How to Calculate a Stripping Ratio
The stripping ratio is the open pit's heartbeat: how much waste you move to uncover a unit of ore. Get it — and its economic ceiling, the break-even stripping ratio — and you understand why pits are shaped the way they are and when they stop.
01Define it in the units your mine uses
SR = waste moved ÷ ore recovered. Coal and quarries usually run BCY of waste per ton of product; metals often use tons per ton. State the units every time — a '3:1' in mixed units has started more bad meetings than any number in mining.
02Compute the simple case
A pushback moves 4.5 million BCY of overburden to release 1.5 million tons of stone: SR = 4.5M ÷ 1.5M = 3.0 BCY/ton. That's the overall SR for the phase.
03Separate overall from instantaneous
The life-of-mine average hides the story. Early benches strip light; the last pushback against the final wall strips heavy. Plot instantaneous SR by phase — the schedule's job is to smooth that curve so year 12 doesn't quietly eat year 3's profits.
04Compute the ceiling: break-even SR
BESR = (revenue per ton of ore − ore-based costs per ton) ÷ stripping cost per unit of waste. Ore nets $28/ton, mining + processing costs $16/ton, stripping runs $2.50/BCY: BESR = (28 − 16) ÷ 2.50 = 4.8 BCY/ton. That's not a target — it's the property line.
05Apply the decision rule
Mine while actual SR < BESR; every such ton adds margin. When a pushback's SR crosses BESR, that rock only makes money if price rises, costs fall, or the pit goes underground. The final pit wall sits, by definition, where SR meets BESR — the whole science of pit optimization is finding that surface in 3D.
06Stress it before you trust it
BESR moves with price and diesel. Rerun at price −15% and stripping cost +20% before committing a pushback — a phase that only works at peak price is a bet, and you should at least know you're making one.
Practice the exact exam form of this in the free Study Vault — SR and BESR problems with the traps labeled.
Studying for the PE Mining exam?
The free Study Vault has exam-style problems with worked solutions, trap warnings, and rules of thumb.
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