Interactive Engineering Tool
Back-of-the-Envelope Capacity Math Calculator
Derive Peak QPS, Bandwidth, Daily Storage, 5-Year Storage Growth, and 80/20 RAM Caching budgets in seconds for your system design interview.
Interactive Calculator
Set your assumptions
Choose a scenario or set your own inputs. Each estimate includes the reasoning to explain it.
Start with a scenario
Illustrative assumptions for practice. Choose a starting point, then adjust it to fit your design.
Your capacity estimate
Use the numbers to explain your trade-offs.
| Metric | Estimate | How to explain it |
|---|---|---|
| Average throughput | 11,574 QPS | Daily requests ÷ 86,400 seconds. QPS means queries per second. |
| Peak throughput | 28,935 QPS | 2.5× average traffic. A starting point for compute and load balancing. |
| Read / write throughput | 9,259 / 2,315 QPS | Average reads / writes per second. Discuss replicas and write partitioning. |
| Peak Bandwidth | 56.51 MB/s | Peak requests × payload size. Use this to discuss network capacity. |
| Daily storage growth | 381 GB / day | Daily writes × payload size, before replicas and indexes. |
| 5-year database storage | 680 TB | Five years of steady writes, with no deletion or compression. |
| RAM cache budget | 305 GB RAM | Cache the hottest 20% of the daily read volume. |
| Storage IOPS at peak | 10,417 IOPS | Peak writes plus the 20% of peak reads that miss the cache. Assumes one storage operation per request. |
Model assumptions: 2.5× peak traffic, 80% cache hit rate and five-year retention. Units use 1,024-based conversions. These estimates exclude replication, indexes and protocol overhead.
Capacity math in live practice
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