I don't know the exact width. I drive a Chrysler 300, and I found I could not put this side-by-side with a Braco Nautilus (harnessed booster) seat. So it wouldn't work as part of a trio with a bigger seat. But if you're talking triplets, so three of same seat, maybe - I think the toddler seat was the bigger of the two and what stopped mine from working next to one another. This seems pretty narrow compared to other infant carrier seats I've seen.