To follow up, here are the sizes of doubles and long doubles on 10 of Debian's architectures. I now regret not checking float for completeness. arch double long double ==== ====== =========== alpha 8 8 arm 8 8 hppa 8 8 i386 8 12 ia64 8 16 m68k 8 12 mips 8 8 powerpc 8 8 s390 8 8 sparc 8 8