estimation-techniques

# Estimation Techniques

This skill provides frameworks for back-of-envelope calculations essential for system design and capacity planning.

## When to Use This Skill

**Keywords:** back-of-envelope, estimation, QPS, storage calculation, bandwidth, latency, capacity planning, scale estimation

**Use this skill when:**

- Estimating system capacity requirements
- Calculating storage needs for a feature
- Determining bandwidth requirements
- Sizing infrastructure for expected load
- Justifying architectural decisions with numbers
- Preparing for system design interviews

## Core Principle

**Estimation is not about precision, it's about order of magnitude.**

Getting within 10x is usually good enough for architectural decisions. The goal is to identify if you need:

- 1 server or 100 servers
- 1 GB or 1 TB of storage
- 10 ms or 1 second latency

## Essential Numbers to Know

### Powers of 2

| Power | Value | Approximate |
| ----- | ----- | ----------- |
| 2^10 | 1,024 | ~1 Thousand (KB) |
| 2^20 | 1,048,576 | ~1 Million (MB) |
| 2^30 | 1,073,741,824 | ~1 Billion (GB) |
| 2^40 | 1,099,511,627,776 | ~1 Trillion (TB) |

### Time Conversions

| Unit | Seconds | Useful For |
| ---- | ------- | ---------- |
| 1 minute | 60 | Short operations |
| 1 hour | 3,600 | Batch jobs |
| 1 day | 86,400 (~100K) | Daily aggregations |
| 1 month | 2,592,000 (~2.5M) | Monthly calculations |
| 1 year | 31,536,000 (~30M) | Annual projections |

### Availability Targets

| Availability | Downtime/Year | Downtime/Month | Downtime/Day |
| ------------ | ------------- | -------------- | ------------ |
| 99% (two 9s) | 3.65 days | 7.31 hours | 14.4 min |
| 99.9% (three 9s) | 8.76 hours | 43.8 min | 1.44 min |
| 99.99% (four 9s) | 52.6 min | 4.38 min | 8.64 sec |
| 99.999% (five 9s) | 5.26 min | 26.3 sec | 864 ms |

### Latency Numbers Every Programmer Should Know

**See full reference:** `references/latency-numbers.md`

Quick reference:

| Operation | Latency | Relative |
| --------- | -------