Mortgage interest deduction: how the TCJA cap actually affects you

The mortgage interest deduction is the most over-cited tax benefit in real estate. Since the 2017 Tax Cuts and Jobs Act doubled the standard deduction, the majority of homeowners get zero net benefit from it — they take the standard deduction either way. Before you assume the deduction shaves a point off your effective mortgage rate, run the actual numbers. They’re often less impressive than your loan officer suggested.

This is a guide, not tax advice. Tax law is fact-specific; consult a CPA before making decisions in the gray zones.

The basics

Mortgage interest is deductible on acquisition indebtedness — debt used to buy, build, or substantially improve a qualified home secured by that home. There are two caps depending on when the loan was originated:

• $750,000 of loan balance for loans originated after Dec 15, 2017 ($375k if married filing separately).
• $1,000,000 for loans originated on or before Dec 15, 2017 — grandfathered under prior law ($500k MFS).

The cap is on the loan balance, not the interest. If your balance is under the cap, all interest is potentially deductible. If your balance is above, only the pro-rated portion of interest is deductible.

You can apply the cap across two qualified homes — primary residence plus one second home. Combined balance under the cap. A third home doesn’t qualify (though you can swap which two you elect year to year).

Refinances and the grandfathered cap

If you refinance a pre-Dec 2017 loan, you keep the $1M cap as long as:

• The refi balance is at or below the original loan balance at the time of refinance, AND
• You don’t take cash out for non-acquisition use.

Take cash out and use it for a kitchen remodel? That portion is also acquisition debt and stays under the $1M cap. Take cash out to pay off credit cards or fund a wedding? That portion gets reclassified as non-deductible personal interest, even if your total balance is under $1M.

Home equity loans and HELOCs (the post-TCJA change)

Pre-TCJA, you could deduct interest on up to $100k of home equity debt regardless of use. That’s gone. Now: HELOC and home equity loan interest is deductible only if proceeds are used to buy, build, or substantially improve the home securing the loan, and the combined loan balance stays under the $750k/$1M cap.

Use a HELOC to add a bathroom: deductible. Use it to buy a car or consolidate credit card debt: not deductible. Track the use of every draw if you ever co-mingle.

Why most homeowners don’t actually benefit

This is the part loan officers skip. To get any benefit from the mortgage interest deduction you must itemize, which means your total Schedule A deductions must exceed the standard deduction.

The standard deduction (2026 estimates, post-TCJA-extension):

• Single: ~$15,500
• Married filing jointly: ~$31,000

The big buckets that get you over standard:

• Mortgage interest
• State and local taxes (SALT) — capped at $10,000 combined ($5k MFS) for property tax + state income or sales tax
• Charitable contributions
• Medical expenses above 7.5% of AGI

For high-tax-state filers (CA, NY, NJ, IL, CT), the SALT cap is the killer. Property tax of $15k plus $40k of state income tax totals $55k, but the combined deduction is capped at $10k. The remaining $45k of state and local tax is permanently lost.

Worked example: deduction does help

MFJ couple in a moderate-tax state. $500,000 mortgage at 6.5% (year-1 interest ~$32,500). $10k SALT (capped). $5k charitable.

• Itemized total: $32,500 + $10,000 + $5,000 = $47,500
• Standard deduction (MFJ): $31,000
• Net benefit from itemizing: $47,500 − $31,000 = $16,500

At a 24% federal marginal rate, that’s ~$3,960/yr in tax savings, or about $330/mo. Their after-tax mortgage rate drops from 6.50% to roughly 5.70%.

Worked example: deduction does nothing

Same couple, but $250,000 mortgage at 6.5% (year-1 interest ~$16,250). Same $10k SALT, same $5k charitable.

• Itemized total: $16,250 + $10,000 + $5,000 = $31,250
• Standard deduction (MFJ): $31,000
• Net benefit from itemizing: $250

At 24%, $60/yr in tax savings. The mortgage interest deduction is effectively worthless to them — they’d itemize basically for the price of a pizza.

This is the reality for tens of millions of homeowners. The deduction is real on paper but produces no actual cash.

Run your specific numbers in the Mortgage Interest Deduction calculator.

Your true after-tax mortgage rate

The honest formula isn’t “nominal rate × (1 − marginal rate).” That math assumes every dollar of interest is deductible. In reality, the standard deduction acts as a floor that swallows most of the benefit for smaller mortgages.

Better formula:

After-tax rate = nominal rate × (1 − effective marginal benefit)

Where:

Effective marginal benefit = (tax saved from interest deduction) ÷ (interest paid)

For the $500k mortgage example: $3,960 saved ÷ $32,500 interest = 12.2% effective. After-tax rate = 6.50% × (1 − 0.122) = 5.71%.

For the $250k example: $60 ÷ $16,250 = 0.4%. After-tax rate = 6.50% × 0.996 = 6.47%. Practically identical to the nominal rate.

The smaller your mortgage relative to your other deductions, the less the deduction is actually worth.

Points: a separate deduction

Discount points — prepaid interest to buy down your rate — have their own tax treatment, separate from regular mortgage interest:

• Points on a purchase: fully deductible in the year paid if the loan is secured by your primary residence, the points are customary for the area, and a few other conditions in IRS Pub 936 are met. Most purchases qualify.
• Points on a refinance: must be amortized over the life of the loan. On a 30-year refi, you deduct 1/30th of the points each year. If you refi or pay off early, the unamortized balance is deductible in that final year.
• Origination fees that are real fees (processing, underwriting, application, doc prep): NOT deductible. Only true points (a charge for a rate reduction, expressed as a percentage of the loan) qualify.

If you’re weighing whether to buy points, use the points calculator and factor the year-1 deduction into the breakeven.

Property tax (lumped into SALT)

Property tax on your primary and second home is deductible as part of the $10k SALT cap, combined with state and local income or sales tax. There is no separate property tax deduction for personal homes.

Rental property tax is fully deductible against rental income on Schedule E and is NOT subject to the SALT cap. See the property tax guide and the rental tax basics guide.

PMI is not deductible (anymore)

The mortgage insurance premium deduction (PMI on conventional, MIP on FHA, the VA funding fee, USDA guarantee fee) expired after the 2021 tax year and has not been reinstated as of early 2026. If a guide or calculator written before 2022 tells you PMI is deductible, it’s out of date.

If Congress reinstates it — it’s been done several times historically — we’ll update. For now: assume zero.

The “married filing separately” trap

Married couples filing separately split each cap in half:

• Post-TCJA: $375k each (vs. $750k MFJ)
• Pre-TCJA grandfathered: $500k each (vs. $1M MFJ)

If both spouses are on the loan and they file separately, they can allocate the deductible interest between themselves — but the combined cap is the MFS cap, not 2× the cap. Filing separately when both are on a large mortgage usually costs more than it saves.

The marriage penalty

Two single homeowners with their own primaries each get a $750k cap — $1.5M of acquisition debt across the household. The same two people, once married and on one title, get one $750k cap.

In high-cost-of-living areas this is real money. Two single buyers each financing $750k get full deduction of all interest. Married, they deduct interest on the first $750k only; the rest is non-deductible.

Not a tax planning recommendation, just a fact.

Refinance specifics worth nailing

Cash-out refi

The new loan balance has to be split conceptually between:

• The portion that paid off the old acquisition debt (still acquisition debt; deductible)
• The portion that funded acquisition or substantial improvement of the home (acquisition debt; deductible)
• The portion that funded anything else (non-acquisition; not deductible at all under current law since home equity debt for non-acquisition use was eliminated)

Track use of cash carefully. If you cashed out $50k and used $30k to add a primary suite and $20k to pay off student loans, only 60% of the “cash out” portion of interest is deductible.

Rate-and-term refi within original cap

Just refinancing for a better rate, no cash out, balance at or below the existing balance? The character of the debt is unchanged. Deductibility carries over identically.

Refi of a pre-Dec 2017 loan

Keep the $1M cap as long as you don’t increase the principal beyond what was outstanding at refi time. Any new borrowing on top gets the new $750k treatment, layered. Your CPA can do the math; the tracking gets messy when refis stack.

For more on whether to refi at all, see the refinance guide.

OBBBA (July 2025): TCJA was extended

The One Big Beautiful Bill Act averted the scheduled Dec 31, 2025 TCJA sunset. The provisions that govern this guide were largely made permanent or extended:

• Standard deduction: doubled-from-pre-TCJA structure made permanent (~$15.5k single / $31k MFJ for 2026, indexed)
• $750k mortgage interest cap: continues for post-12/15/2017 loans
• $10k SALT cap: continues (with minor OBBBA adjustments — verify current state for your year)
• Personal exemptions: stay eliminated
• Home equity debt limited to acquisition use: continues

Net effect: the post-2017 status quo on the mortgage interest deduction was not reversed. Most homeowners still won’t benefit from itemizing. If you previously planned around a TCJA sunset (e.g., expected to itemize again in 2026), update those plans.

The post-OBBBA mortgage interest deduction picture: the current rules largely persist.

Verify current law before relying on either set of rules. This guide reflects current statute as of April 2026.

Common mistakes

• Assuming you benefit. Run the math. For most homeowners with <$400k of mortgage debt and modest other deductions, the standard deduction wins.
• Forgetting the SALT cap kills high-state filers. A $40k state income tax bill plus $15k property tax doesn’t mean $55k of deduction — it means $10k.
• Not separating cash-out refi proceeds. The IRS requires you to trace use. Co-mingling means you may have to defend an allocation in audit.
• Assuming PMI is still deductible. It expired after 2021.
• Ignoring the standard deduction floor in your “after-tax rate” math. Effective benefit is almost always less than (rate × marginal rate).
• Deducting points on a refi all in year one. Refi points must amortize over the loan life.
• Filing separately while both on a big mortgage. The MFS caps are half. Usually a worse outcome.
• Not consulting a CPA on a complicated refi or HELOC use case. Tracing rules and acquisition-debt classification get fact-specific fast.

Tools you’ll use

• Mortgage Interest Deduction calculator — model your specific itemized vs. standard
• Points calculator — including the year-1 deduction on a purchase
• Mortgage Calculator — full PITI on the loan you’re evaluating
• Refinance calculator — pair with the deduction math when you cash out

Related reading: property tax basics, refinancing your mortgage, rental tax basics, and closing costs.