Hmm. This is the downfall of many a relationship. Money.
Now, if you are just living together (not married) then you have two options.
1. Equality
2. Proportionality
1. Regardless of salary you split all household bills 50/50. A sound argument seeing as you both live there. However this may breed resentment if one of you is one a far higher wage/salary that the other. This may lead to "its not fair" arguments, When one of you has very little fun money at the end of the month.
2. On the basis of your annual take home wage/salary you then split the bills into an fair ratio. This may be the more fair option as you will both be left with fun money, and be seen to be contributing your "fair share".
If you are married/single but jointly paying a mortgage, then other legal (on divorce/separating) considerations need to be considered with the above.
i.e if a partner is not directly contributing to mortgage payments, their other payments ( food, bills, maintenance etc.. ) will be taken into consideration on the division of assets (the house) when you split up. Yes, now cohabiting partners have rights to your property on separation. Don't need to be married.
Jonathan.