Childcare would be the biggest difference here. I assume the other parent is taking care of the kids which reduces wages needed to cover that expense.

If you scroll down they have a breakdown, but that big increase in expenses is that a single working adult with a child would need to pay for childcare. If there is 2 adults and only 1 working it is assumed the non working adult would handle childcare.

Thank you. I missed this!

In addition to what other people said, taxes are much lower for a one income married couple vs a single person.

Childcare costs. It’s cheaper to feed and house two people than to pay for childcare in the US.

No, because being a single parent is far more expensive due to the need for daycare/babysitting/etc...

The model assumes the non-working adult is able to provide child care.

I think it means per person working and then one providing childcare.

because the calculator is sponsored by big co’s

Because childcare is way more expensive than you'd think, and children are far cheaper to take care of than you'd think.