Cathodic protection makes the whole of a structure a cathode by supplying it with electrons from somewhere else — a metal that is willing to corrode instead. Designing it is three questions in a fixed order: how much current does the structure need, can the anodes deliver it, and is there enough anode metal to keep delivering it for the design life.

The demand. $I = A\,i\,f$ — I equals A i f. $I$ is the total protection current in amps. $A$ is the structure's whole surface area in m² — the cathode, coated or not. $i$ is the protection current density the environment demands, in mA per square metre of BARE steel: roughly 10 to 30 in soil, 20 to 100 in seawater. $f$ is the coating breakdown factor, the fraction of the surface actually through to metal — 1 for uncoated steel, a few percent for a good new coating. The whole reason a coating is worth money is that it multiplies this current by a small number.

The output. $I = \dfrac{\Delta E}{R}$ — I equals delta-E over R. Here $I$ is the current one anode delivers, $\Delta E$ is the NET driving voltage — the anode's operating potential less the potential the structure has already polarised to — and $R$ is the total circuit resistance in ohms: anode-to-earth, plus cable, plus the structure's own resistance to earth. That word NET is where the money is. Once buried steel is polarised to the conventional −0.85 V criterion, zinc has only about 0.2 V of drive left and magnesium about 0.7 V. That difference is the entire reason magnesium is used in high-resistivity soil and zinc is not.

The metal. $W = \dfrac{I\,t}{C\,u}$ — W equals I t, over C u. $W$ is the net anode mass in kg. $I$ is the MEAN current over the life — not the initial and not the final, because a coating breaks down and those are three different numbers. $t$ is the design life, and the capacity below is quoted in amp-HOURS, so the years have to arrive as hours: take the year as 8766. $C$ is the alloy's practical current capacity in A·h/kg — zinc 780, magnesium 1100, aluminium 2500. $u$ is the utilisation factor, the fraction of the anode that can actually be consumed before the metal near the core insert loses contact and falls off: 0.90 for slender stand-off shapes, 0.80 for bracelets.

Both denominators are where optimism costs a system its life. Note that the practical capacities above are already below the theoretical ones Faraday's law gives — 820 for zinc, 2980 for aluminium, 2200 for magnesium — because current efficiency is never 100 %. And the nugget: an anode that lasts is not the same as an anode that works. Mass answers the life question; the voltage over the resistance answers the output question, and a system can pass one and fail the other on the same day.