I've tried a whole bunch of keychain tape measures. I find the plastic ones too delicate for keychain carry, and the metal ones too large and heavy, not to mention ugly. Until I found the Ti-Tape Titanium Tape Measure by TEC Accessories. It's expensive, but worth it. Really, really small and light, but very high quality, with both imperial and metric measurements. They even sell a replacement tape for a few bucks and it's user replaceable. Really highly recommended.