How to set line width and opacity

Make every line on a layer thicker or thinner, and more or less transparent. Works with one colour, categories, and class breaks.

Line width and opacity apply to every line on the layer, whatever colours the lines get. Use a thick line for main routes and a thin line for dense networks, and lower the opacity so the basemap shows through. Open the layer’s Style tab, drag the line width slider, and set the opacity. Where you set the opacity depends on the colour driver: the colour picker for one colour, or the Line opacity slider for a field.

Before you start

The layer must be a line layer. For polygon outlines, see How to style polygon outlines. For marker size, see How to set a uniform point style.
You need edit access to the map.
Line width is one value for the whole layer. You cannot make a line’s width depend on a field.

Open the Style tab

In the layer panel, select your line layer. Click the Style tab.

Set the line width

The line width slider is below the colour controls. It is in the same place with Uniform colour and with a field in Colour lines by.

Drag the slider, or type a value in the box. The range is 0.5 to 20 px, in steps of 0.5 px. A new line layer is 2 px wide.

Here both sliders are on a layer coloured by category:

Style section of the Roads — Category layer with Colour lines by set to class, the Colour scheme dropdown, the Line opacity slider at 100%, the line width slider at 2.5 px, More line settings, and Customise categories open with rows Motorway to Primary road

A few starting points:

Dense networks (streets, streams): 0.5–1.5 px.
Main routes (highways, rail): 3–6 px.
A single route on a quiet basemap (a trail, a delivery run): 6–10 px.

A wide Dashed or Dotted line also gets longer dashes, bigger dots, and wider gaps, because the pattern grows with the width. See How to style line patterns and casing.

Set the opacity

How you set the opacity depends on Colour lines by:

`Colour lines by`	Where to set it
`Uniform colour`	In the colour picker. Click the line colour swatch and lower the transparency. There is no opacity slider.
A field (categories or class breaks)	The opacity slider below the `Colour scheme` dropdown. The range is 0 to 100%.

With a field, the Line opacity slider changes every category or class break, except the ones you gave a custom colour in Customise categories or Customise Class Breaks. Those keep their own opacity. To change one, open its swatch and set the transparency in the colour picker.

Save

Click Save at the bottom of the panel.

Verify the result

The map redraws with the new width and opacity. Zoom in and out: the width stays the same number of pixels on screen at every zoom.

Map close-up of central Singapore with the Roads — Uniform layer drawn as 4 px blue lines at 70% opacity, the basemap showing through the lines and darker where they cross, and the Legend open with Roads — Uniform turned on

Common problems

Lines look darker where they cross. With an opacity below 100%, two lines on top of each other add up. Use 100% opacity for even colour at crossings.

Lines are too thick when zoomed out. The width does not change with zoom. Use a thinner line, or show a thinner copy of the layer at low zoom: duplicate the layer and give each copy its own zoom range. See How to set the zoom range a layer is visible at.

What this doesn’t cover

Line pattern, cap, join, and casing. See How to style line patterns and casing.
Line colour from a field. See How to style features by category and How to style features by quantity.