Strip Ratio Explained: Why Waste Movement Decides Open-Pit Economics
Strip ratio is the tonnes of waste moved per tonne of ore. It is the clearest single predictor of open-pit operating cost, and the most common place where a study quietly understates what a mine will cost to run.
Strip Ratio Explained: Why Waste Movement Decides Open-Pit Economics
Summary box
- Strip ratio is waste tonnes divided by ore tonnes. A 4:1 ratio means four tonnes of rock moved and dumped for every tonne fed to the mill.
- It is the dominant driver of open-pit mining cost, because moving waste costs nearly the same per tonne as moving ore and generates no revenue.
- Life-of-mine strip ratio is an average. Incremental strip ratio — the ratio in any given year — is what determines cash flow in that year.
- Pre-stripping happens before production and is capital, not operating cost. It is a real cash outflow that never appears in a steady-state cost table.
- A strip ratio that falls between PEA and feasibility deserves scrutiny; detailed geotechnical work usually flattens pit walls, which raises it.
The definition, and the trap inside it
Strip ratio = waste tonnes ÷ ore tonnes.
The trap is that this can be quoted several ways and they are not interchangeable:
- Life-of-mine (LOM) strip ratio. Total waste over total ore across the whole pit. The headline number.
- Incremental or period strip ratio. The ratio in a specific year or pushback. Varies enormously.
- Strip ratio by volume rather than tonnage. Waste and ore often have different densities. A volumetric ratio and a tonnage ratio for the same pit give different numbers.
Always confirm which is being quoted. A project advertising "2.5:1" LOM may be running 6:1 in years one through four while it opens the pit, and 1:1 at the bottom.
Why it dominates cost
An open-pit operation's mining cost is roughly a per-tonne charge on everything moved — drill, blast, load, haul — applied to ore and waste alike. Processing cost applies only to ore.
So for a mine at $3.00/t mined and $14.00/t processed:
| Strip ratio | Mining cost per tonne of ore | + processing | Total per ore tonne |
|---|---|---|---|
| 1:1 | $6.00 | $14.00 | $20.00 |
| 3:1 | $12.00 | $14.00 | $26.00 |
| 6:1 | $21.00 | $14.00 | $35.00 |
| 10:1 | $33.00 | $14.00 | $47.00 |
Illustrative, using $3.00/t mined applied to ore plus its associated waste.
Going from 1:1 to 6:1 raises cost per ore tonne by 75% before a single other input changes. That flows straight into cut-off grade, which changes the reserve, which changes mine life.
This is why two deposits with identical grade can have completely different economics. Grade tells you what a tonne of ore is worth. Strip ratio tells you what it costs to get at.
Pre-stripping: the cost that hides in capital
Before an open pit produces its first ore, waste sitting above the orebody has to be removed. That is pre-stripping, and it is capitalised as part of initial capital expenditure rather than expensed as operating cost.
Two consequences:
- It inflates initial capex in a way that is easy to miss when comparing projects. A deposit under 40 metres of overburden and one at surface can have very different capital requirements at identical grade.
- It is excluded from steady-state operating cost tables, so the mine looks cheaper to run than it was to start.
Accounting treatment of production-phase stripping also differs by regime. Under IFRS, deferred stripping costs for a producing open pit can be capitalised. Under US GAAP they cannot, per EITF 04-6. The same pit, the same shovels, different reported cost — a point the World Gold Council's AISC guidance makes explicitly when explaining why AISC is not directly comparable across reporting regimes.
Why strip ratio moves between study stages
Strip ratio is an output of pit design, and pit design is an output of geotechnical work. As a project matures:
- Slope angles get refined. Detailed geotechnical drilling and rock-mass characterisation often force flatter walls than the assumption used at PEA. A flatter wall means a wider pit at the top, which means more waste. Strip ratio rises.
- Pit optimisation reruns at updated prices and costs. Higher costs shrink the economic pit shell, which usually lowers LOM strip ratio while also lowering reserves.
- Geological reinterpretation can change the orebody shape and with it the waste envelope.
The base rate is that strip ratio goes up from PEA to feasibility, because early-stage slope assumptions tend to be optimistic. When a feasibility study reports a lower strip ratio than the PEA it superseded, that is not automatically wrong — pit optimisation genuinely can improve — but it warrants finding the explanation in the report. If the study does not explain it, that silence is the finding.
Reading the waste-movement schedule
A good technical study publishes annual tonnes mined split into ore and waste. Read it for:
Peak total material movement. This sets the required fleet size, and therefore capital. A schedule that peaks at 45 Mtpa total movement and averages 25 Mtpa is buying equipment for the peak.
Front-loaded waste. High strip in early years destroys early cash flow, which a DCF punishes hardest. Two mines with identical LOM strip ratios can have very different NPVs depending on when the waste moves.
Fleet-availability assumptions. Total movement divided by fleet capacity implies a utilisation rate. Rates above roughly 85% availability on a large haul fleet deserve justification.
Haul distance growth. As a pit deepens, haul cycles lengthen and cost per tonne rises. A schedule holding mining cost flat across a 300-metre-deep pit is assuming that away.
Underground has no strip ratio — it has dilution
The underground analogue is not strip ratio but dilution: waste rock that unavoidably comes with the ore because stope shapes cannot perfectly trace the mineralised envelope.
Planned dilution of 5% on a narrow-vein deposit is optimistic. Fifteen to twenty per cent is closer to typical, and actual dilution frequently exceeds the plan. Because dilution lowers head grade, a study with understated dilution overstates every downstream economic output in exactly the same way an understated strip ratio does.
For a comparison of the two mining methods and what drives the choice, see the note on mining method in our feasibility study stages guide.
Checklist
- [ ] Is the quoted ratio LOM or incremental, tonnage or volume?
- [ ] Is pre-stripping quantified separately and included in initial capex?
- [ ] Does the annual schedule show total material movement, not just ore?
- [ ] How front-loaded is the waste, and what does that do to early cash flow?
- [ ] Did the strip ratio move between study stages, and is the change explained?
- [ ] What slope angles were assumed, and are they supported by geotechnical drilling?
- [ ] Does mining cost per tonne stay flat as the pit deepens? If so, why?
- [ ] Is the fleet sized for peak movement or average?
How Mining Terminal handles strip ratio
Strip ratio is extracted from technical reports alongside the mining method, pit design parameters and the production schedule that produced it, with each value traced to its source document and effective date. Because the figure is meaningless without its basis, LOM and period ratios are kept distinct rather than blended into one field.
Where a report quotes a ratio without stating whether it is tonnage or volume, that ambiguity is preserved rather than resolved by assumption.
To compare strip ratios and waste-movement schedules across an open-pit peer set, get in touch or ask Nara.
Related reading
Sources
- World Gold Council, all-in sustaining costs guidance and FAQs, including the IFRS/US GAAP stripping treatment difference — gold.org
- CIM Definition Standards, 10 May 2014 — BCSC-hosted copy
- The JORC Code, 2012 Edition — jorc.org
This article is educational and is not investment advice. Mining Terminal is a data platform, not a broker, dealer or investment adviser.