# Prompt cache cost calculator > Enter your prefix size and how many calls land inside one window. Below two calls on a five-minute cache, caching costs you money. URL: https://getculpa.com/prompt-cache-cost-calculator Last reviewed: 2026-08-02 ## Answer This prompt cache cost calculator takes your shared prefix size and the number of calls that land inside one cache window, then prices the cached path against the uncached one. Culpa, a local-first LLM cost, margin, and forecast ledger, computes both in exact decimal, so you can see whether the write premium earns itself back on your own traffic. ## Why this happens The input that decides this answer is the one people get wrong. Caching gets evaluated on how many calls share a prefix, and the arithmetic only cares how many of them arrive close enough together to hit a live entry. So the field above asks for calls inside one window rather than calls a month, and entering a monthly figure there reports a triumph on an endpoint that may be losing money. A five-minute entry needs a second call before the write premium is repaid, and a one-hour entry needs a third. ## What this usually looks like - Your caching decision was made on monthly call volume. - Nobody has grouped your calls by arrival time inside a window. - Caching is on everywhere by default rather than chosen per endpoint. - Cache write charges sit close to your total call count on the invoice. ## Common mistakes - Entering monthly call volume into a per-window question. Why it hurts: It turns a losing endpoint into a spectacular saving on paper and changes nothing in reality. Do instead: Count calls sharing a prefix inside one window, then enter that number. - Picking the one-hour window because it sounds safer. Why it hurts: The write premium doubles, so it takes a third call rather than a second to break even. Do instead: Try both above. The calculator prices the premium either way. - Treating one answer as covering every endpoint. Why it hurts: Arrival patterns vary between endpoints far more than prefix sizes do. Do instead: Run it per endpoint. The losing ones are usually the quiet, spread-out ones. ## Self-check - Pick one endpoint and count calls sharing a prefix in a busy five-minute window. - Enter that count above with your real prefix size. - If the result comes back a loss, switch caching off for that endpoint. - Try the one-hour window and see whether it turns the answer around. - Repeat on your quietest endpoint, which is where caching usually loses. ## One prefix, three arrival patterns (illustrative) A 50,000-token shared prefix on Claude Sonnet 5 at a real rate of $2.00 per million input, effective 2026-07-02. Anthropic bills cache reads at 0.10 times input and five-minute writes at 1.25 times. Arrival patterns are modelled. Uncached, per call: 50,000 x $2.00 per million = $0.1000 Five-minute write: 50,000 x $2.50 per million = $0.1250 Cached read: 50,000 x $0.20 per million = $0.0100 One call in the window: $0.1250 cached against $0.1000 uncached, so caching loses $0.0250 Two calls: $0.1250 + $0.0100 = $0.1350 against $0.2000, so caching saves $0.0650 Ten calls: $0.1250 + (9 x $0.0100) = $0.2150 against $1.0000, a 78.5% saving One call and caching is a 25% surcharge. Two calls and it saves 32.5%. The whole decision turns on a number most teams have never counted. ## Cost figures Every figure carries its confidence and its source. No figure on this site is provider-reported. - 2 calls — calls inside one five-minute window at which caching starts to pay [calculated] Source: A write costs 1.25 times input and a read 0.10 times, so M = 1.25 + (M - 1) x 0.10 solves to M = 1.28. Anthropic's multipliers from the price book, effective 2026-07-02. - $0.1350 to $1.0000 — modelled monthly cost of one prefix across the teardown's arrival patterns, cheapest to dearest [estimated] Source: Endpoints from the teardown arithmetic at a real Claude Sonnet 5 rate. A range because the arrival patterns are modelled. ## FAQ Q: What number goes in the calls field? A: Calls sharing the same prefix that arrive inside one cache window, so within five minutes of each other on the short entry. A monthly total belongs nowhere near this field, and one sitting there explains most caching estimates that come out wrong. Q: Why does the calculator only offer Anthropic models? A: Because those are the cache write rates Culpa's price book carries. OpenAI's GPT-5.6 family also charges to create a cache as of 2026-08-02 and the book has yet to seed those rates, while Gemini bills caching by storage time rather than by written tokens, which sits outside token accounting. Q: Does a bigger prefix change the break-even? A: No. The break-even is a ratio between the write premium and the read discount, so it lands at two calls on a five-minute entry whatever the prefix size. A bigger prefix changes how much you win or lose, not which side of the line you land on. ## Sources - Anthropic pricing: https://platform.claude.com/docs/en/docs/about-claude/pricing Run the free Cost Leak Scan: https://app.getculpa.com/scan?source=pseo&slug=prompt-cache-cost-calculator&cluster=calculator Machine-readable index of every guide: https://getculpa.com/api/pages Human-readable index of every guide: https://getculpa.com/guides Site overview: https://app.getculpa.com/llms.txt Privacy: Culpa runs on your infrastructure. Your prompts and responses never leave it. Culpa counts calls to run your plan, and it fails open, so if it ever breaks your app keeps running.