calculators
Numbers you can check yourself.
Every calculator here sits inside a post that explains where its numbers come from, with the provider's own pricing page or documentation linked. Nothing you type leaves your browser.
ai engineering
Do you need an LLM gateway?
What an LLM gateway gives you, what it costs in fees and in latency, and a checklist for choosing between a library, a self-hosted proxy and a hosted one.
Sep 2026
What one agent task costs, step by step
An agent resends its history on every step, so cost grows with the square of the step count. What a task costs, and where to put the caps.
Sep 2026
How many test cases does your eval need?
Your eval went from 80% to 85% on 40 cases. That is noise. The sample size maths for LLM evals, with a calculator and the intervals nobody prints.
Sep 2026
Retrieval or the whole corpus in the prompt?
Retrieval, fine-tuning and a million-token context window solve different problems. A cost comparison, the break-even corpus size, and when each one fails.
Sep 2026
What prompt caching saves on your traffic
How prompt caching works on all three big providers: minimum prefixes, lifetimes, write premiums, prompt ordering, and the hit rate where caching starts paying.
Sep 2026
Scope your chatbot, see what it costs
Agency guides quote anything from $5,000 to $300,000. Here is where the money actually goes, line by line, with an estimator and the monthly bill after launch.
Sep 2026
What your AI feature costs per user
Tokens per call, calls per action, actions per user: a calculator for what an AI feature costs each user, and why the heaviest 10% decide whether the plan works.
Sep 2026
What tracing your LLM calls will store, and what each call costs
Trace model calls with OpenTelemetry's GenAI conventions, attribute cost per user at 2026 prices, and size trace storage before prompt payloads become most of it.
Mar 2026
architecture
Order a strangler fig migration
How to move off a legacy monolith without a rewrite: the facade, what order to move modules in, a wave planner, and the reasons migrations stall.
Sep 2026
Build it or buy it, over three years
Auth, billing, search or notifications: what building really costs over three years once maintenance is counted, and the test that comes before the money.
Sep 2026
Freelancer, agency, fractional CTO or a hire
Four ways to build or fix a product, compared on published rates: what each delivers in six months, what each costs an hour, and how each one fails.
Sep 2026
What an hour of downtime costs you
Surveys put an hour of downtime above $300,000. Here is the arithmetic for a startup instead: interrupted revenue, SLA credits, people, support and churn.
Sep 2026
Score your company before a reviewer does
What investors and acquirers check in your tech: architecture, security, data, delivery, bus factor and cost. A 20-question scorecard and what to fix first.
Sep 2026
How many customers are you charging twice?
A timeout does not mean the request failed. How idempotency keys stop retries creating duplicate charges and orders, what to store, and a calculator for the damage.
Mar 2026
Are you ready for microservices?
Every service on a request path multiplies failure and slow tails. A readiness scorecard, the distributed tax in numbers, and when a modular monolith is the answer.
Mar 2026
messaging
Guess the category before Meta does
Meta's own test for marketing, utility and authentication templates, what gets re-categorised, and an exercise: guess the category before you see the price.
Sep 2026
What a month of OTPs costs on each channel
Twilio's SMS rates against Meta's WhatsApp authentication rates in eight countries, the fallback nobody budgets for, and a calculator for your own volume.
Sep 2026
Go direct on the Cloud API, or pay a provider?
Meta charges the same per message either way. The real choice is a provider's cut against building it yourself. Current BSP prices and a break-even calculator.
Sep 2026
What your WhatsApp bill does on 1 October
From 1 October 2026 Meta bills every free-form WhatsApp reply and every in-window utility template. Official rates, a calculator, and four real-shaped bills.
Sep 2026
real-time
How many partitions does this topic need?
The formula is max(t/p, t/c), but the number you pick is close to permanent: partitions can be added and never removed, and adding them re-maps your keys.
Sep 2026
How many WebSocket connections will this box hold?
File descriptors, memory, the event loop or the link: one of them stops you first. Measured memory per idle connection, and a calculator for your own box.
Sep 2026
system design
What a rate limiter does to a burst
A token bucket forgives a burst and still holds the average. Fixed windows let a client send double at the boundary. A simulator, and the headers to send back.
Sep 2026
Will your pools fit in max_connections?
Eight app instances with pools of ten is eighty connections to a database that can usefully run eight. The sizing formula, max_connections and pgbouncer.
Sep 2026
What fan-out does to your tail latency
A backend that is slow 1% of the time makes 63% of user requests slow when one request waits on a hundred of them. The fan-out maths, and how hedging fixes it.
Sep 2026
How many workers does this queue need?
One formula sizes worker pools, connection pools and queues: L = λW. What it does and does not tell you, why the last worker matters most, and a calculator.
Sep 2026
What happens when everyone retries at once
Retries without jitter arrive in waves and stretch a ten-second outage into four and a half minutes. The four backoff formulas, a simulator, and what to set.
Sep 2026
What can you actually promise?
Every service a request touches multiplies into your uptime. A composite SLA calculator, the published numbers for AWS, GCP, Azure and Twilio, and what to promise.
Sep 2026
How long until your queue runs out of memory?
A queue with no ceiling grows at the difference between two rates until the process dies. The arithmetic, a simulator, and the four places the extra work can wait.
Mar 2026
Will this backlog ever drain?
A queue drains only by the capacity above its incoming rate. Work out drain time, message wait and consumers needed, and where Kafka's partitions stop you.
Mar 2026
voice ai
Where your voice agent's response time goes
People answer each other in about 200 ms. Voice agents aim for 800 and often land past 1,400. A stage-by-stage latency budget, with a calculator for your own stack.
Sep 2026
What one minute of an AI phone call costs
One minute of an AI phone call, priced part by part: line, speech-to-text, model and voice. Where building beats Vapi, Retell and Bland, and where it doesn't.
Sep 2026
Need the number for your own system?
The calculators cover the common shapes. If yours is stranger than that, tell me what you are building and I'll work it through with you on a call.