How many reuses it takes for prompt caching to actually save money — write premium, read discount and the 5-minute TTL included.
A cached prefix only pays off if you hit it again before it expires. For a busy shared system prompt that is a certainty; for occasional calls it can cost more than not caching. This calculator shows exactly where your break-even sits. Compare with the plain prompt caching savings calculator and batch savings.
Free prompt cache break-even calculator — the cache-write premium and cheap cache reads to the number of reuses before caching pays off, TTL included.
Caching charges a one-time premium to write the prefix (about 1.25× base input at Anthropic) and then bills reads at roughly 10% of base. You break even once the discount from enough reuses outweighs that write premium — often after just one or two hits. Below that, or if the cache expires (5-minute default TTL) before a reuse, the write premium is wasted.
The time-to-live is how long a cached prefix stays warm — 5 minutes by default, with a longer 1-hour option at higher write cost. Every reuse resets or extends it. If your requests are spaced further apart than the TTL, each call re-writes the cache and you pay the premium repeatedly instead of the cheap read, killing the savings.