# Convert px to rem or em from an AI agent

You need a CSS length in px turned into rem or em for a given font size, rounded to a set number of decimals.

Use `css-px-to-rem@1.0.0`. It is free and needs no key or sign-up. Convert px to rem or em with a supplied root or parent font size; returns an exactly rounded relative length.

## Why call a tool for this

The font size the conversion depends on is part of the call, so the assumption is visible. For example, 24 px with a root font size of 16 px and 2 decimals returns 1.50 rem.

## Call it over HTTP

```sh
curl -X POST https://computefirst.net/v1/tools/css-px-to-rem/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"px":"24","relative_unit":"rem","root_font_size_px":"16","decimals":2}'
```

Response:

```json
{
  "result": {
    "value": "1.50",
    "unit": "rem",
    "standard_version": "CSS Values and Units Level 4 WD 2024-03-12",
    "data_version": "css-absolute-length-ratios-2024-03-12"
  }
}
```

## Call it over MCP

Connect `https://computefirst.net/mcp` ([setup](/docs#connect)), then call `execute` with:

```json
{
  "id": "css-px-to-rem",
  "version": "1.0.0",
  "input": {
    "px": "24",
    "relative_unit": "rem",
    "root_font_size_px": "16",
    "decimals": 2
  }
}
```

## Check the result

For rem pass root_font_size_px; for em pass parent_font_size_px. Passing the wrong one is rejected as invalid_input. Browsers often use 16 px as the default root size, but check what the site actually sets. To go the other way, use css-rem-to-px.

Full contract, limits and unsupported cases: [Css px to rem](/tools/css-px-to-rem).

## Related tools

- [Css rem to px](/tools/css-rem-to-px)
- [Css length convert](/tools/css-length-convert)