The reserve, and what it costs

The reserve has a job, or it is just idle money

A plan’s reserve is a deliberate cash holding, typically 5% to 10% of the account, that exists to do two specific things. Neither of them is “feel safer”.

Stop you being a forced seller

The one way a long-horizon plan reliably fails is selling assets at a bad price to meet a bill. Cash that covers a known number of months removes that path entirely.

Fund the rungs

When a decline reaches the levels your plan named in advance, the reserve is what makes acting on that possible. Without it the ladder is a document rather than a mechanism.

Both jobs require the size and the trigger to be written down in advance. A reserve with no stated job is not a reserve; it is money that has not been allocated, and it will quietly grow every time the market is frightening.

The price, named

Holding cash inside a plan costs return in most years. That is not a caveat, it is the entire trade, and a product that presents the reserve as free is not being straight with you.

The arithmetic is simple enough to do in your head. A 10% reserve gives up a tenth of whatever the rest of the plan earned that year. In a year the invested part gained 20%, the reserve cost you about 2% of the account. In a year it gained nothing, the reserve cost you nothing but the erosion covered in debasement, measured honestly.

When the premium buys something

The reserve earns its keep in exactly one situation: the year you would otherwise have sold. Everything else it does is a side effect.

That is a narrow claim on purpose. Deploying cash into declines is not a way to earn more than simply staying invested — the evidence on that is unambiguous and it is set out in the ladder is insurance, not alpha. The reserve is worth its cost because of what it stops you doing, not because of what it earns.

Which means the honest way to size it is behavioural rather than financial. How many months of expenses would you need covered before a 40% fall stopped being a threat to your plan? That number, in cash, is your reserve. If it is larger than 10% of the account, the allocation is probably more aggressive than your temperament, and the allocation is the thing to change.