Stripping Ratio: Formula, Units and How to Calculate It

Stripping ratio = waste removed ÷ ore recovered. A pit that moves 3 million tonnes of waste to mine 1 million tonnes of ore has a stripping ratio of 3, usually written 3:1.

Waste here means everything inside the excavation that is not sent to the plant: overburden, barren rock, and mineralised blocks that fall below the cut-off grade. That last part matters — raise the cut-off and some ore becomes waste, so the same pit has a higher stripping ratio without a single block having moved.

The ratio is quoted in two ways. In metal mining both sides are tonnages, so the ratio is tonnes of waste per tonne of ore and carries no unit. In coal and many industrial-mineral operations the waste is quoted as an in-place volume: cubic metres per tonne, or cubic yards per ton.

The two are linked by the density of the waste: ratio in t/t = ratio in m³/t × waste density. Take 1,200,000 m³ of waste and 1,000,000 t of ore. By volume the ratio is 1.2 m³/t. If the waste rock weighs 2.5 t/m³, that is 3,000,000 t of waste and the ratio is 3.0 t/t. Same pit, same day, and one figure is two and a half times the other — which is why a stripping ratio without its unit cannot be compared with anything.

The overall stripping ratio is total waste over total ore for the whole pit. It describes the deposit as designed and is the figure quoted in a study.

The incremental — or instantaneous — stripping ratio is the ratio of one slice: the next bench down, or the next pushback of the wall. It is the waste that slice exposes over the ore that slice releases, and it normally rises with depth, because each bench has to carry the pit walls further out to reach it.

The break-even stripping ratio is an economic limit, not a measurement: the ratio at which the margin on a tonne of ore is spent entirely on the waste moved to reach it. Break-even ratio = (recoverable value per tonne of ore − cost of mining and processing that tonne) ÷ cost of stripping one tonne of waste. If a tonne of ore returns 25, costs 8 to mine and process, and waste costs 3 a tonne to move, the break-even ratio is (25 − 8) ÷ 3 = 5.7 t/t.

The comparison that decides how deep a pit goes is the incremental ratio against the break-even ratio, not the overall ratio. A pit can have a comfortable overall ratio and still be losing money on its last bench.

Tabulate waste and ore bench by bench and two columns follow. The bench ratio is the incremental ratio of that bench. The cumulative ratio is all waste down to and including that bench over all ore down to it — the overall ratio the pit would have if it stopped there.

Read together, they show where the pit stops paying: the cumulative ratio moves slowly, the bench ratio moves first. The bench where the bench ratio crosses the break-even ratio is the one to look at, whatever the cumulative column says.

Waste and ore are not measured separately; they are a split of one volume. The excavation is the volume between the ground as surveyed and the pit design. The ore is the part of a block model that falls inside that excavation and above the cut-off grade, converted to tonnes with the density stored in the model. Waste is the rest of the excavation.

So the ratio is only as good as three inputs: the survey that fixes today's surface, the design that fixes the final one, and the block model that says what lies between them. A ratio computed against last year's surface counts waste that has already been moved.

Mixing the conventions is the common one: a volume-based ratio from one report set against a tonnage-based ratio from another. Using the ore density for the waste is the quiet one — the two rocks rarely weigh the same, and the error passes straight into the t/t figure.

Loose volumes are the third. A stripping ratio uses in-place volume; truck counts and dump surveys measure material after swell, which is larger. And where a bench contains no ore, its ratio is undefined, not zero — a zero would read as a bench with no waste.

STREAM's block model module reports the stripping ratio as part of the bench plan. It intersects an imported block model with the surveyed terrain and the pit design, and for every bench gives ore tonnage, waste and the stripping ratio, with cumulative ore and cumulative ratio beside them. The unit is chosen explicitly — t/t or m³/t — and printed next to every figure; until it is chosen, no ratio is written.

Waste tonnage uses the waste density set on the design, not the density of the ore blocks, and a tonnage-based ratio stays blank when no waste density has been given rather than being filled with an assumption. STREAM does not compute a break-even ratio and does not optimise the pit limit; it reports the ratio of the pit you designed.