A rain sensor just tells you if it’s raining or not. A rain gauge will give you cumulative rainfall values. I know there are DIY projects to make HA-compatible gauges but I’d rather have something out of the box, at least in terms of hardware.
I found this on Amazon, and it appears to be be a clone of a more expensive station by Ambient Weather. It looks like it could be made to work with HA as it has a web interface, but I’m not sure if an app is required for setup. The rain gauge also doesn’t seem super accurate, which is frustrating as that’s the only thing I really care about.
I ended up going with this one. I can’t confirm the accuracy of the gauge as it hasn’t rained much yet and I don’t have an analog gauge to compare, but I suppose it will work fine as a rough estimator if it isn’t accurate.
There’s no app or account required which is the biggest selling point for me.