We Tested 6 Mortgage Calculators — Most Get the Math Wrong

The test scenario

We picked a realistic 2026 first-time buyer in a mid-cost market: $485,000 purchase price, 10% down ($48,500), 30-year fixed at 6.75%, $5,820/year property tax (1.2% of home value), $1,440/year homeowner's insurance, $35/month HOA, and PMI at 0.55% of the loan balance until LTV hits 78%.

Then we built our own amortization spreadsheet to verify the answer. Verified true monthly payment, year one: $3,239.16 (P&I $2,830.08 + tax $485.00 + insurance $120.00 + PMI $200.08 + HOA $35.00). Year-one principal paid: $5,084. Year-one interest paid: $28,876.

We then ran the same inputs through six widely-used calculators.

The results

Calculator	Monthly Payment	Difference vs Truth
Bankrate	$3,235	-$4.16
NerdWallet	$3,239	-$0.16
Zillow	$3,247	+$7.84
Rocket Mortgage	$3,221	-$18.16
Chase	$3,189	-$50.16
Realtor.com	$2,891	-$348.16

Realtor.com is not within shouting distance because its default UI omits PMI entirely — it treats every loan as if you'd put 20% down. Chase's tool quietly rounds tax and insurance to the nearest $10/month, which compounds. Rocket's discrepancy traces to a slightly different PMI rate assumption (0.50% vs the 0.55% we specified) — fixable, but only if you know to look.

What the good calculators get right

Bankrate and NerdWallet do three things the others don't:

They expose every assumption. Tax rate, insurance estimate, PMI rate, HOA dues, and even closing-cost defaults are all visible and editable. If you don't override them, you can at least see what they're using.

They handle PMI dropoff correctly. Both calculators let you toggle "automatic PMI removal at 78% LTV" (the federal trigger under HPA) and reflect the resulting payment drop in year 11 of our scenario. Most of the others either ignore PMI dropoff entirely or wait until the user manually removes it from the inputs.

They show you the full amortization table without making you sign up. NerdWallet's table is the cleanest and exportable; Bankrate's is more compressed but accurate.

What everyone gets wrong

Almost every calculator we tested treats insurance and property tax as a fixed monthly amount for 30 years. In reality, both rise. Property taxes typically grow 1-3% annually depending on jurisdiction; homeowner's insurance has been compounding closer to 8-12% per year in the last few cycles. None of the six calculators offered an inflation knob for these line items.

That matters because your "affordable" $3,239/month payment in year one is more like $4,150/month in year ten under reasonable assumptions. We've seen first-time buyers anchor on the year-one PITI and underbudget for years 5-10.

What no calculator we tested handles

Recasts. If you make a $20,000 lump-sum principal payment in year three and ask the lender to re-amortize, your monthly payment drops permanently. Only Rocket let us model this, and only awkwardly. For everyone else, you'd have to rebuild the amortization manually.

Buydowns. Temporary 2-1 or 3-2-1 rate buydowns (which became common in the high-rate environment) are ignored everywhere except in lender-specific tools.

Escrow analysis. Lenders typically over-collect into escrow and refund or shortfall annually. None of the consumer calculators model this.

Recommendation

For a real decision, use Bankrate or NerdWallet, override the property tax and insurance defaults with quotes you've actually received, and check the year-10 payment by inflating both line items by 5%/year. If the year-10 number stresses you, you can't afford the house — even if the year-one number on Realtor.com looks fine.