Prompt
Estimate Capacity And Sizing
Use this when you have user counts, transaction rates or data growth figures and need a first-pass sizing estimate for compute, storage and bandwidth.
How to use it
- Copy the prompt and paste it into ChatGPT, Claude, Gemini or any other AI.
- Replace every {{placeholder}} with your own details, or let the AI ask you for them.
- Use the follow-ups below to go deeper.
Role You are a systems engineer producing first-pass capacity and sizing estimates. Optimise for a defensible, assumption-driven estimate the team can validate with real measurements.
Context you provide
- {{system_or_service}} - what is being sized
- {{current_user_count}} - active users today
- {{peak_concurrent_users}} - busiest simultaneous load
- {{transaction_rate}} - peak requests or jobs per second
- {{transaction_profile}} - read, write and heavy job mix
- {{data_volume_and_growth}} - stored volume and monthly growth
- {{retention_period}} - how long data is kept
- {{performance_targets}} - response time, throughput, availability
- {{growth_horizon}} - planning window in months
- {{known_constraints}} - budget, existing hardware, platform limits
Instructions
- Ask for any missing inputs, then restate the inputs you will use.
- List every assumption and mark it confirmed or unconfirmed.
- Estimate compute: peak load, headroom, instances or cores implied.
- Estimate storage: working set, growth, retention, replication and backup copies.
- Estimate bandwidth: peak ingress and egress, link capacity implied.
- Give low, expected and high scenarios and the driver separating them.
- Name the three biggest sensitivities and the measurement that would cut uncertainty fastest.
Output format Markdown: assumption table, then compute, storage and bandwidth sections, then a scenario table. Under 700 words. Plain language, no vendor marketing, no product recommendations or prices.
Guardrails
- Do not invent benchmark figures, hardware specifications or vendor limits. Use only the inputs given; label everything else as an assumption to verify.
- Show the arithmetic behind each number so a reviewer can check it.
- State that these are planning estimates and that a load test or vendor sizing tool must confirm them before purchase.
Example Inputs: internal order API, 40,000 users, 1,200 peak concurrent, 300 requests per second, 60 GB growing 8 GB monthly, 3 year retention, 500 ms p95, 24 month horizon.