In reply to gavinpeach: My 3/4 self inflating mat fits okay in there with enough room to loft the bag but I have concerns about ripping the bivi bag too. I've taken a cut down closed cell foam mat before (I realise this is carrying two mats...) and I've also slept on some cut up pond liner which did a pretty good job as I could fold up an end an keep my boots dry too. Pond liner/thin tarp was my favourite as it folded down to nothing, weighed nothing and I acquired it for nothing from my father in law...