Table of Contents

Specify SrtSet on Responsive Images API

Both <img> as well as <source> tags can have a srcset attribute.


If you are new to responsive images, we suggest you read the MDN Guide to get familiar with this

The Responsive Images API generates these automatically for you. In many cases, the configuration for this comes from the settings - like from Settings.Images.Content.

But often you may want to configure it yourself - either by reconfiguring the Settings in the Admin-UI, or by supplying in in the code.


The way the generator works is that you give it the rules you want based on 4 options:

  1. An exact width in pixels (usually with the w parameter in srcsets)
  2. A multiple based on the device pixel density (usually with the x parameter)
  3. A smaller size based an a multiple (like half the original if it's half the size)
  4. The fallback / default picture


  • 1000w, 800w, 600w, 400w
    Generate these four widths as exact pixel sizes
  • 1x, 1.5x, 2x
    Generate these three sizes for screen pixel densities
  • 1*, 0.75*, 0.5*, 0.25*
    Generate exact pixel sizes based on the initial size, so if the initial image was 1200px, it is the same as 1200w, 800w, 600w, 400w
  • d Generate a default line (without any w or x specifier)

A lot is auto-detected, so if you want to, you can leave away the w and x and * in almost all cases. So this is valid too (see below for auto-detection rules):

  • 1000, 800, 600, 400
  • 1, 1.5, 2
  • 1*, 0.75, 0.5, 0.25
  • 1/1, 1/2, 1/3, 1/4
  • (empty string) - defaults to d

In addition to the standard factor you can also set what size should be used exactly - otherwise it's auto-calculated:

  • 1x=1200, 1.5x=1800, 2x=2400, 2.5x=2400, 3x=2400
  • 1000=1000, 1600=1200
  • 1x=1200:600, 1.5x=1800:600, 2x=2400:600 - height can also be set

SrcSet Parameter Specs

  1. It is always one or more values, separated by a comma ,
  2. Each value has a number like 1, 1.5, 1200 or a fraction like 1/2 or 1:2 - or empty for d (default)
  3. Each value can have a specifier like w, x, * or d - or it's auto detected
  4. It can be followed by an = to specify exact resizing rules
    1. After the = it can have a width
    2. After that it can have a : followed by the resize height

Auto Detected Types w, x, * and d

These are the rules if the type is not specified:

  • If the number is smaller than 1 it must be a multiplier of the original size, so * is assumed
  • If the number is written as a fraction like 2/3 or 4:3 it is assumed to be a multiplier *
  • If the number is greater than 1 and less than 10 a pixel density is assumed x
  • If the number is 10 or greater than width is assumed w
  • If nothing (not even a number) is specified, default d is assumed


  1. Introduced in 2sxc 13.01