Unit Converter

Let people convert a value between units without leaving the page they are on.

simple Forms & Input

What it adds

An inline converter that takes a typed value in one unit and shows the equivalent in another, in place.

What your agent is told to do

1
Start from the units the app actually deals in. A shipping tool needs weight and length; a recipe tool needs volume and temperature. Ship those and leave the rest out.
2
Convert as the user types, with the result visible beside the input rather than behind a submit button. Let either side be edited so the conversion runs in both directions.
3
Reuse the app's existing number parsing and display formatting rather than writing new ones. If Locale-Aware Numbers and Dates or Number Formatting Component is already present, the converter must render through it so grouping and decimal marks match the rest of the app.
4
Hold the canonical value in a single base unit internally and convert out to whatever is displayed, so switching units repeatedly does not accumulate error.
5
Do not implement conversion as a flat table of pairwise multipliers. It grows quadratically, and the pairs nobody thought to add will silently return nothing.

Edge cases it handles

Converting a value out and back must return the value the user typed. Rounding the display separately from the stored value produces drift that shows up as 1 becoming 0.99999 after three switches.
People type decimals with a comma as well as a period depending on their locale. Accept both on input, and do not treat a comma as a thousands separator in a field that expects a single number.
Do not report more significant figures than the input justifies. Two inches converted to millimetres is not 50.800000 mm, and showing it that way implies a precision the user never gave.
Temperature and some pressure scales have an offset as well as a ratio. A converter that only multiplies will get Celsius to Fahrenheit wrong for every value except zero.
A request to convert between incompatible dimensions, such as kilograms to metres, must be refused with a clear message rather than producing a number. Only offer units in the same dimension in the target selector.
Empty, partial, and non-numeric input must show nothing rather than an error or a zero. A user mid-keystroke has not made a mistake yet.
Very large and very small magnitudes must stay readable. Decide whether to switch to exponential notation or to a larger unit, and apply the rule consistently.
The converted value must be announced to screen readers when it changes, not only rendered visually next to the field.

Definition of done

A value converted to another unit and back matches the original exactly.
Both comma and period decimal separators are accepted on input.
Displayed precision reflects the precision of the input rather than the float behind it.
Offset scales such as temperature convert correctly across their whole range, including negatives.
Only units in the same dimension are offered, and incompatible conversions are refused with an explanation.
Numbers render through the app's existing formatting rather than a second implementation.
Result changes are announced accessibly.
The feature matches the existing design system.
No existing functionality is broken.

Related features

Code Editor Field

Voice Dictation

Form Field Shell

How it works

1

Copy the link

Grab the Markdown instruction URL for this feature.
2

Give it to your AI

Paste it into Claude Code, Cursor, v0, Lovable — whatever you build with.
3

It inspects, then implements

Your agent reads your existing app first, then adds the feature to fit it.

Works with your stack

These instructions are written to adapt. They tell the agent to detect your framework, match your existing design system, and reuse what you already have — rather than assuming a particular stack.

Need it tighter than that? Customize the feature and tell it exactly what you're running.