Where do recommendations come from regarding the percent of their incomes should Americans spend on their mortgage?

I interviewed nine real-estate experts to help me understand why the numbers vary so much and, I hoped, help me figure out the right one to use for myself. They confirmed that, yes, the mortgage-affordability numbers are all different, and though some lenders use them to approve mortgages, they are basically guesstimates. “To some extent, they’re plucked out of the air,” Robert Van Order, an economics professor at George Washington University, told me. “A lot of these numbers are pretty arbitrary,” added Edward Seiler, the associate vice president of housing economics at the Mortgage Bankers Association. “It’s just based on people staring at data and thinking, What are the tipping points that force people into delinquency?” If the percentages don’t seem ironclad, it’s because they aren’t.

If these numbers are at the upper end of what people should spend, what do people actually spend?

Despite hearing the 30 percent figure from many of the experts I talked with, I was surprised to learn that most current homeowners actually spend much less on their housing. So do most renters. The median homeowner with a mortgage spends 16 percent of their gross income on their house payment, including taxes and insurance. That number is higher—24 percent—for low-income households, but it’s still less than 30 percent. Renters spend an average of 26 percent of their income on housing. In other words, if you take the mortgage calculators at their word and spend 28 percent, you’re paying much more for a house than the average American does.

Medians can disguise a lot of variability. In certain housing markets or in certain economic conditions or certain personal circumstances, the top end percent might be very helpful. In other situations, it may not matter as much.

Even with the variation in recommendations, it appears they roughly fall into a range of 25-35% of income. Would it be better then to suggest to people that they should aim to spend at most a quarter to one-third of their income on housing? This does not have the convenience of a single number but the range could fit a broader set of conditions and circumstances.