Don't know for sure about JetBlue, since we usually do SW, which as PP mentioned are pretty lax about your carry-ons and personal items, another reason we like them! But I do not believe they are going to be whipping out a tape measure. I think as long as it is a standard size that fits in the bin you will be OK. I know I did the same thing when DS and DH had to fly Spirit one leg of their trip this yr., and just took carry-ons for a soccer tournament to only pay for one piece of luggage. I was unsure the measurements would work as ours seemed bigger, even though they were standard carry-on bags, and even for personal items, which seemed like a very small number that might not include DS's backsack! They were all fine, so I am guessing you will be OK. Hopefully an experienced JetBlue flyer will comment for you.