freesewing/markdown/dev/reference/api/config/options/pct/toabs/en.md

---
title: Reporting a percentage option value in millimeter
---

Percentage options are great for parametric desing, but not always
very intuitive for the user. For example: Setting the `chestEase`
option to `9%` is not very meaningful unless you happen to know
what that percentage is based on.

To address this common grievance, FreeSewing allows you to add a
`toAbs` method that should return the value of the option in
millimeter.

## Structure

The `toAbs` property should hold a function with the following
signature:

```js
function toAbs(percentage, settings) {
  // return value in millimeter here
}
```

The first parameter is the percentage value provided by the user (for example
`0.5` for `50%`).
The second parameter is the pattern's run-time configuration
or [settings](/reference/api/settings) which holds -- among other things -- the
measurements provided by the user.

## Example

In our example above, let's say that the `chestEase` option is
a simple percentage of the `chest` measurement. Our option
configuration could like like this:

```js
chestEase: {
  pct: 8,
  min: 0,
  max: 20,
  toAbs: function(value, settings) {
    return settings.measurements.chest * value
  }
}
```

With object destructuring and fat-arrow notation,
you can write it a bit terser like this:

```js
toAbs: (val, { measurements }) => measurements.chest * val
```

## Using pctBasedOn for simple measurement fractions

Many percentage options represent a simple fraction of a measurement
(chest circumference in the example above).

As this scenario is so common, `@freesewing/core` exports a `pctBasedOn` method
that will do the work for you:

```js
// First import the method
import { pctBasedOn } from '@freesewing/core'

const config = {
  // ...
  options: {
    chestEase: {
      pct: 8,
      min: 0,
      max: 20,
      // Pass the measurement name as parameter
      // and spread the return value into your option
      ...pctBasedOn('chest')
    }
  }
}
```

This will not only add an `toAbs()` method to your option -- one that will return
the value in millimeter of whatever percentage the option is set to -- it will
also add a `fromAbs()` method that does the inverse: return the percentage of
any millimeter value passed into it. See [Setting a value in millimeter as a
percentage option](/api/config/options/pct/fromabs) for details.