I thought this was obvious...

If you are paying a mortgage at a certain rate r% and the rate changes to r + δr, then how much will your payment increase?

Well, it's complicated but to first approximation:

δp/p ≈ δr/r

Which means if the rates go from 5% to 6%, your payment doesn't go up by 1%; it goes up by 20%. (120% of 5 = 6.)

I thought this was obvious. I have been informed that this is not obvious, and the vast majority of people think it goes up 1%.

Crikey! These fucked borrowers are truly fucked!