ton or more is machine work.
Watch the ground. Heavy boulders can crack patios, sink into soft lawns and crush buried lines. Keep them off septic tanks, drain fields, shallow pipes and irrigation, and have underground utilities marked before you dig a seat for the stone.
Seat it on a firm base. Dig a shallow bed, set the boulder, then backfill and tamp around it so it cannot rock. A layer of compacted gravel under a heavy stone helps on soft ground; run the amount in the gravel calculator.
On a slope or wall, boulders that hold back soil are a retaining structure. Tall or loaded walls may need a design and a permit.

Boulders in other projects

Dry creek beds: a few large accent boulders mixed with river rock and cobbles; see rock for a dry creek bed.
Gabion walls: cages filled with smaller stone weigh a lot too; see how much stone fills a gabion basket.
Gravel and river rock around boulders: see how much a yard of river rock weighs and gravel weight per yard.
Getting rid of a boulder: if a stone has to go rather than come in, Dumpmapper’s guide to getting rid of landscaping rocks covers giving it away and where clean rock is accepted.

Common mistakes

Guessing from the visible part of a stone that is half buried.
Using a box volume (length × width × height) without a shape factor, which overstates the weight.
Assuming a stone twice as wide is twice as heavy; it is closer to eight times.
Loading a trailer or small loader to its limit on an estimate.

Questions people ask

How much does a 3 foot boulder weigh?

A round granite boulder about 3 feet across weighs roughly 2,300 pounds, a little over a ton. A flatter 3 x 3 x 2 foot stone is closer to 1,500 to 2,000 pounds, depending on how blocky it is.

How do you calculate the weight of a boulder?

Multiply length x width x height in feet, then multiply by about 0.52 for a rounded stone or about 0.6 to 0.7 for a blocky one to get cubic feet. Multiply that by the rock's weight per cubic foot, about 165 for granite, and divide by 2,000 for tons.

How much does a cubic foot of rock weigh?

Solid granite is about 165 to 172 pounds per cubic foot, basalt about 175 to 187, limestone about 145 to 168 and sandstone about 137 to 156. Porous lava rock can be much lighter.

How many boulders are in a ton?

Divide 2,000 pounds by the weight of one boulder. That works out to about three 2-foot boulders, about seven 18-inch boulders, or a single 3-foot boulder, which is already slightly over a ton.

How much does a large boulder weigh?

A 4-foot round granite boulder is about 5,500 pounds, a 5-foot one about 5.4 tons and a 6-foot one about 9 tons. Large feature stones like these need machinery rated for the load.

Can I move a boulder by hand?

Stones up to a few hundred pounds can be shifted with a pry bar, rollers or a heavy hand truck and a helper. A boulder approaching half a ton or more is a job for a loader, excavator or a delivery truck with a boom.

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