Is the Back Of The Envelope skill free?

Yes. The skill itself is free from proyecto26/system-design-skills. SkillProof publishes the install command and an independent test verdict at no cost.

What is the Back Of The Envelope SkillProof Score?

9.2/10 — installs cleanly 5/5, triggers reliably 5/5, output vs. baseline 8/10, docs & honesty 5/5.

How do I install Back Of The Envelope?

Copy the install command from this page, run it in your terminal, and restart Claude Code. Skills live in ~/.claude/skills/ (global) or .claude/skills/ inside a project.

Can I use Back Of The Envelope with Cursor, Copilot, Gemini CLI, Codex or other AI tools?

The SKILL.md format is native to Claude (Claude Code, Desktop, claude.ai). The instructions inside adapt to other assistants: Cursor rules, GitHub Copilot instructions, Windsurf rules, Custom GPTs, AGENTS.md for OpenAI Codex, and GEMINI.md for Google Gemini CLI — our conversion guides cover each, and the free converter on the tools page does the wrapping for you.

Back Of The Envelope

Turns "high traffic" into QPS, storage, bandwidth and server-count numbers, with a script to check the math.

by proyecto26 · proyecto26/system-design-skills

Tested · Works

Install — copy the command Download source ↗

Test report

Verdict: Tested · Works
Score: 9.2/10
Tested: Jul 15, 2026
Environment: Claude Code 2.x (agent harness)
Upstream re-checked: Aug 10, 2026 · 29b900d

Ran the bundled botec.py on a 50M-DAU photo-app scenario: it produced peak QPS, TB/day storage, PB total, and 1.2GB/s bandwidth with labeled peak factor and units, where an unaided baseline only got avg QPS and total storage — genuinely more complete. Own test_botec.py golden-fixture check passed live. Minor flaw: server-count rounds to "0" instead of "1" for light workloads.

Scored on four weighted criteria — install, triggering, output vs. baseline, docs. How scoring works

Installs cleanly 5/5
Triggers reliably 5/5
Output vs. baseline 8/10
Docs & honesty 5/5

What Back Of The Envelope does

Guides QPS, storage, bandwidth and server-count estimation for system design using standard rule-of-thumb tables (per-server QPS, latency numbers, powers of two). Triggers on estimate/capacity-planning language, and even implicitly whenever a design choice hinges on scale. Ships a deterministic Python calculator (botec.py) with a golden-fixture test.

How to install Back Of The Envelope

git clone https://github.com/proyecto26/system-design-skills
mkdir -p ~/.claude/skills
cp -r system-design-skills/skills/back-of-the-envelope ~/.claude/skills/back-of-the-envelope

Skills live in ~/.claude/skills/ (global) or .claude/skills/ (per-project). Restart Claude Code after installing.

Commands — how to trigger Back Of The Envelope

/back-of-the-envelope Turns "high traffic" into QPS, storage, bandwidth and server-count numbers, with a script to check the math.

It also activates on plain-language prompts like these:

estimate the QPS and storage needs for 50 million daily users
how many servers would I need to handle this traffic load
calculate bandwidth and storage for a photo-sharing app at scale

Frequently asked questions

Is the Back Of The Envelope skill free?: Yes. The skill itself is free from proyecto26/system-design-skills. SkillProof publishes the install command and an independent test verdict at no cost.
Does Back Of The Envelope work with Claude Code?: We tested it with Claude Code 2.x (agent harness) on Jul 15, 2026. Verdict: Tested · Works. Ran the bundled botec.py on a 50M-DAU photo-app scenario: it produced peak QPS, TB/day storage, PB total, and 1.2GB/s bandwidth with labeled peak factor and units, where an unaided baseline only got avg QPS and total storage — genuinely more complete. Own test_botec.py golden-fixture check passed live. Minor flaw: server-count rounds to "0" instead of "1" for light workloads.
What is the Back Of The Envelope SkillProof Score?: 9.2/10 — installs cleanly 5/5, triggers reliably 5/5, output vs. baseline 8/10, docs & honesty 5/5.
How do I install Back Of The Envelope?: Copy the install command from this page, run it in your terminal, and restart Claude Code. Skills live in ~/.claude/skills/ (global) or .claude/skills/ inside a project.
Can I use Back Of The Envelope with Cursor, Copilot, Gemini CLI, Codex or other AI tools?: The SKILL.md format is native to Claude (Claude Code, Desktop, claude.ai). The instructions inside adapt to other assistants: Cursor rules, GitHub Copilot instructions, Windsurf rules, Custom GPTs, AGENTS.md for OpenAI Codex, and GEMINI.md for Google Gemini CLI — our conversion guides cover each, and the free converter on the tools page does the wrapping for you.