Interest-Only Mortgage Balloon Calculator
Interest-only mortgages skip principal during a 5-10 year window — low payments now, balloon-sized recast later. This calculator shows your IO payment, the post-IO recast, and lifetime cost vs a standard amortizing loan.
| Standard amortizing payment (full term) | — |
| IO period total paid | — |
| Principal balance entering amort phase | — |
| Total paid over loan life | — |
| Total paid — standard amortizing | — |
| Extra cost of going IO | — |
The interest-only mortgage balloon calculator is a free, browser-based tool that shows your payment during the interest-only period, the balloon or recast payment when that period ends, and the total interest cost against a standard amortizing loan. Enter the loan amount, rate, IO period and full term — nothing is uploaded.
Interest-only mortgages defer principal repayment for typically 5 to 10 years, cutting the monthly payment by 20% to 40%. What catches borrowers out is what happens next. Last updated: 10 August 2026.
How Interest-Only Mortgages Work
During the interest-only window you pay only the interest accrued each month, calculated as the loan balance multiplied by the monthly rate. The principal does not move. A $600,000 loan at 6.5% costs $3,250 a month interest-only, against $3,792 fully amortizing over 30 years — a saving of $542 a month, or roughly $32,500 across a five-year IO period. The catch is that after five years you still owe the full $600,000, while the amortizing borrower has paid it down to about $560,000 and built $40,000 of equity from payments alone.
Recast vs Balloon: Two Very Different Endings
This is the distinction that decides whether an IO loan is manageable or dangerous, and most calculators blur it. A recast means the loan re-amortizes the untouched principal over the shorter remaining term. Take the same $600,000 at 6.5% on a 30-year note with a 5-year IO period: at recast, the full $600,000 amortizes over the remaining 25 years, and the payment jumps from $3,250 to about $4,051 — a 25% increase, permanent, on a date you already know. A balloon is more severe: the entire principal falls due as a single lump sum at the end of the IO period. On that same loan you would owe $600,000 in cash on one date, with only three ways out — refinance, sell, or pay it. Check your note for the words “balloon payment” and the maturity date, because the monthly payment looks identical under both structures right up until the day it does not.
The Real Balloon Risk Is Correlated, Not Random
The danger with a balloon is not that you might be unable to refinance — it is that the conditions that block a refinance tend to arrive together. Rates rise, so the new payment is higher than you modelled. Values soften, so your loan-to-value fails the lender’s test. Lending standards tighten in the same cycle, so the non-QM product you used the first time is withdrawn. And because you paid no principal, you have no equity buffer to absorb any of it. That is precisely what happened to the 2005–2007 IO cohort. The practical defence is to start arranging your exit 18 to 24 months before the IO period ends rather than 3 months before, and to make voluntary principal payments during the IO window — most IO notes permit them without penalty, and every dollar paid reduces both the balloon and the recast payment proportionally.
Who Interest-Only Mortgages Actually Suit
IO makes sense when the payment gap is being deployed, not consumed. That fits high earners with lumpy income who repay in annual bonus lump sums, investors with a defined sale or refinance date inside the IO window, jumbo borrowers with a documented plan for the deferred principal, and borrowers whose alternative use of the cash reliably out-earns the mortgage rate. It suits almost nobody who is using IO to afford a house they otherwise could not, because that borrower is structurally unable to pay the recast. A simple test before signing: can you afford the post-recast payment today, at today’s income? If not, you are relying on a future refinance that the market is under no obligation to provide.
Regulatory and Tax Position in 2026
Interest-only loans do not meet the Consumer Financial Protection Bureau’s definition of a Qualified Mortgage, so they are non-QM and sit outside QM safe-harbor protections. Lenders still owe you the ability-to-repay obligation and must underwrite you at the fully amortizing payment, not the interest-only one. In practice most IO programmes require a 720+ FICO score and 20% to 25% down. On tax, two things changed in your favour for 2026. The One Big Beautiful Bill Act (Pub. L. 119-21, July 2025) made the $750,000 acquisition-debt cap on deductible mortgage interest permanent, removing the scheduled reversion that would have restored the older $1,000,000 limit — relevant to IO borrowers because your deductible interest stays at its maximum for the whole IO period while principal never falls. Separately, mortgage insurance premiums became permanently deductible as qualified residence interest from 1 January 2026, subject to an income phase-out beginning at $100,000 AGI and ending at $109,000. Interest on an investment-property IO loan remains fully deductible as a business expense and is not subject to the $750,000 cap.
Last updated 10 August 2026. Sources: IRS Publication 936, Home Mortgage Interest Deduction; CFPB Regulation Z ability-to-repay and Qualified Mortgage rule, 12 CFR §1026.43; One Big Beautiful Bill Act (Pub. L. 119-21) §70108. Figures are illustrative — confirm terms against your own note.