Measure it three ways, say roughly what shape it is and what it is made of. You get a weight as a range — because nobody weighs a rock to the pound with a tape — and, if you give it your machine's rating, whether the heavy end of that range is a lift you should make.
Worked through the calculator for a roughly round granite boulder — the usual landscape rock — as long, as wide and as tall as the size in the first column. The range is the ±25% the calculator carries: the heavy end is the one to plan the lift around.
| Boulder | Volume | Likely weight | Range | Heavy end |
|---|---|---|---|---|
| 2 ft | 3.0 ft³ | 507 lb | 380–633 lb | 0.32 tons |
| 3 ft | 10.2 ft³ | 1,710 lb | 1,282–2,137 lb | 1.07 tons |
| 4 ft | 24.1 ft³ | 4,053 lb | 3,040–5,067 lb | 2.53 tons |
| 5 ft | 47.1 ft³ | 7,917 lb | 5,937–9,896 lb | 4.95 tons |
| 6 ft | 81.4 ft³ | 13,680 lb | 10,260–17,100 lb | 8.55 tons |
Weight grows with the cube of size. Double a boulder's size and it weighs eight times as much — which is why a "3 foot" rock you would move with a skid-steer becomes a "6 foot" rock that needs an excavator.
Solid-rock densities — the stone itself, not a heap of it. These are the numbers the calculator uses.
| Stone | lb per ft³ | kg per m³ |
|---|---|---|
| Unknown / average rock | 175 | 2,803 |
| Granite | 168 | 2,691 |
| Basalt / trap rock | 188 | 3,011 |
| Fieldstone / glacial granite | 168 | 2,691 |
| Limestone | 162 | 2,611 |
| Sandstone | 145 | 2,323 |
| Marble | 169 | 2,716 |
| Quartzite | 163 | 2,612 |
| Slate | 168 | 2,691 |
| Bluestone | 162 | 2,608 |
| River rock (one solid stone) | 165 | 2,643 |
Two things to know. Porous stones soak water — wet sandstone is roughly 8 to 15 lb heavier per cubic foot than dry. And a pile of loose rock weighs far less per cubic foot than one solid stone, about 60% of it, because most of a pile is air. If you do not know what the rock is, "unknown" deliberately sits above granite: guessing heavy is the safe mistake.
Written out: weight = π/6 × L × W × H × shape × density, then ×0.75 and ×1.25 for the range.
The verdict compares the heavy end of the weight with the capacity you enter, and only calls it safe when it is under 80% of that capacity. That 80% is a working margin on top of the manufacturer's number, not instead of it — so the number you enter has to be the right one:
The rock you can see is not always the whole rock. A boulder sitting in the ground can have a good share of itself below grade, and nothing measured from above can see it. Dig around it before you trust any estimate — this one included.
BoulderLift does the measuring from a scan: walk round the rock with an iPhone Pro's LiDAR scanner and it rebuilds the shape, works out the volume, and applies the same densities, the same ±25% and the same three-band verdict against the machines you have saved. On an iPhone without LiDAR, type the three measurements in as you would here. The weight and the verdict are free; the one-time Pro unlock saves rocks to a library and exports a PDF lift card.
Last reviewed 22 September 2026