How to share costs

[Stepdadfindinghisway]

New step dad recently married and moved in. How do we fairly share costs? My lovely wife has a 19 year old son and a 12 year old daughter. Am happy to take on half the costs for the 12 year old daughter as although I’m not her dad I’ve taken on the family and she is young enough for me to have some influence. Her son is 19 and has no job, eats for Britain, leaves every electrical appliance on and leaves mess everywhere he goes. I can’t criticise as “I’m not his dad”. My wife is happy to pay for him but it gets very difficult about how to split bills. How do other people manage it?

Congratulations, although this sounds like a conversation that should have been done and dusted before you moved in together?

I don't think you can calculate bill splits in the way you're implying - it's both too complex to do realistically and too transactional.
At those ages and you're happy to pay half for the 12yo, it's either:

Treat as 4 people, you pay 1.5, wife pays 2.5, so percentage wise: You 37.5, wife 62.5%,

Or

Treat as 3 adults, you pay one, wife pays two, so 33.4% & 66.6%.

That assumes you're earning roughly equal though. Otherwise there's a whole different conversation about what's fair in terms of relative splits from income, and how much each person has left for themselves...

And separate to the above, you need to have a conversation with your wife about how she's going to give the 19yo more responsibilities, insist that he contributes to the household fully, even if not financially, and encourage him to either go into studying or employment, with a backstop that neither is not an acceptable answer.

Thankyou. The question still remains though whether I have moved in or not. What is a fair way to share step children costs?

Have you moved into their house? Them into yours? All into a new house together? I think this really matters.

If she wants to pay for her lazy adult child that’s her choice, but she can’t expect you to.If you’ve moved into their place you’re on shakier ground for complaining, but you still shouldn’t be paying more than your share, imo.

I think probably the majority of married couples pool most of their money rather than splitting bills. For example, my H & I both have monthly direct debits from our personal accounts into a joint account and all the household expenses get paid out of that joint account. What’s left is our personal spending money and we make sure that’s fairly equal.

The step son you resent is the spoke in the wheel here but that’s about way more than money.

I don’t think I would be paying for an adult that consistently contributes nothing? Do they do housework? Cook? If not I think I would be laying down some ground rules.
out of interest ‘whose house are you living in?’ Is it owned- if so by who? Does their own dad contribute? Who earns what?