Cost-to-Capacity Calculator
Extrapolate equipment and plant capital cost from a known reference using the Power Sizing Model (Six-Tenths Rule). Adjust for inflation via CEPCI. Useful for pre-feasibility studies, budgeting, and bid benchmarking.
Cost-to-Capacity Formula
Equipment / Asset Type
Known Reference
Target
Scaling Exponent (x)
0.60Economies of scale — doubling capacity costs 52% more, not 100%
Inflation Adjustment
Apply a cost index (CEPCI / Marshall & Swift)
Estimated Capital Cost (C₂)
—
Capacity Ratio (Q₂/Q₁)
—
Scale Factor (ratio)ˣ
—
Exponent (x)
0.60
Scaling Curve
Sensitivity — Estimated Cost by Exponent
| Exponent x | Scale Factor | Est. Cost |
|---|---|---|
| Enter values above to see sensitivity analysis | ||
Power Sizing Model (Six-Tenths Rule) — C₂ = C₁ · (Q₂/Q₁)ˣ · (I₂/I₁). Typical exponents: 0.60 vessels & tanks · 0.67 centrifugal pumps · 0.70 centrifugal compressors · 0.75 reciprocating compressors · 0.65 complete ETP/STP plants. CEPCI reference: ~567 (2015), ~820 (2023). Results are order-of-magnitude estimates — apply site factors, vendor quotes, and contingency before budgeting.
The Power Sizing Model: How It Works
When you need to estimate the capital cost of a plant or piece of equipment at a size different from a known reference, the Power Sizing Model (also called the Capacity Exponent Method or Six-Tenths Rule) gives a reliable first approximation. The formula is:
Here, C₁ is the known cost at reference capacity Q₁, C₂ is the estimated cost at your target capacity Q₂, x is the capacity exponent (typically 0.5–0.8 for process plant equipment), and the inflation fraction I₂/I₁ adjusts for cost escalation between the reference year and today using an index such as CEPCI.
Capacity Exponent: Why Not 1.0?
If cost scaled linearly with capacity (x = 1.0), doubling plant size would double the cost. In practice, larger equipment shares the same foundations, control systems, civil structures, and some electrical infrastructure. This is the economy of scale in capital cost — and the exponent x captures it.
For most process plant equipment, x falls between 0.6 and 0.75. Storage tanks and simple vessels are closer to 0.5–0.6 (very strong economies of scale). Complex systems like reciprocating compressors or highly instrumented plants approach 0.75–0.85. The famous “six-tenths rule” uses x = 0.6 as a default for first-pass estimates when no better data is available.
| Equipment Type | Typical Exponent (x) | Notes |
|---|---|---|
| Spherical / Process Vessels | 0.60 | Shell, pressure vessel |
| Heat Exchangers | 0.60 | Shell-and-tube |
| Centrifugal Pumps | 0.67 | Including motor |
| Centrifugal Compressors | 0.70 | Multistage |
| Reciprocating Compressors | 0.75 | Less economy of scale |
| ETP / STP Plants | 0.65 | Overall system |
| Evaporators / MEE | 0.65 | Multiple effect |
| Membrane Systems (RO/UF) | 0.70 | Including skids |
| Complete Chemical Plant | 0.65 | Integrated facility |
Using CEPCI for Inflation Adjustment
The Chemical Engineering Plant Cost Index (CEPCI), published monthly by Chemical Engineering magazine, tracks changes in the cost to build process plants in the United States. It accounts for changes in material costs, labour rates, and equipment pricing. To adjust a historical cost quote to current prices:
For example, a plant budgeted in 2015 (CEPCI ≈ 567) can be brought to 2023 terms (CEPCI ≈ 820) by multiplying by 1.45 — representing roughly 45% cost escalation over that period due to material, labour, and equipment price inflation.
Note: CEPCI is US-centric. For Indian projects, it can be used as a proxy for global equipment cost trends, but local factors (rupee/dollar exchange rate, Indian steel and civil costs, regional labour rates) should be applied separately.
When to Use (and When Not to Use) This Model
The Power Sizing Model is appropriate for pre-feasibility studies, budget screening, and bid benchmarking where order-of-magnitude accuracy (±30–50%) is acceptable. It is most reliable when:
- ✓The reference and target plants use the same technology and process type
- ✓The capacity ratio Q₂/Q₁ is between 0.1× and 10× (ideally 0.5×–5×)
- ✓The reference cost is from a reliable, well-documented source
- ✓Site conditions and location are broadly similar
Do not use this model as a substitute for a detailed Bill of Quantities, vendor quotations, or Detailed Project Report (DPR) when making a final investment decision. The method does not account for site-specific factors, local civil costs, regulatory compliance add-ons, or technology changes between reference and target.
Frequently Asked Questions
What is the Six-Tenths Rule?
The Six-Tenths Rule states that equipment cost scales with capacity raised to the power of 0.6. It is a rule of thumb from engineering economics, where the exponent reflects economies of scale in equipment manufacturing and plant construction.
What capacity exponent should I use for ETP/STP cost scaling?
For ETP and STP plants, 0.65 is a common starting point. Biological treatment systems tend toward 0.65–0.70; ZLD with evaporators toward 0.70–0.75; membrane systems around 0.65–0.70. Validate against actual project data for your region.
How do I find a reliable reference cost?
Use actual project quotes you have received, published case studies, equipment vendor data, or authoritative references such as Peters & Timmerhaus (Plant Design and Economics for Chemical Engineers) or Matches Process Equipment Cost Estimating.
Can this model be used for Indian project costs?
Yes, provided your reference cost is also from India. If using a foreign reference, adjust for exchange rate differences and Indian construction cost factors (civil labour, local steel prices, duties) before applying CEPCI.
What is the accuracy of this method?
Order-of-magnitude accuracy of ±30–50% is typical. Accuracy improves with closer matching of technology, geography, and capacity ratio between reference and target. It is suitable for pre-feasibility and budget planning, not for final investment decisions.
Related Tools
Need a Detailed Cost Estimate?
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