Business loan calculator

Principal that is not strictly positive fails with invalid_input instead of inventing a schedule.

What a business loan monthly payment means and how the amortizing formula works

A business-loan monthly payment is the constant installment a borrower sends so that a principal P at annual nominal rate i percent is fully amortised in n months on a reducing-balance schedule. Money fields round to two decimal places so they match currency. Origination fees, packing, and balloon residuals are not invented: add financed fees to principal if they go into the note, otherwise leave them out of the principal. The monthly rate r is the annual percent divided by twelve hundred: r equals annual_rate divided by twelve divided by one hundred. When r is greater than zero the closed form is monthly_payment equals P times r times (1 plus r) to the n, divided by (1 plus r) to the n minus one. That identity is the standard amortizing installment banks, credit unions, and SBA-style lenders publish for term facilities and equipment notes: each installment covers interest on the outstanding principal first, then reduces the balance, so the interest share falls and the principal share rises even though the cash payment never changes. When the annual rate is zero the denominator would be zero, so the calculator switches to the honest special case monthly_payment equals P divided by n, which is how genuine interest-free vendor notes are billed. The response also reports total_repayment, which is monthly_payment times term_months, and total_interest, which is total_repayment minus principal, so an owner can see the true extra cost of stretching a thirty-six-month note to sixty months.

Input contract, response shape, and error behaviour

Principal is the field the product contract singles out: it must be strictly positive. Zero, negatives, and non-numeric values never produce a fake schedule. POST a JSON body with three required fields: principal, the financed amount as a finite number strictly greater than zero (aliases amount and loan_amount); annual_rate, the annual nominal interest as a percent such as eight for eight percent per year, finite and at least zero (aliases rate and interest_rate); and term_months, the term as a positive integer number of months from one through three hundred sixty (aliases months, term, and tenure). Each numeric field may arrive as a JSON number or as a numeric string. Missing fields, empty strings, non-finite values, a non-positive principal, a negative annual_rate, and any term_months that is not a positive integer—zero, a fraction such as twelve point five, a boolean, or a non-numeric string—fail with invalid_input naming the field and are not billed. On success the body echoes principal, annual_rate, and term_months, and adds monthly_rate (annual_rate divided by twelve hundred), monthly_payment, total_repayment, total_interest, and a formula string restating the amortizing identity and the zero-rate special case. Example: principal fifty thousand, annual_rate eight, term_months thirty-six yields a monthly rate of one twelfth of eight percent and a monthly payment rounded to two decimals, with total_repayment equal to that payment times thirty-six. Pair the figure with your own fee table if the lender adds documentation charges outside the loan; this endpoint computes installment math only and does not emit an amortization schedule row by row.

Where a business loan calculator earns its keep

Educators teaching amortizing versus interest-only or merchant-cash-advance quotes can seed classroom examples that fail loudly when principal is not positive, instead of silently treating a zero principal as a free lunch. Failed validation does not consume balance. Keep principal strictly positive, pin the rounded money fields in tests, and you get reproducible business-loan payments for widgets, LMS modules, and edge workers without a loan-origination stack of your own. Quote tools and CRM pipelines can call the same pure module from the browser for free previews and from the API when storing a scenario, guaranteeing that the payment a visitor saw on a landing page matches the payment stored after they save a quote. Controllers and bookkeepers already know the financed amount; posting principal, annual_rate, and term_months fills a card with monthly payment, total repayment, and total interest so a founder can see whether sixty months is cheaper in cash than thirty-six at a higher monthly hit. Content teams can illustrate zero-percent vendor financing with annual_rate zero, where monthly_payment collapses to principal over term_months without a divide-by-zero.