Choose a plugin

Start with the unit that appears in your source CSS and the unit your runtime expects. The focused plugins keep familiar APIs; postcss-rule-unit-converter is the composition layer when one stylesheet needs several conversion rules.

Conversion map

Source	Target	Package	Best for
rem	px / rpx	postcss-rem-to-responsive-pixel	Existing rem-to-pixel pipelines and Tailwind output
rem	vw / vh	postcss-rem-to-viewport	Viewport-based responsive layouts
px	rpx / rem / vw	postcss-pxtrans	Weapp, H5, React Native, Quick App and Harmony presets
Any supported unit	px	postcss-units-to-px	A small legacy API with a configurable unitMap
Any supported unit	Any target	postcss-rule-unit-converter	Composable rules, grouped presets and custom transforms
Utility building blocks	—	postcss-plugin-shared	Writing a plugin that shares matching and rounding behavior

Pick the smallest stable API

Use a focused package when its source and target match your project. Use the rule engine when you need to process px, rem, rpx, vw or vh in one PostCSS pass, or when the conversion policy belongs in a reusable preset.

Next step

Read PostCSS integration, then open the plugin page for its complete options and examples.