How do you use a converter on a trigger

You can’t.

You can however use a data trigger and set the binding RelativeSource to Self. Data Triggers allow binding and bindings lets you have converters. Yey

Example:

       <Button Content="I change colour depending on my width for some reason">

            <Button.Triggers>

                <DataTrigger

                    Binding="{Binding

                    Path=Width,

                    RelativeSource={RelativeSource Self},

                    Converter={StaticResource isLessThanConverter},
                    ConverterParameter=50}"

                    Value="True">

                    <Setter Property="Button.Background" Value="Red" />

                DataTrigger>

            Button.Triggers>

        Button>