The sx prop

The `sx` prop is a shortcut for defining custom style that has access to the theme.

The property is a superset of CSS that packages all the style functions that are exposed in @mui/system. You can specify any valid CSS using this prop.

Example

import * as React from 'react';
import { Box, ThemeProvider, createTheme } from '@mui/system';

const theme = createTheme({
  palette: {
    background: {
      paper: '#fff',
    },
    text: {
      primary: '#173A5E',
      secondary: '#46505A',
    },
    action: {
      active: '#001E3C',
    },
    success: {
      dark: '#009688',
    },
  },
});

export default function Example() {
  return (
    <ThemeProvider theme={theme}>
      <Box
        sx={{
          bgcolor: 'background.paper',
          boxShadow: 1,
          borderRadius: 1,
          p: 2,
          minWidth: 300,
        }}
      >
        <Box sx={{ color: 'text.secondary' }}>Sessions</Box>
        <Box sx={{ color: 'text.primary', fontSize: 34, fontWeight: 'medium' }}>
          98.3 K
        </Box>
        <Box
          sx={{
            color: 'success.dark',
            display: 'inline',
            fontWeight: 'medium',
            mx: 0.5,
          }}
        >
          +18.77%
        </Box>
        <Box sx={{ color: 'text.secondary', display: 'inline', fontSize: 12 }}>
          vs. last week
        </Box>
      </Box>
    </ThemeProvider>
  );
}

On the example above, you can notice that some of the values are not valid CSS properties. This is because the sx keys are mapped to specific properties of the theme. In the following sections, you will learn how different sx properties are mapped to specific parts of the theme.

Theme aware properties

Borders

The border property can receive only a number as a value. It creates a solid black border using the number as the width.

<Box sx={{ border: 1 }} />
// equivalent to border: '1px solid black'

The borderColor property can receive a string, which represents the path in the theme.palette.

<Box sx={{ borderColor: 'primary.main' }} />
// equivalent to borderColor: theme => theme.palette.primary.main

The borderRadius properties multiples the value it receives by the theme.shape.borderRadius value (the default for the value is 4px).

<Box sx={{ borderRadius: 2 }} />
// equivalent to borderRadius: theme => 2 * theme.shape.borderRadius

Head to the borders page for more details.

Display

The displayPrint property allows you to specify CSS display value, that will be applied only for printing.

<Box sx={{ displayPrint: 'none' }} /> // equivalent to '@media print': { display: 'none' }

Head to the display page for more details.

Grid

The grid CSS properties gap, rowGap and columnGap multiply the values they receive by the theme.spacing value (the default for the value is 8px).

<Box sx={{ gap: 2 }} />
// equivalent to gap: theme => theme.spacing(2)

Head to the grid page for more details.

Palette

The color and backgroundColor properties can receive a string, which represents the path in the theme.palette.

<Box sx={{ color: 'primary.main' }} />
// equivalent to color: theme => theme.palette.primary.main

The backgroundColor property is also available trough its alias bgcolor.

<Box sx={{ bgcolor: 'primary.main' }} />
// equivalent to backgroundColor: theme => theme.palette.primary.main

Head to the palette page for more details.

Positions

The zIndex property maps its value to the theme.zIndex value.

<Box sx={{ zIndex: 'tooltip' }} />
// equivalent to zIndex: theme => theme.zIndex.tooltip

Head to the positions page for more details.

Shadows

The boxShadow property maps its value to the theme.shadows value.

<Box sx={{ boxShadow: 1 }} />
// equivalent to boxShadow: theme => theme.shadows[1]

Head to the shadows page for more details.

Sizing

The sizing properties: width, height, minHeight, maxHeight, minWidth and maxWidth are using the following custom transform function for the value:

function transform(value) {
  return value <= 1 ? `${value * 100}%` : value;
}

If the value is between [0, 1], it's converted to percent. Otherwise, it is directly set on the CSS property.

<Box sx={{ width: 1/2 }} /> // equivalent to width: '50%'
<Box sx={{ width: 20 }} /> // equivalent to width: '20px'

Head to the sizing page for more details.

Spacing

The spacing properties: margin, padding and the corresponding longhand properties multiply the values they receive by the theme.spacing value (the default for the value is 8px).

<Box sx={{ margin: 2 }} />
// equivalent to margin: theme => theme.spacing(2)

The following aliases are available for the spacing properties:

Prop	CSS property
m	margin
mt	margin-top
mr	margin-right
mb	margin-bottom
ml	margin-left
mx	margin-left, margin-right
my	margin-top, margin-bottom
p	padding
pt	padding-top
pr	padding-right
pb	padding-bottom
pl	padding-left
px	padding-left, padding-right
py	padding-top, padding-bottom

Head to the spacing page for more details.

Typography

The fontFamily, fontSize, fontStyle, fontWeight properties map their value to the theme.typography value.

<Box sx={{ fontWeight: 'fontWeightLight' }} />
// equivalent to fontWeight: theme.typography.fontWeightLight

The same can be achieved by omitting the CSS property prefix fontWeight.

<Box sx={{ fontWeight: 'light' }} />
// equivalent to fontWeight: theme.typography.fontWeightLight

There is additional typography prop available, which sets all values defined in the specific theme.typography variant.

<Box sx={{ typography: 'body1' }} />
// equivalent to { ...theme.typography.body1 }

Head to the typography page for more details.

Responsive values

All properties as part of the sx prop also have a support for defining different values for specific breakpoints. For more details on this, take a look at the Responsive values section.

Callback values

Each property in the sx prop can receive a function callback as a value. This is useful when you want to use the theme for calculating some value.

<Box sx={{ height: (theme) => theme.spacing(10) }} />

TypeScript usage

A frequent source of confusion with the sx prop is TypeScript's type widening, which causes this example not to work as expected:

const style = {
  flexDirection: 'column',
};

export default function App() {
  return <Button sx={style}>Example</Button>;
}
//    Type '{ flexDirection: string; }' is not assignable to type 'SxProps<Theme> | undefined'.
//    Type '{ flexDirection: string; }' is not assignable to type 'CSSSelectorObject<Theme>'.
//      Property 'flexDirection' is incompatible with index signature.
//        Type 'string' is not assignable to type 'SystemStyleObject<Theme>'.

The problem is that the type of the flexDirection prop is inferred as string, which is too wide. To fix this, you can cast the object/function passed to the sx prop to const:

const style = {
  flexDirection: 'column',
} as const;

export default function App() {
  return <Button sx={style}>Example</Button>;
}

Alternatively, you can pass the style object directly to the sx prop:

export default function App() {
  return <Button sx={{ flexDirection: 'column' }}>Example</Button>;
}

Performance

If you are interested in the performance tradeoff, you can find more details here.