Savings & Interest

Idle cash in a savings account earns interest every turn. There is no feature flag: savings interest is always on. The rate is tied to the central bank's prime rate, accrues continuously, and credits to your balance on a quarterly schedule.

The rate#

The annual percentage yield (APY) paid on savings is half the real rate: the prime rate minus local inflation, not the prime rate on its own.

realRate = max(0.5, primeRate - inflation)
APY      = realRate / 2 + centralBankDepositBonus

So a 4.42% prime rate does not pay 2.21% APY unless inflation is zero. With 2% inflation the real rate is 2.42% and the APY is 1.21%.

Using the real rate is what closes the carry trade. A currency with a high nominal prime rate normally also has high inflation, so its real spread, and therefore its savings APY, lands near everyone else's. Parking money in a high-nominal-rate currency earns no free real return.

The real rate never falls below a 0.5 percentage point floor, so every currency pays at least 0.25% APY. The floor is the same everywhere, so it gives no currency an edge.

During a central-bank pricing phase-in, central-bank-held deposits receive a temporary bonus in addition to this formula. The current rollout adds up to 0.25 percentage points over 8 turns. Private-bank deposit rates are set by their own charters and do not receive this central-bank bonus.

When the Central Bank Chair raises rates, your savings yield rises with them, but only to the extent the rise beats inflation.

Per-turn accrual#

Interest accrues every turn but is only credited quarterly (every 12 turns, a quarter of the 48-turn game year). The per-turn accrual is:

perTurnAccrual = balance × (APY / 100) / 48

With 48 turns per year and a real rate of 4% (APY 2%):

perTurnAccrual = balance × 0.02 / 48 = balance × 0.0004167

A ¥1,000,000 balance accrues roughly ¥41.67 per turn, credited as a ¥500 lump every 12 turns.

Rounding#

Accruals are rounded to the currency's native precision:

This keeps credited interest matching the ledger's expected precision: no fractional yen, no floating-point drift on dollar/euro balances.

Crediting schedule#

Accrued-but-uncredited interest is tracked internally; if you close or move the account mid-quarter, the pending accrual is settled at that point so nothing is lost.

Practical notes#

• Rate-following: Because APY = real rate / 2, savings is sensitive to both policy rates and inflation. In a high-rate environment it is a genuine yield instrument; in a low-rate environment it barely beats stuffing cash under a mattress.
• No lock-up: Unlike bonds, savings can be withdrawn at any time. The cost of that liquidity is the half-real-rate haircut versus buying sovereign bonds directly.
• Compound effect: Credited interest becomes part of the balance for the next quarter's accrual, so savings compounds quarterly, not just per-turn.
• Whale cap: Only the lesser of your balance and 25% of the whole currency's national savings pool earns interest. This closes a carry-trade loophole where a single huge account could farm interest on nearly 100% of a currency's savings; ordinary balances well under a quarter of the pool are unaffected.

See also: Central Banks, Sovereign Bonds, Line of Credit, Currency Exchange