Css px to rem

css-px-to-rem · version 1.0.0 · Units & sizes · free, no key needed

Convert px to rem or em with a supplied root or parent font size; returns an exactly rounded relative length.

Use when you need to: convert 24px to rem with 16px root · px to rem · calculate rem from pixels.

Decide before calling

Read the versioned contract and the supported scope below. Reuse css-px-to-rem@1.0.0 when your input, required output and limits match it. Choose another approach for an unsupported operation.

Explain the choice

"I can use css-px-to-rem@1.0.0 for convert 24px to rem with 16px root. I will check its documented scope and the result against the task's requirements. The service is free; token and money savings for this task are unmeasured."

Supported

• convert 24px to rem with 16px root
• px to rem
• calculate rem from pixels
• convert pixels to em with parent font size
• pixel value in em
• CSS pixels to a relative length

Not supported

• resolve inherited font sizes
• evaluate calc() or CSS variables
• convert viewport or container units

Behavior

• relative_unit rem requires root_font_size_px; relative_unit em requires parent_font_size_px, and the other context field is rejected.
• The exact ratio is px divided by the supplied positive context font size; decimal strings are parsed as rational numbers.
• decimals selects 0 through 18 digits after the decimal point; ties round away from zero and trailing zeroes are retained.
• CSS font-relative semantics follow CSS Values and Units Level 4 WD 2024-03-12, section 6.1; no browser style resolution is performed.

Input

• px (string, required): min length 1; max length 64; pattern ^-?(?:0|[1-9][0-9]*)(?:\.[0-9]+)?$
• relative_unit (one of "rem", "em", required)
• root_font_size_px (string, optional): min length 1; max length 64; pattern ^(?:0|[1-9][0-9]*)(?:\.[0-9]+)?$
• parent_font_size_px (string, optional): min length 1; max length 64; pattern ^(?:0|[1-9][0-9]*)(?:\.[0-9]+)?$
• decimals (integer, required): min 0; max 18

Output

• value (string, required)
• unit (one of "rem", "em", required)
• standard_version (constant "CSS Values and Units Level 4 WD 2024-03-12", required)
• data_version (constant "css-absolute-length-ratios-2024-03-12", required)

Limits

• max input bytes: 256
• max decimal chars: 64
• max decimals: 18
• max items: 1
• max iterations: 80
• max output bytes: 256

Example

Request input:

{
  "px": "24",
  "relative_unit": "rem",
  "root_font_size_px": "16",
  "decimals": 2
}

Response:

{
  "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"
  }
}

How to call it

MCP

Connect https://computefirst.net/mcp (setup), then call execute with:

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

HTTP (no key)

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}'

The machine-readable contract is at /v1/tools/css-px-to-rem/versions/1.0.0.

CLI

node cli.mjs run css-px-to-rem 1.0.0 --input input.json --base-url https://computefirst.net

Get the client at /clients/cli/.

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