What to wear in Paris in May

I'd definitely vote for the closed toe and long pants. We were there in the very end of April last year, maybe even a day or two in May, and we not only wore long pants, but carried jackets everywhere and often used them. On rainy days, it was chilly and was never hot, even in bright sun. It was fun to see the chesnut trees in bloom, and we particularly enjoyed the cruise from the Cite de la Musique on the St. Martin's Canal and then along the Seine. Enjoy your time there!
Laurel

Laurel - do you mean the Cite des Sciences? That's close to the arrival/departure point for the St. Martin canal boat tour - which is well worth doing.

Claire - no one here can predict the weather for you that closely. Expect cool weather and thunderstorms. Expect warm sunny days. Expect dog poop...

It won't be COLD, but bring light layers. Early morning you'll probably want a light sweater or cardigan and in the evening it might be a little chilly. I would just bring all long pants, because I doubt it would be hot enough to make wearing them uncomfortable.

We were in Paris last year 1 May to 7 May, and in the Loire for 4 days later. It warmed up as time went by, but days often began cool. Overall I would plan on cooler rather than warmer, but this is my meteorological opinion as I am the last person on earth to consult with about fashion.

I think cropped pants with sandals is too casual a look for Paris, unless it is very hot, which it isn't likely to be in early May. You will probably be happier if you dress a little nicer. You can still be comfortable, I've done it many times.

We day tripped Paris from London last year, late May. I just went and looked at the pictures and in the morning almost everyone has a light jacket or sweater on, by the afternoon, we were down to a short sleeve polo shirt as were others in the background of the pictures. It was a sunny day the entire day, but cool until around 1:30pm or so. Dress with layers. Have fun.

We were in Paris May 19 to 23 last year and the two pairs of crops and short sleeve shirts that I took along never left the suitcase. I wore long pants(the same pair every day), tank top,long-sleeved shirt, and carried a medium-weight sweater plus a pashmina when we left the hotel each morning. The tank top never saw the light and the sweater and pashmina were out of the backpack and on me every afternoon. It was just lovely! No sweat! :) Be casual but be decent. Black works everywhere. Enjoy your visit!

Just got back from Paris and Provence. My recommendation for dress in Paris is long pants and closed toe shoes. Black, black, black. Add grey for color. Har. No , seriously, if you want to be comfortable and not stand out wear slimmer cut black pants and euro-style black shoes. I am so glad I brought stylish black boots I could tuck skinny dark denim jeans and stretchy black pants into.
Don't bare too much skin. I would not wear tank tops. And cropped pants are ok but stay away from the safari look. Make sure they are black or brown!
Like others say, light layers! It can be cool outside but air conditioning is spotty inside some places. I almost melted a few times.
I think too much khaki and pastels spells 'American Tourist'. Not that there is anything wrong with that.