Waterproof jacket
Not a wardrobe. One set to ride in, one set that stays dry.
What a plan says about it
The planner writes this from the ride you describe, so there is no version of it that is true for everybody. Here it is worked against one ride, named below, with a single answer changed at a time to show what moves.
2 nights · wild camping · cold but not freezing (8 °C) · gravel and tracks · towns daily
You said rain is likely. Wet plus moving air is how riders get cold in summer, and it is much harder to recover from than to prevent.
On that ride the planner marks it essential.
Ways to satisfy it
One object doing one job, so this is a choice between them rather than a shopping list. Every one carries what it costs you as well as what it gets you.
Membrane hardshell
Keeps real rain out for hours rather than minutes.
but You sweat in it, and the coating wears out with washing.
A plain web search. We earn nothing from it.
Find one ↗Packable shell
Weighs so little you always have it, which is most of the battle.
but An hour of steady rain and it wets through.
A plain web search. We earn nothing from it.
Find one ↗
Where to look
We hold measured dimensions for bags and for nothing else, so this hands the requirement to a search rather than picking something for you. Nothing on this page has a price, a weight or a maker attached to it, because none of those were measured.
A plain web search. We earn nothing from it.
Search for Waterproof jacket ↗Which one do I need?
That depends on the ride, and this page does not know it. The interview does, and it is the only thing here that can answer the question — no account, and every line it gives you names the answer that produced it.