Ask HN: Alternative to pen and paper for calculations by hand?

When you need to do a calculation by hand, do you have any good alternative to pen and paper?

I've tried Khan Academy a few times, but what kills me is their scratchpad: it's nice and all, but I suck very much at using it, and I never carry pen and paper with me (other than for calculations, I only use pen and paper for long grocery lists; for taking text notes, a keyboard is way better than paper).

I've tried TeX and a few equation editors, but nothing I've seen so far is nearly as fast as pen and paper.