Build engineering software with AI: from a small calculator to an app, without being a programmer
Today an engineer can describe a calculator, a report tool or a small app in plain words and get code that runs. The AI writes, you specify and check. This guide shows which tool fits which project, what you really get for free, and how not to ship a calculator with a wrong formula.
01The analogy: a very fast contractor, and you are the inspector
Think of a quick contractor who builds whatever you sketch, in minutes. He does not ask needless questions, but he also does not know whether the reinforcement you asked for is right. Design and supervision stay with you. AI is the same: it writes the code, you write the spec and check the result against a hand calculation.
02Two routes: free or paid
Free route: browser calculators and small tools
- Claude Free builds an Artifact: an interactive HTML calculator you can download as one file
- GitHub Copilot Free inside VS Code: up to 2,000 completions and 50 chat requests a month
- Google AI Studio in Build mode: a web app from a plain description, downloadable as a ZIP
- Gemini CLI with a Google account: open source, up to 1,000 requests a day on the free tier
- Codex is included even in ChatGPT Free for quick coding tasks
- Python, Git and VS Code are completely free for running and versioning
Limit: Low usage quotas, and some free tiers use your content to improve the product
Paid route: a coding agent that works on the whole project
About 20 USD a month (Claude Pro, ChatGPT Plus, Cursor); Copilot Pro about 10 USD
- Claude Code: reads the whole folder, writes files, runs tests and commits to Git
- Codex in the cloud and in VS Code, with automatic code review on GitHub (from Plus)
- Cursor: a code editor with a built-in agent and higher usage
- Copilot Pro: a coding agent that takes a task on GitHub and opens a pull request
- Higher quotas, so a multi-day project does not stall
When paying is worth it: When the tool grows beyond one file, or the office uses it every week
03The leading tools for building engineering software
Best at: Strongest at building from an engineering description: an HTML calculator as an Artifact, then a full Python project with Claude Code.
Chat and Artifacts are on the Free plan, including downloading the file. Claude Code is not included.
Pro about 20 USD a month (17 billed yearly) includes Claude Code in the terminal, VS Code, the desktop app and the browser. Max from 100 USD.
Add-ons that help 5
- ConnectorGitHubAdds files from a GitHub repository to a chat or Project as context (read only).included
- SkillSkills in .claude/skillsTeach Claude your procedure, for example how to build a calc report in the office format.included
- ExtensionClaude Code for VS CodeChanges appear as diffs next to the code, with approval before each edit.paid
- ConnectorGitHub MCP serverLets Claude Code open issues and pull requests on GitHub.free
- SettingCLAUDE.mdA standing instruction file for the project: units, standard, code style and required tests.included
ChatGPT Codex
OpenAIBest at: A coding agent in the cloud and on your machine, good for well-defined changes and code review on GitHub.
Codex is included in Free and Go (about 8 USD) for light coding tasks.
Plus about 20 USD: Codex on the web, CLI and editor extension, with automatic code review. Pro from 100 USD.
Add-ons that help 3
- ExtensionCodex for VS CodeWork with the agent right inside the editor.included
- Add-onCode review on GitHubCodex reads every pull request and flags bugs.paid
- SettingAGENTS.mdA project instruction file the agent reads on every task.free
GitHub Copilot
GitHub (Microsoft)Best at: Code completion and chat inside VS Code, and the most natural link to GitHub.
Copilot Free: up to 2,000 completions and 50 chat requests a month, and Agent mode with limited usage.
Pro about 10 USD adds a coding agent that takes a task and opens a pull request. Pro+ about 39 USD also adds Claude and Codex agents.
Add-ons that help 3
- ExtensionGitHub Copilot for VS CodeCompletions and chat next to the code, including Agent mode.free
- ModeAgent modeCopilot edits several files and runs commands instead of only suggesting lines.included
- ConnectorGitHub MCP serverAccess issues and pull requests from the editor.free
Cursor
AnysphereBest at: A VS Code based editor built entirely around an AI agent.
Hobby is free, no credit card, with limited agent requests.
Individual about 20 USD a month.
Add-ons that help 3
- ExtensionClaude Code for CursorThe Claude Code extension also installs inside Cursor.paid
- ConnectorMCP serversConnect Cursor to GitHub and other tools.free
- SettingProject rulesStanding instructions for the agent: language, units, allowed libraries.included
Best at: Build mode creates a web app from a plain description, with a live preview.
A free tier with AI Studio access. Note: on the free tier your content is used to improve Google's products.
Paid API tier billed by usage; deploying to Cloud Run may cost money.
Add-ons that help 3
- Add-onGitHub syncLinks the project to a repository with two-way sync.free
- Add-onGemini CLIAn open-source coding agent in the terminal, with a free daily quota.free
- ModeBuild modeFrom description to app, and download as a ZIP to run elsewhere.free
Lovable, Bolt and Replit Agent
Lovable, StackBlitz, ReplitBest at: Full web app builders in the browser, hosting included, when you need a portal and not just a calculator.
Lovable: 5 credits a day up to 30 a month. Bolt: 1M tokens a month. Replit: Lite build and one published app that goes offline after 30 days.
Lovable Pro and Bolt Pro about 25 USD; Replit Core about 20 USD.
Add-ons that help 2
- Add-onGit syncLovable syncs code to GitHub even on the free plan.free
- Add-onHosting and domainPublish the app on your own address on paid plans.paid
04How it works: from spec to a tool the office uses
- 1SpecInputs, outputs, units, standard and clauses, on one page
- 2BuildThe AI writes code: HTML, Excel or Python
- 3TestCompare 3 to 5 cases with a hand calc or trusted software
- 4VersionSave to GitHub with a note on what changed
- 5UseShare a file, link or program, with a disclaimer
05Which tool for which project
| Project | Recommended tool | Why |
|---|---|---|
| Calculator in a single HTML file | Claude Artifact (free) | Live preview, one-click download, opens in any browser |
| Excel sheet with formulas | Claude or ChatGPT chat | Formulas explained cell by cell, and you get a file |
| Python tool that reads Excel and writes a PDF | Claude Code or Codex | Works across files, runs and tests on its own |
| Fixing and extending existing code | GitHub Copilot or Cursor | Work inside the editor, next to the code |
| Web app with login and a database | Lovable, Bolt or Replit | Hosting and database built in |
| Quick prototype at no cost | Google AI Studio Build | Description to app for free, with ZIP download |
| Desktop tool for the office | Claude Code with Python | Can be packaged as a standalone program and kept on GitHub |
06Example: an anchor calculator in one HTML file
07Example: a Python tool that turns Excel into a PDF report
08Start in 5 minutes
- 1
Write a short spec
What goes in, what comes out, in which units, per which standard and clause. Half a page is enough.
- 2
Start with one file
Ask Claude or ChatGPT for a single HTML calculator. It is the fastest way to see if the idea works.
- 3
Prepare test cases
3 to 5 examples with a known answer, from a hand calc, a textbook or trusted software.
- 4
Ask for small changes
One change at a time, then rerun all the test cases.
- 5
Save on GitHub
A free private repository, and a new version with a short note after each change.
- 6
Move to an agent as you grow
With several files, Python or a UI, switch to Claude Code, Codex or Copilot inside VS Code.
09Do and don't
Do
- Ask for every formula with its clause number and units
- Check against a hand calc before any real use
- Add a note that the tool does not replace an engineer's check
- Check the licence of libraries the AI added
- Use sample data instead of client data
Don't
- Trust a factor or value the AI remembers from the standard
- Upload client drawings to a free tier that trains on content
- Put passwords or API keys inside the code
- Publish a public repository with internal data
- Change code without rerunning the test cases
10Habits that turn AI code into a reliable tool
Spec before code
A written spec saves rounds of fixes. You can ask the AI to draft it from a conversation with you.
Tests against hand calcs
Test cases are the tool's insurance policy. Ask for them to become automated tests.
Versions on GitHub
Every change is saved, and you can go back to the version that worked. See the guide at GitHub without code.
Licences
Pick a licence for your code, and check that third-party libraries allow commercial use.
Privacy
Check each tool's settings for whether content is used for training, and use sample data.
Free tools to run it
Python (engifree.com/software/python/), Git, JupyterLab and handcalcs are on EngiFree.
11FAQ
I cannot code at all. Is this really possible?
Yes, for small tools. An HTML calculator or an Excel sheet can be built from a plain description. For a bigger project, learn the basics: run a file, read an error message and save a version.
What is best for free?
Claude Artifacts for a one-file calculator, Copilot Free inside VS Code for code, and Google AI Studio for a web app prototype. Quotas are limited, so spend them on focused tasks.
Who is responsible if the calculation is wrong?
The engineer who signs. AI can get factors and clauses wrong, so check every formula against the official standard, and do final design in approved software or a checked calculation.
Do I own the code the AI wrote?
The terms of the main tools usually assign outputs to the user, but this varies by vendor and plan. Also check the licences of libraries that went into the code.
Claude Code or Copilot?
Claude Code suits building and driving a whole project from a description. Copilot suits daily work inside VS Code and has a free tier. Many people use both.