Why does this give a range instead of a number?

Because a range is what is actually known, and a single number would be a claim nobody can support. Two separate things are uncertain and neither is resolved by measuring more carefully. The first is SHAPE: three measurements describe the box the rock would fit in, not the rock, and a stone fills somewhere between about half and four fifths of its box depending on how rounded it is — you cannot tell which by looking. The second is DENSITY: rock types are bands, not values. Limestone spans roughly 2300 to 2700 kg per cubic metre depending on how porous the bed was, and sandstone spans more than that. Multiply two uncertainties and the honest output is wide: across every rock and shape this page offers, the narrowest band is still 43 per cent of its own lower end and the widest is 73, so the top of the range can be half as much again as the bottom. Narrowing that would mean inventing precision, and on this particular subject invented precision is the dangerous kind.

Can I use this to check whether my machine can lift it?

No, and this is the one question the page is built to refuse. A lift is planned from a load that has been weighed, or from an engineered assumption that carries its own margin — never from an estimate made by looking at a rock from outside. The failure mode is specific rather than theoretical: a number that reads "about 1,100 kg" next to a machine rated at 1,200 invites the comparison, and the rock that was about 1,100 turns out to be 1,400 because its hidden third was bigger than it looked or its core was saturated. Machines tip, slings part, and people are standing nearby. That is why this page gives an upper end rather than a middle, states a band rather than a value, and does not link to the lift, sling or crane capacity calculators that exist elsewhere on this site. If you need to move it, weigh it or have someone competent plan the lift.

Which measurement do people get wrong?

The third one, almost always, and almost always low. A boulder sitting in the ground shows its top and some of its sides, and the part below grade is invisible; the visible portion is frequently the smaller part of the rock. People measure what they can see, enter it as the depth, and get an answer for a rock that does not exist. The fix is to dig around it before deciding anything, which also tells you the other thing the measurements cannot: what it is sitting on and how much soil is packed against it. The second most common error is taking the length and the width along the same visual axis rather than at right angles — that always reads low as well, because it misses the rock's true breadth. Both errors push the same direction, which is why a boulder is so reliably heavier than expected rather than lighter.

Does wet rock weigh more?

Yes, and by more than most people expect for the porous types. Density figures are usually quoted for dry rock, but a sandstone with fifteen per cent porosity that has been sitting in wet ground has that pore space full of water, and water in the voids adds real mass — enough to push a specimen past the top of its own dry band. Limestone behaves similarly where the bed is porous. Dense igneous rock like granite and basalt has very little pore space and barely changes. The practical consequence is that a sandstone or limestone boulder dug out of wet ground in winter can be meaningfully heavier than the same rock after a dry summer, and the range on this page is quoted for the rock rather than for the water it is carrying. It is one more reason the upper end is the figure to plan around.

Materials & Quantities

Boulder Weight Range Calculator

The range a single boulder's weight falls in, from its three measurements and rock type: a band, since its shape and density cannot be known from outside.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

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Market

Imperial · sales tax

Longest measurement

The longest dimension across the rock.

Measure the rock where it is biggest, not where it is convenient. All three measurements together describe the BOX the rock would fit inside, and the calculation then estimates how much of that box the rock actually fills.

Width across

The width at right angles to the longest measurement.

At ninety degrees to the first measurement and again at the widest point. Taking both measurements along the same visual axis is the commonest way this comes out badly wrong, and it always errs low.

Depth or thickness

The third dimension, at right angles to both of the others.

The one people guess, because a rock sitting in the ground hides it. A buried boulder is frequently deeper than it looks from above, and guessing this low is the single biggest reason a rock turns out heavier than expected.

Rock type

The rock it appears to be. Each type is a band, not a number.

If you are not certain, pick the heavier candidate. Basalt and other dark volcanic rock is the one most often mistaken for something lighter, and it is the densest common rock on the list.

Shape

How much of its bounding box the rock fills.

A smooth river boulder is close to an ellipsoid and fills about half its box. A blasted block keeps its corners and fills far more. This is the larger of the two uncertainties and the one no amount of measuring resolves.

Could weigh as much as

3,700 lb

Low confidence

Somewhere between 2435 and 3671 lb — a band about 51% wide, and that width is the honest answer rather than a shortcoming. Granite's density band is one of the tighter ones here, though still a twelfth wide. The shape factor is the larger uncertainty on top of it. PLAN AROUND THE UPPER END, and never against a machine's rated capacity: a lift is planned from a weighed load, not from an estimate made by looking at a rock.

Lower end of the range: 2,434.69 lb
Upper end of the range: 3,670.76 lb
Width of the range: 50.77 %
Bounding box, all three measurements: 30 ft³
Estimated rock volume, lower: 15 ft³
Estimated rock volume, upper: 21 ft³

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Volume is estimated as the bounding box times a SHAPE FACTOR. An ellipsoid fills π/6 ≈ 0.524 of its box, which anchors the rounded end; angular quarried stone retains more of its corners and fills more. The factor is given as a band because no measurement from outside resolves it
Rock densities are the bands each type spans rather than single figures — granite roughly 2600 to 2800 kg/m³, basalt 2700 to 3100, limestone 2300 to 2700, sandstone 2000 to 2600. Porosity and weathering move a specimen within its band and cannot be seen from the surface
THIS FIGURE IS NOT A LIFT PLAN. A lift is planned from a weighed load or from an engineered assumption carrying its own margin. Nothing estimated from outside a rock belongs in a comparison against a machine's rated capacity, which is why this page does not link to one

Inputs used

Longest measurement: 4 ft
Width across: 3 ft
Depth or thickness: 2.5 ft
Rock type: Granite or gneiss
Shape: Irregular — a typical landscape boulder

Intermediate steps

Lower end of the range: 2,434.69 lb
Upper end of the range: 3,670.76 lb
Width of the range: 50.77 %
Bounding box, all three measurements: 30 ft³
Estimated rock volume, lower: 15 ft³
Estimated rock volume, upper: 21 ft³

Final result3,670.76 lb

Confidence note: Somewhere between 2435 and 3671 lb — a band about 51% wide, and that width is the honest answer rather than a shortcoming. Granite's density band is one of the tighter ones here, though still a twelfth wide. The shape factor is the larger uncertainty on top of it. PLAN AROUND THE UPPER END, and never against a machine's rated capacity: a lift is planned from a weighed load, not from an estimate made by looking at a rock.

What this calculation does not cover

THIS IS NOT A LIFT PLAN AND MUST NOT BE USED AS ONE. A lift is planned from a load that has been weighed, or from an engineered assumption carrying its own margin. Comparing an estimate made from outside a rock against a machine's rated capacity is how machines tip and slings fail.
The range is the answer. A boulder's weight genuinely cannot be pinned down from outside, because neither how much of its box it fills nor exactly how dense it is can be seen — and the two uncertainties multiply. Reporting a single number here would be a false precision, not a convenience.
The upper end is the figure to plan around, and even it can be exceeded. A rock denser than its type's usual band, or fuller than its shape suggests, lands above this; so does one with a wet, saturated core, which a porous sandstone or limestone readily has.
The third measurement is usually a guess and usually a low one. A boulder sitting in the ground hides its depth, and what is visible is frequently the smaller part of it — dig before deciding anything, rather than measuring what is showing.
Nothing here knows what the rock is sitting on, what it is frozen to, or how much soil is packed around it. Breaking a boulder free takes force that has nothing to do with its weight, and a rock being dragged is resisting far more than its own mass.
Rock type is being identified by eye, which is unreliable. Dark, fine-grained volcanic rock is the one most often mistaken for something lighter, and it is also the densest here — if in doubt, take the heavier option.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-15 · v1.0.0

Regulatory standards & verification citations3

Volume is estimated as the bounding box times a SHAPE FACTOR. An ellipsoid fills π/6 ≈ 0.524 of its box, which anchors the rounded end; angular quarried stone retains more of its corners and fills more. The factor is given as a band because no measurement from outside resolves it
Rock densities are the bands each type spans rather than single figures — granite roughly 2600 to 2800 kg/m³, basalt 2700 to 3100, limestone 2300 to 2700, sandstone 2000 to 2600. Porosity and weathering move a specimen within its band and cannot be seen from the surface
THIS FIGURE IS NOT A LIFT PLAN. A lift is planned from a weighed load or from an engineered assumption carrying its own margin. Nothing estimated from outside a rock belongs in a comparison against a machine's rated capacity, which is why this page does not link to one

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

The data behind it: Bulk densities of construction materials

How to calculate boulder weight range in 6 steps

Longest measurementThe longest dimension across the rock.
Width acrossThe width at right angles to the longest measurement.
Depth or thicknessThe third dimension, at right angles to both of the others.
Rock typeThe rock it appears to be. Each type is a band, not a number.
ShapeHow much of its bounding box the rock fills.
Could weigh as much asThe tool computes the could weigh as much as from those figures and shows the formula, its sources, and a confidence rating alongside it.

Could weigh as much as by longest measurement

Page defaults, not your figures above.

Longest measurement	Could weigh as much as (lb)
2 ft	1,659
3 ft	2,489
4 ft	3,319
5 ft	4,149
6 ft	4,978
7 ft	5,808

Frequently asked questions

Why does this give a range instead of a number?: Because a range is what is actually known, and a single number would be a claim nobody can support. Two separate things are uncertain and neither is resolved by measuring more carefully. The first is SHAPE: three measurements describe the box the rock would fit in, not the rock, and a stone fills somewhere between about half and four fifths of its box depending on how rounded it is — you cannot tell which by looking. The second is DENSITY: rock types are bands, not values. Limestone spans roughly 2300 to 2700 kg per cubic metre depending on how porous the bed was, and sandstone spans more than that. Multiply two uncertainties and the honest output is wide: across every rock and shape this page offers, the narrowest band is still 43 per cent of its own lower end and the widest is 73, so the top of the range can be half as much again as the bottom. Narrowing that would mean inventing precision, and on this particular subject invented precision is the dangerous kind.
Can I use this to check whether my machine can lift it?: No, and this is the one question the page is built to refuse. A lift is planned from a load that has been weighed, or from an engineered assumption that carries its own margin — never from an estimate made by looking at a rock from outside. The failure mode is specific rather than theoretical: a number that reads "about 1,100 kg" next to a machine rated at 1,200 invites the comparison, and the rock that was about 1,100 turns out to be 1,400 because its hidden third was bigger than it looked or its core was saturated. Machines tip, slings part, and people are standing nearby. That is why this page gives an upper end rather than a middle, states a band rather than a value, and does not link to the lift, sling or crane capacity calculators that exist elsewhere on this site. If you need to move it, weigh it or have someone competent plan the lift.
Which measurement do people get wrong?: The third one, almost always, and almost always low. A boulder sitting in the ground shows its top and some of its sides, and the part below grade is invisible; the visible portion is frequently the smaller part of the rock. People measure what they can see, enter it as the depth, and get an answer for a rock that does not exist. The fix is to dig around it before deciding anything, which also tells you the other thing the measurements cannot: what it is sitting on and how much soil is packed against it. The second most common error is taking the length and the width along the same visual axis rather than at right angles — that always reads low as well, because it misses the rock's true breadth. Both errors push the same direction, which is why a boulder is so reliably heavier than expected rather than lighter.
Does wet rock weigh more?: Yes, and by more than most people expect for the porous types. Density figures are usually quoted for dry rock, but a sandstone with fifteen per cent porosity that has been sitting in wet ground has that pore space full of water, and water in the voids adds real mass — enough to push a specimen past the top of its own dry band. Limestone behaves similarly where the bed is porous. Dense igneous rock like granite and basalt has very little pore space and barely changes. The practical consequence is that a sandstone or limestone boulder dug out of wet ground in winter can be meaningfully heavier than the same rock after a dry summer, and the range on this page is quoted for the rock rather than for the water it is carrying. It is one more reason the upper end is the figure to plan around.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.