What is OPEX — and why does it matter?

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Operating Expenditure (OPEX) is the total recurring cost of running a chemical process over a defined period. Unlike capital expenditure (CAPEX), which covers one-time investments in plant and equipment, OPEX encompasses everything that must be paid continuously to keep the process running: energy, labour, maintenance, materials and consumables, utilities, and waste disposal. It is one of the most important metrics in process economics and a direct indicator of manufacturing competitiveness and sustainability.

GoalMinimise OPEX per unit of product — reducing operating costs while maintaining or improving output quality and environmental performance.

WhyHigh OPEX reduces profit margins, makes processes economically unviable at scale, and often correlates with high energy use and waste — undermining green chemistry goals.

HowImprove energy efficiency, optimise labour scheduling, extend catalyst and equipment lifetimes, recover and reuse materials, and reduce waste treatment burdens.

The formula

$$\text{OPEX} = \sum_{i} C_i$$

$$\text{OPEX}_{\text{unit}} = \frac{\text{OPEX}}{Q}$$

Symbol	Term	Units
$C_i$	Cost of operating item $i$ in the reporting period (energy, labour, maintenance, etc.)	Currency (e.g. £, $, €)
$\text{OPEX}$	Total operating expenditure for the reporting period	Currency / period
$Q$	Total production output in the same reporting period	kg, L, mol, units, etc.
$\text{OPEX}_{\text{unit}}$	Operating cost per unit of product	Currency / unit output

All cost items must be expressed for the same reporting period (e.g., all annual costs for an annual OPEX). The production output must also be for the same period. If some costs recur on different timescales (e.g., monthly catalyst replacement, annual equipment service), convert each to the reporting period before entering.

OPEX components by category

Category	Typical examples	% of OPEX (chemical industry)
Energy	Electricity, natural gas, steam, heating/cooling	20–50%
Labour	Process operators, supervisors, safety personnel	15–35%
Raw materials	Feedstocks, reagents (ongoing consumables)	10–40%
Maintenance	Scheduled servicing, spare parts, repairs	5–15%
Utilities	Water, compressed air, nitrogen, waste treatment	3–10%
Waste disposal	Effluent treatment, solvent incineration, landfill	2–8%

Strengths and limitations

Strengths

Directly comparable across processes, facilities, and time periods
Per-unit OPEX enables benchmarking against competitors and industry standards
Identifies the dominant cost drivers, guiding targeted improvement
Captures the economic dimension of green chemistry decisions (e.g., energy recovery)
Applicable from laboratory scale through to full industrial production

Limitations

Does not include capital costs (CAPEX) — use alongside capital efficiency metrics
Cost figures vary significantly with geography, market conditions, and scale
Shared facility costs (overhead allocation) can be difficult to assign fairly
Does not capture environmental or social costs unless explicitly included
Raw material costs fluctuate — snapshot OPEX may not represent long-term economics

OPEX in context: complementary economic metrics

Metric	What it measures	Relationship to OPEX
CAPEX	One-time capital investment in plant, equipment	Separate from OPEX; amortised over asset lifetime
Raw Material Cost (RMC)	Cost of feedstocks and reagents per mass of product	A component of OPEX; excludes energy, labour, etc.
Space–Time Yield (STY)	Mass of product per reactor volume per time	Higher STY distributes fixed costs over more output
E-factor	kg waste per kg product	High E-factor typically increases waste disposal OPEX
Energy Efficiency (EE)	Useful energy out vs total energy in	Low EE drives up the energy component of OPEX

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Process details

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Process / facility name

Analyst(s)

Reporting period (start date)

Facility / institution

Process type

Scale

Reporting period

Currency

Reference / source

Notes

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Operating costs

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Enter all recurring operating costs for the reporting period selected above. Include energy (electricity, gas, steam), labour (operators, supervisors), maintenance (scheduled servicing, spare parts), materials and consumables (catalysts, reagents, packaging), utilities (water, waste treatment), and any other ongoing costs. Do not include capital expenditure (equipment purchase, construction) — use CAPEX metrics for those.

Cost item	Category	Cost (per period)	Notes / basis

Σ Total OPEX — per period

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Production output

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Enter the total production output for the same reporting period. This is used to calculate OPEX per unit of product. Leave blank if you only want the total OPEX for the period.

Total output quantity

Output unit

Product name

OPEX per unit — per unit output

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Results

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Total OPEX

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per period

OPEX per unit

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per unit output

Energy costs

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of total OPEX

Labour costs

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of total OPEX

Energy fraction of OPEX (lower fraction = more efficient energy use)

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0%25%50%75%+

OPEX by category

Cost breakdown (top items)

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Detailed breakdown & interpretation

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Cost item	Category	Cost (per period)	% of OPEX	Notes	Visual
Enter operating cost items above to see breakdown.

Interpretation

Enter your operating cost items and production output above to generate an interpretation.

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Save & load sessions

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No saved sessions yet.

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Export

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Export your OPEX calculation as a PDF report or CSV data file. PDF opens in a new tab and uses your browser's print function. CSV downloads directly.

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Where can I read more?

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References are sorted alphabetically by first author.

P. T. Anastas and J. C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, 1998. ISBN 978-0-19-850698-0. — Original statement of the 12 Principles; Principle 6 addresses energy efficiency and its economic implications.
M. Dunnair, J. M. Humphrey, I. Laird, A. Lengyel and G. A. Sherwood, Org. Process Res. Dev., 2020, 24, 1481–1492. DOI. — CHEM21 metrics toolkit and economic considerations in green process design.
M. S. Peters, K. D. Timmerhaus and R. E. West, Plant Design and Economics for Chemical Engineers, 5th edn, McGraw-Hill, 2003. ISBN 978-0-07-239266-1. — Standard reference for OPEX estimation, cost indices, and economic analysis in chemical engineering.
R. A. Sheldon, ACS Sustainable Chem. Eng., 2018, 6, 32–48. DOI. — Metrics and economics of green and sustainable chemistry.
R. Turton, J. A. Shaeiwitz, D. Bhattacharyya and W. B. Whiting, Analysis, Synthesis, and Design of Chemical Processes, 5th edn, Prentice Hall, 2018. ISBN 978-0-13-478684-3. — Comprehensive treatment of OPEX estimation including labour, utilities, and maintenance correlations.
G. D. Ulrich and P. T. Vasudevan, Chemical Engineering Process Design and Economics: A Practical Guide, 2nd edn, Process Publishing, 2004. ISBN 978-0-9708768-2-3. — Practical OPEX cost factors and utilities costing methodology.

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Contributors

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Roles follow the CRediT taxonomy (Contributor Roles Taxonomy), adapted for educational software. Hover a contributor's name for a summary, or a column header for the definition of that role.

Contributor

This portal was built with the assistance of a large language model (Claude, Anthropic), which was used to generate and refine code, articulate and structure contributed ideas within the defined page format, and support iterative design decisions. All scientific content, conceptual frameworks, pedagogical choices, and final outputs were directed, reviewed, and verified by the contributors listed above.

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How do I cite this page?

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If you use this tool in teaching or published work, please cite the DodecaGreen portal as the source.

Reference

BibTeX

Operating Cost (OPEX) calculator.