{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"interest-only-loan"},"result":{"entity_type":"calculator","id":"interest-only-loan","calculator_id":"interest-only-loan","canonical_url":"https://tttkmbb.com/finance/interest-only-loan","name":"Interest-Only Loan Calculator","title":"Interest-Only Loan Calculator – Interest-Only Payment, Amortizing Payment After the IO Period and Total Interest","category":"finance","category_name":"Finance","tool_name":"calculate_interest_only_loan","featured_mcp_tool":false,"description":"Computes the payments of an interest-only loan: the interest-only monthly payment during the initial period, the higher amortizing payment that repays the full principal over the remaining term, the interest paid in each phase and the extra cost versus a fully amortizing loan.","use_when":"You need the monthly payment during and after the interest-only period of a mortgage or HELOC-style loan, the payment jump at the end of the IO period, or the total interest of the structure.","do_not_use_when":"The loan amortizes from the first payment (use loan-payment), or the principal is due as a lump sum instead of being amortized after the IO period (use balloon-loan). Informational; not financial advice.","inputs":[{"name":"loan_amount","label":"Loan amount","type":"number","required":true,"max":1000000000000,"exclusive_min":0,"description":"Principal borrowed; unchanged during the interest-only period.","example":400000},{"name":"interest_rate_percent","label":"Annual interest rate","type":"number","unit":"%","required":true,"max":100,"exclusive_min":0,"description":"Fixed nominal annual rate in percent, assumed the same in both phases; the monthly rate is this / 12.","example":5},{"name":"interest_only_years","label":"Interest-only period","type":"number","unit":"years","required":true,"max":40,"exclusive_min":0,"description":"Years during which only interest is paid; must be shorter than total_term_years.","example":5},{"name":"total_term_years","label":"Total term","type":"number","unit":"years","required":false,"default":30,"max":100,"exclusive_min":0,"description":"Total loan term including the interest-only period; the principal is amortized over the remaining years.","example":30}],"outputs":[{"name":"interest_only_monthly_payment","label":"Interest-only monthly payment","type":"number","decimals":2,"description":"loan_amount × interest_rate_percent / 1200."},{"name":"amortizing_monthly_payment","label":"Amortizing monthly payment","type":"number","decimals":2,"description":"Level payment that repays loan_amount over the remaining term after the IO period."},{"name":"payment_increase_percent","label":"Payment increase","type":"number","unit":"%","decimals":2,"description":"(amortizing − interest-only payment) / interest-only payment × 100."},{"name":"interest_during_io_period","label":"Interest during IO period","type":"number","decimals":2,"description":"interest_only_monthly_payment × 12 × interest_only_years."},{"name":"interest_during_amortization","label":"Interest during amortization","type":"number","decimals":2,"description":"amortizing_monthly_payment × remaining months − loan_amount."},{"name":"total_interest","label":"Total interest","type":"number","decimals":2,"description":"Interest of both phases."},{"name":"total_cost","label":"Total cost","type":"number","decimals":2,"description":"loan_amount + total_interest."},{"name":"fully_amortizing_monthly_payment","label":"Fully amortizing payment (for comparison)","type":"number","decimals":2,"description":"Level payment of a standard loan amortized over total_term_years from the start."},{"name":"extra_interest_vs_fully_amortizing","label":"Extra interest vs fully amortizing loan","type":"number","decimals":2,"description":"total_interest − interest of the fully amortizing loan over the same total term."}],"input_schema":{"type":"object","properties":{"loan_amount":{"description":"Principal borrowed; unchanged during the interest-only period.","type":"number","maximum":1000000000000,"exclusiveMinimum":0,"examples":[400000]},"interest_rate_percent":{"description":"Fixed nominal annual rate in percent, assumed the same in both phases; the monthly rate is this / 12. Unit: %.","type":"number","maximum":100,"exclusiveMinimum":0,"examples":[5],"x-unit":"%"},"interest_only_years":{"description":"Years during which only interest is paid; must be shorter than total_term_years. Unit: years.","type":"number","maximum":40,"exclusiveMinimum":0,"examples":[5],"x-unit":"years"},"total_term_years":{"description":"Total loan term including the interest-only period; the principal is amortized over the remaining years. Unit: years.","type":"number","maximum":100,"exclusiveMinimum":0,"default":30,"examples":[30],"x-unit":"years"}},"additionalProperties":false,"required":["loan_amount","interest_rate_percent","interest_only_years"]},"output_schema":{"type":"object","properties":{"interest_only_monthly_payment":{"description":"loan_amount × interest_rate_percent / 1200.","type":"number"},"amortizing_monthly_payment":{"description":"Level payment that repays loan_amount over the remaining term after the IO period.","type":"number"},"payment_increase_percent":{"description":"(amortizing − interest-only payment) / interest-only payment × 100. Unit: %.","type":"number","x-unit":"%"},"interest_during_io_period":{"description":"interest_only_monthly_payment × 12 × interest_only_years.","type":"number"},"interest_during_amortization":{"description":"amortizing_monthly_payment × remaining months − loan_amount.","type":"number"},"total_interest":{"description":"Interest of both phases.","type":"number"},"total_cost":{"description":"loan_amount + total_interest.","type":"number"},"fully_amortizing_monthly_payment":{"description":"Level payment of a standard loan amortized over total_term_years from the start.","type":"number"},"extra_interest_vs_fully_amortizing":{"description":"total_interest − interest of the fully amortizing loan over the same total term.","type":"number"}}},"formula":"i = interest_rate_percent / 1200; m = 12 × interest_only_years; n = 12 × (total_term_years − interest_only_years); io_payment = P × i; amortizing_payment = P × i / (1 − (1 + i)^−n); interest_io = io_payment × m; interest_amort = amortizing_payment × n − P; total_interest = interest_io + interest_amort; total_cost = P + total_interest; increase = (amortizing_payment − io_payment) / io_payment × 100","method":"Fixed rate in both phases, end-of-month payments, no principal prepayments during the IO period. Real interest-only products often reset to an adjustable rate after the IO period, so the amortizing payment can differ from this estimate.","sources":[{"name":"CFPB – What is an \"interest-only\" loan?","url":"https://www.consumerfinance.gov/ask-cfpb/what-is-an-interest-only-loan-en-101/","type":"government","retrieved_at":"2026-09-24"},{"name":"Wikipedia – Interest-only loan","url":"https://en.wikipedia.org/wiki/Interest-only_loan","type":"reference","retrieved_at":"2026-09-24"},{"name":"Wikipedia – Amortization calculator (annuity payment formula)","url":"https://en.wikipedia.org/wiki/Amortization_calculator","type":"reference","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"400,000 at 5 %, 5 years interest-only, 30-year term","inputs":{"loan_amount":400000,"interest_rate_percent":5,"interest_only_years":5,"total_term_years":30},"expected":{"interest_only_monthly_payment":1666.67,"amortizing_monthly_payment":2338.36,"payment_increase_percent":40.3,"interest_during_io_period":100000,"interest_during_amortization":301508.05,"total_interest":401508.05,"total_cost":801508.05,"fully_amortizing_monthly_payment":2147.29,"extra_interest_vs_fully_amortizing":28484.91},"url":"https://tttkmbb.com/api/v1/calculate/interest-only-loan?loan_amount=400000&interest_rate_percent=5&interest_only_years=5&total_term_years=30"},{"name":"250,000 at 6.5 %, 10 years interest-only, 30-year term","inputs":{"loan_amount":250000,"interest_rate_percent":6.5,"interest_only_years":10,"total_term_years":30},"expected":{"interest_only_monthly_payment":1354.17,"amortizing_monthly_payment":1863.93,"payment_increase_percent":37.64,"interest_during_io_period":162500,"interest_during_amortization":197343.88,"total_interest":359843.88,"total_cost":609843.88},"url":"https://tttkmbb.com/api/v1/calculate/interest-only-loan?loan_amount=250000&interest_rate_percent=6.5&interest_only_years=10&total_term_years=30"}],"faq":[{"q":"Why does the payment jump so much after the IO period?","a":"The full principal must now be repaid over the shorter remaining term: 400,000 over 25 instead of 30 years at 5 % costs 2,338 per month instead of 1,667, a 40 % increase."},{"q":"Is an interest-only loan more expensive overall?","a":"At the same rate, yes: no principal is repaid during the IO period, so interest accrues on the full balance for longer. Here the structure costs 28,485 more than a 30-year amortizing loan."},{"q":"Does the calculator handle a rate change after the IO period?","a":"No. Enter the expected post-IO rate separately in loan-payment with the remaining term to see the amortizing payment under a different rate."}],"tags":["interest only loan","interest only mortgage","interest only payment","payment after interest only period","io period","heloc payment"],"related":[{"calculator_id":"loan-payment","reason":"Amortizing payment for any balance, rate and term."},{"calculator_id":"balloon-loan","reason":"Lump-sum repayment instead of amortization after the initial period."},{"calculator_id":"amortization-schedule","reason":"Principal and interest split of the amortizing phase."}],"links":{"html":"https://tttkmbb.com/finance/interest-only-loan","markdown":"https://tttkmbb.com/finance/interest-only-loan.md","json":"https://tttkmbb.com/finance/interest-only-loan.json","api":"https://tttkmbb.com/api/v1/calculate/interest-only-loan","schema":"https://tttkmbb.com/api/v1/calculators/interest-only-loan","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T03:46:08Z"}