Concept
The six-tenths rule estimates the cost of equipment at a new size by scaling a known cost at a reference size, raising the size ratio to an exponent of roughly 0.6. It captures economy of scale: doubling capacity costs less than double, because cost tracks surface-like quantities while capacity tracks volume-like ones.
A power-law scaling between two sizes of the same equipment:
C₂ = C₁ · (S₂ / S₁)ⁿC₁ is the known cost at reference size S₁; C₂ the estimate at new size S₂; n the scaling exponent (n ≈ 0.6 on average). S is any capacity measure (volume, area, power, mass flow, duty), with S₁ and S₂ in the same unit, and C₁, C₂ on the same basis — currency, cost year, battery-limits scope (see Accounting).
Why ~0.6. Equipment cost is often set by the material in a shell or vessel, scaling with surface area (~length²), while capacity scales with volume (~length³). The ratio of exponents is 2/3 ≈ 0.67; empirical fits cluster a little below.
The exponent is class-specific. 0.6 is an aggregate default; each class has its own — compressors ~0.62, vessels and reactors ~0.6, with most classes in the ~0.4–0.9 band. Use the class value when available; 0.6 is the fallback. The rule scales a single item once sized; scaled costs then aggregate toward total capex. Splitting capacity across multiple identical trains is the separate question of numbering up / down (cost ~linear in the count, no economy of scale).
S₂ is extrapolated, the more it degrades as the true curve bends away from one exponent.(S₂/S₁)^Δn, negligible at 2× but material at 10× or 100×.C₁ gives a 25%-off C₂ before any exponent error. The size basis of S₁ and S₂ must also match (volume vs. throughput, nameplate vs. operating).