Interest-Only Mortgage Calculator

You pay only the interest on the loan for the first 5 to 10 years (the IO period), so the monthly payment is just loan amount times monthly rate. The principal balance does not move during that time. When the IO period ends, the loan starts amortizing the original balance over whatever years are left in the term, which produces a sharply higher monthly payment. On a $500,000 loan at 7%, the IO payment is about $2,917 per month for the first 10 years, then jumps to roughly $3,877 per month for the remaining 20 years to pay the loan off by year 30.

Yes, they were a major contributor. Most pre-crisis IO loans were also adjustable-rate (IO ARMs) with low 2 or 3 year teaser rates, so when the IO period ended the borrower hit a payment shock from two sides at once: principal started amortizing and the rate reset higher. Many borrowers had only qualified at the teaser payment, home values had stalled or fallen so they could not refinance, and a 2010 Federal Reserve study found that IO and option-ARM loans had foreclosure rates several times higher than standard fixed-rate mortgages. Post-2008 Dodd-Frank and the CFPB's Qualified Mortgage rule effectively pushed most IO products off the conforming market.

A narrow group. They can work for borrowers with lumpy but reliable income (commission salespeople, bonus-heavy finance and tech workers, business owners with seasonal cash flow) who want a low fixed minimum and plan to throw extra principal at the loan when bonuses land. They can also fit short-term owners (someone who knows they will sell or refinance within the IO period, like a corporate relocation or a fix-and-flip) and high-net-worth investors using the IO structure as a deliberate leverage and tax-planning tool. They are a poor fit for buyers who are stretching to afford the IO payment itself, since the post-IO jump is the cliff that ended a lot of homeownership stories in 2008.

Because you carry the full principal balance for the entire IO period instead of grinding it down. On a $500,000 loan at 7% over 30 years, a traditional amortizing loan costs about $698,000 in total ($198,000 in interest). The same loan with a 10-year IO period costs about $1,280,000 in total ($780,000 in interest), roughly $582,000 more, because every dollar of principal accrues interest for 10 extra years. The slightly cheaper monthly payment in years 1 to 10 is more than offset by the bigger payment in years 11 to 30 plus the extra decade of unpaid principal.

Larger than most borrowers expect. The new payment has to amortize the entire original loan balance over a shorter remaining term, not the full 30 years. On the $500,000 / 7% / 10-year-IO / 30-year-total example, the payment jumps from $2,917 to $3,877, a roughly 33% increase, overnight. Shorter IO terms relative to total term produce smaller jumps; longer IO terms (especially the 10-year-IO / 15-year-total products that briefly existed pre-2008) can more than double the payment. Always model what you will actually owe after the IO period before signing, not just what you will pay during it.