Stone Slab Weight Calculator — Worktops, Flags and Panels

Frequently asked questions

Why does this give a number when the boulder page gives a range?

Because a slab's shape is known and a boulder's is not. Three measurements of a rock describe the box it would fit in, and how much of that box the rock actually fills is a guess worth tens of per cent — that uncertainty, multiplied by the density band, is why the boulder page reports a range half again as wide as its own lower end. A slab is a rectangular prism cut to a nominal thickness, so its volume is simply length times width times thickness with nothing to estimate. That leaves only density, and within a processed stone type that spans a few per cent rather than a third. Quoting a range as wide as the boulder page's here would be its own kind of dishonesty: it would suggest an uncertainty that does not exist and make the number less useful than it should be.

Why does the page make so much of the weight per square metre?

Because it is the figure that decides whether the thing underneath will hold, and the total is not. A cabinet run, a joisted floor or a cladding substrate is designed against a load per unit area, and that load does not change with how the stone was cut: a 30 mm granite worktop puts about 80 kilograms on every square metre whether the piece is two metres long or four. The total weight answers a different and more immediate question — how many people it takes to get it through the door — and people reach for it because it is the number that feels urgent. Both matter, but they matter to different people at different moments, which is why the page reports the areal density alongside rather than burying it. If you are asking whether an existing carcass will take a stone top, the square-metre figure is the one to take to that question.

Why will you not tell me if two people can lift it?

Because that question has no answer that is both short and honest. Manual handling limits are set in regulation, and every published figure comes with conditions attached: the weight a person can safely handle depends on how high the lift starts and ends, how far the load is held from the body, whether they have to twist, how often the lift is repeated, and who is doing it. Reproducing one number here would strip all of that away and turn a conditional guideline into an unconditional threshold, which is exactly the misuse those guidelines warn about. So the page gives you the weight, gives you the share each person would take if the load were even — and it usually is not, because a long slab carried at its ends loads the carriers rather than the middle — and points you at the rule that applies where you are. Most stone of any size is moved with suction lifters and an A-frame for good reasons that have nothing to do with the arithmetic.

Does a sink cutout make the worktop lighter or more fragile?

Both, and the second matters far more. Removing a sink opening takes out real weight — a large undermount cutout can be a fifth of the slab's area — so the finished piece is lighter than this page's figure for the uncut size. But stone is strong in compression and weak in tension, and a cutout is a stress concentration right where the material would otherwise be doing the most work. That is why a worktop with an opening is the piece that breaks: not usually in service, but in transit and during the lift, when it is being flexed by being carried flat. Stone with a cutout should be carried on edge, supported along its length, and never rested on two points with the opening in the middle. It is also why fabricators cut sink openings on site or brace them for transport, and why a slab that survived the journey can still fail when somebody leans on it before the support is fitted.