Which airlines allow an 18 × 14 × 8 in personal item?

It fits the under-seat limit on 3 of the 21 airlines that publish one: American, Frontier and Allegiant. It’s too big for United (4 in too wide), JetBlue (1 in too wide), Sun Country (1 in too tall), Breeze (1 in too tall), Avelo (1 in too tall), Ryanair (2.3 in too tall), easyJet (0.3 in too tall), British Airways (2 in too tall), Wizz Air (2.3 in too tall), Jet2 (2.3 in too tall), AirAsia (4.1 in too deep), Malaysia Airlines (6.1 in over the total), Singapore Airlines (8.5 in over the total), Scoot (4.1 in too deep), Air Canada (1.7 in too deep), WestJet (2 in too deep), Qantas (0.6 in too wide) and Virgin Australia (1 in too wide). Delta, Southwest, Alaska, Hawaiian, Batik Air, Cathay Pacific and Jetstar give no size: it just has to fit under the seat in front of you.

It fits the carry-on limit on every airline here: American, Delta, United, Southwest, JetBlue, Alaska, Frontier, Allegiant, Hawaiian, Sun Country, Breeze, Avelo, Ryanair (right at the limit), easyJet, British Airways, Wizz Air, Jet2, AirAsia, Malaysia Airlines, Batik Air, Singapore Airlines, Scoot, Cathay Pacific, Air Canada, WestJet, Jetstar, Qantas and Virgin Australia.

Free in every fare on American, Delta, Southwest, JetBlue, Alaska, Frontier, Allegiant, Hawaiian, British Airways, Jet2, AirAsia, Malaysia Airlines, Batik Air, Singapore Airlines, Scoot, Cathay Pacific, Jetstar, Qantas and Virgin Australia. Free except in the cheapest fare on United (Basic Economy), Air Canada (Economy Basic) and WestJet (UltraBasic). On Sun Country, Breeze, Avelo, Ryanair, easyJet and Wizz Air it’s a paid cabin bag: only a smaller under-seat bag is free.