Part Two — The Turnover of Capital Chapters 7–17. Turnover time and its number, fixed and circulating capital, the theories of the classical economists, the working and production periods, and the turnover of variable capital and the circulation of surplus-value.

Part One analysed a single circuit. Part Two treats the circuit as a periodic process — one that repeats, and whose speed of repetition governs how much surplus-value a given capital yields in a year. This is the turnover of capital. The turnover time is the whole span from the advance of a capital-value in a given form until its return in that form, and it is composed of production time and circulation time. From this Marx builds a distinction that structures the whole Part and that the classical economists had garbled: the difference between fixed capital, which stays in production and yields its value piecemeal over many turnovers, and circulating capital, whose value returns whole each time. The Part's decisive results come at its end: turnover affects not only the magnitude of capital that must be advanced but the annual rate of surplus-value, so that two capitals of equal size and equal rate of exploitation can yield very different annual surpluses if they turn over at different speeds (Chapter 16); and the circulation of surplus-value raises the question — pursued to its limit in Part Three — of where the money comes from to realize the surplus-value the whole class produces (Chapter 17). The middle chapters develop the working period, production time, and circulation time, and settle accounts with the Physiocrats, Smith, and Ricardo, whose confusion of fixed and circulating capital with constant and variable capital is the negative foil against which Marx's distinction is drawn.

The turnover is the circuit taken as a repeating process. Its unit of measure is the turnover time — the interval from the advance of a capital-value in a given form to its return in that same form — and this is simply the sum of the two times distinguished in Part One.

the overall time of circulation of a given capital is the sum of its circulation time proper and its production time. It is the period of time that elapses from the moment that the capital value is advanced in a particular form until the return of the capital value in process in the same form.

Marx, Capital II, page 233.

The definition of turnover follows directly.

The circuit of capital, when this is taken not as an isolated act but as a periodic process, is called its turnover. The duration of this turnover is given by the sum of its production time and its circulation time. This period of time forms the capital’s turnover time. It thus measures the interval between one cyclical period of the total capital value and the next; the periodicity in the capital’s life-process, or, if you like, the time required for the renewal and repetition of the valorization and production process of the same capital value.

Marx, Capital II, page 235.

Marx takes the year as the natural unit of comparison and defines the number of turnovers as the year divided by the turnover time of a given capital. A capital that turns over four times a year does in three months what another does in a year. This ratio — how many times a capital renews itself annually — is the quantity the rest of the Part is built to analyse, because it, not the mere size of the capital, governs the annual yield.

The measure and the formula are given together.

As the working day forms the natural measuring unit for the function of labour-power, so the year forms the natural measuring unit for the turnovers of capital in process. The natural basis for this measurement is that the most important food crops in the temperate zone, the native ground of capitalist production, are annual products.

If we call the year, as measurement unit of the turnover time, U, the turnover time of a particular capital u, and the number of its turnovers n, then n = U/u. If the turnover time u is three months, for example, then n = 12/3 = 4; the capital completes four turnovers in a year, or turns over four times. If u = 18 months, then n = 12/18 = 2/3; the capital only gets through two thirds of its turnover time in one year. If the turnover time amounts to several years, then it is reckoned in terms of multiples of a year.

Marx, Capital II, page 236.