Bank cubic metres vs. tonnes: what the numbers actually mean

A pit plan says 2.4 million BCM for the quarter. The site's monthly report says 6.1 million tonnes moved. Someone on the finance side asks if those two numbers agree, and the honest answer is: you can't tell yet, not without knowing what density was used to get from one to the other.

This mismatch comes up constantly in reconciliation work, and it's worth having the units straight before you try to check a number against a plan or a covenant.

Bank cubic metres: the rock before anyone touches it

A bank cubic metre (BCM) is a volume measurement of material in its natural, undisturbed state, still sitting in the ground. "Bank" just means in situ: the rock as it was before drilling, blasting, or digging moved it. Mine plans, reserve statements, and most loan covenants are written in BCM (or the tonnage equivalent of BCM) because that's the volume geologists modeled when they built the block model in the first place.

If you're measuring pit progression from elevation surfaces, before-and-after DEMs from satellite or aerial stereo, you're measuring BCM. The ground didn't move itself and swell in the process; you're looking at the void left behind, which is an in-situ volume by definition.

Converting to tonnes: bulk density does the work

Tonnes come from multiplying BCM by bulk density, expressed as tonnes per bank cubic metre (t/BCM). A waste rock unit might run 2.6 t/BCM. An ore zone in the same pit might run 2.9. A clay-rich overburden layer can sit well under 2.0. None of these are universal constants: they come from the resource model, built from drill core density testing across the deposit, and they vary pit to pit and bench to bench within the same pit.

This is the part that trips up a lot of reconciliation work. A site reporting tonnes moved is applying its own density assumptions: blended across material types, updated periodically as new drilling comes in, and sometimes set conservatively for royalty or covenant purposes. If you're trying to check that reported tonnage against an independently measured volume, you need to know which density number got you there, and whether it's the same one the plan used.

Loose cubic metres and why swell factor doesn't belong here

Once rock gets blasted and loaded into a truck, it fluffs up. A cubic metre of bank material typically becomes somewhere between 1.3 and 1.6 loose cubic metres (LCM) once it's broken and sitting in a muck pile or truck bed, depending on rock type and fragmentation. That ratio is the swell factor, and it matters for fleet sizing, truck payload planning, and dump capacity.

It does not matter for a monthly extraction reconciliation. Swell factor describes material moving through the haul cycle, trucks and muck piles between blast and dump. If someone hands you a production number in loose cubic metres and you try to compare it directly to a BCM-based plan without backing out the swell factor first, the two numbers will never line up, and the gap will look like a shortfall or overage that isn't really there.

Reading a monthly number against plan

When you're setting a reported extraction figure against the mine plan or a covenant threshold, the unit has to match before anything else does. An elevation-difference volume from before-and-after surfaces gives you BCM moved, independent of how the site counted trucks or logged shovel passes. Converting that to tonnes for comparison against a covenant written in tonnage means applying a density figure, and stating plainly which one you used, since that's where most of the disagreement between an independent read and a site's own number tends to live.

Pit Progression turns that elevation difference into a monthly moved-material figure, delivered with the two surfaces behind it, so you can see the volume directly rather than inheriting someone else's density assumptions buried in a tonnage report.

If you need a BCM number you didn't have to take anyone's word for, that's the whole offer.