Housing costs are the biggest expense most people face, so knowing how much you can afford to spend can be key to staying on top of your budget. However, that amount is not always clear.

As a rule of thumb, financial planners commonly recommend spending no more than 30% of your gross income on housing, whether that's mortgage or rent costs. This advice is often based on guidelines set in the 1980s by the U.S. Department of Housing and Urban Development, which defined spending beyond 30% as "housing cost burdened."

That means if you earn the U.S. median income of around $80,000, you should aim to keep your housing costs to $2,000 a month.

But with housing costs soaring over the past few years, a significant portion of Americans now spend more than 30% of their income on housing, including nearly half of all renters, according to a recently published U.S. Census report. Among homeowners, 21.1% of those with a mortgage and 11.5% without a mortgage exceed this threshold.

Considering that Americans are routinely spending more than 30% of their income on housing, the guideline can feel more like an ideal than a realistic rule of thumb.

How much you can afford to spend on housing, based on your income

Here's a look at how much you can afford to spend on housing at different income levels if you're aiming to stay within a 30%, 40% or 50% threshold.

The maximum amount you can put toward monthly housing costs without spending more than 30% of your gross income:

• $30,000 income: $750
• $40,000 income: $1,000
• $50,000 income: $1,250
• $60,000 income: $1,500
• $70,000 income: $1,750
• $80,000 income: $2,000
• $90,000 income: $2,250
• $100,000 income: $2,500
• $110,000 income: $2,750
• $120,000 income: $3,000

The maximum amount you can put toward monthly housing costs without spending more than 40% of your gross income:

• $30,000 income: $1,000
• $40,000 income: $1,333
• $50,000 income: $1,667
• $60,000 income: $2,000
• $70,000 income: $2,333
• $80,000 income: $2,667
• $90,000 income: $3,000
• $100,000 income: $3,333
• $110,000 income: $3,667
• $120,000 income: $4,000

The maximum amount you can put toward monthly housing costs without spending more than 50% of your gross income:

• $30,000 income: $1,250
• $40,000 income: $1,667
• $50,000 income: $2,083
• $60,000 income: $2,500
• $70,000 income: $2,917
• $80,000 income: $3,333
• $90,000 income: $3,750
• $100,000 income: $4,167
• $110,000 income: $4,583
• $120,000 income: $5,000

How much should you be spending on housing?

While the 30% rule is still a useful guideline, it's not always practical — especially in urban areas where housing costs are often higher, says Melissa Caro, certified financial planner and founder of My Retirement Network.

For many, "flexibility is necessary," but spending more on housing should still be done carefully, she says. That's because other parts of a household budget, such as discretionary spending on things like entertainment, have some built-in flexibility, while housing costs do not.

"Rent or mortgage payments won't adjust if you face a job loss, so carefully weigh what's essential versus what's ideal," Caro says.

That said, spending 50% or more of your income on housing is a "red line" to avoid since it limits your financial flexibility, Caro says.

Other experts recommend sticking closer to 30%, if you can.

"The 30% rule is still a good starting point for housing costs," says Emmanuel Eliason, a CFP in Colorado. However, in places with high housing costs "something in the range of 35% to 39% could be ideal for most families if they take proactive steps to revert back to the standard 30% housing budget allocation over time."

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