No one said that index funds always go up but they do return more than 4% (nominal) as a long term average. I bet that the calculator uses 4% as the default bec

No one said that index funds always go up but they do return more than 4% (nominal) as a long term average. I bet that the calculator uses 4% as the default because they assume a relatively conservative portfolio such as 50% bonds & 50% stocks. That portfolio may be appropriate for many people, but it is substantially less risky than tying up your entire net worth (plus more?) in a single house.