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loan-calculator

A loan calculator computes the monthly payment, total interest paid over the term, and total cost (principal + interest) for any standard amortising loan — mortgages, auto loans, personal loans, student loans — given principal, annual rate, and term. The ZTools Loan Calculator runs in the browser, supports side-by-side comparison of two or three offers, generates the full amortisation schedule, models extra payments to show interest savings, and surfaces the total-cost-of-credit perspective that monthly-payment thinking alone hides.

Use cases​

Comparing two mortgage offers​

20-year vs 30-year fixed at slightly different rates. Both monthly payments and total interest matter; calculator shows both.

Sizing a personal loan​

Based on a max monthly payment you can afford, work backward to the maximum principal that fits your budget at quoted rate / term.

Refinancing analysis​

Existing loan at 7% vs refinance offer at 5% with $2,000 closing costs. Calculator computes break-even point in months.

Auto loan term tradeoff​

60-month vs 72-month — longer term lowers payment but increases total interest. See the tradeoff numerically.

How it works​

  1. Enter loan parameters — Principal, annual rate (APR), term (months or years).
  2. Compute payment — Standard amortisation: P × r × (1+r)^n / ((1+r)^n − 1) where r is monthly rate.
  3. Show totals — Monthly payment, total interest, total cost.
  4. Compare offers — Side-by-side panel: monthly payment, total interest, total cost, break-even point if there are upfront costs.
  5. Model extra payments — Add a recurring or one-time prepayment; see new term and interest saved.

Examples​

Input: $300,000, 6%, 30 years

Output: Monthly $1,799 · Total interest $347,515 · Total $647,515

Input: Same loan, $200/month extra

Output: New term 24 yr 4 mo · Saves $84,000 in interest

Input: $25,000 auto, 7%, 5 years

Output: Monthly $495 · Total interest $4,702

Frequently asked questions​

What is APR vs interest rate?

Interest rate is the cost of borrowing only. APR includes lender fees and closing costs spread over the term. APR is the more honest comparison metric for loan offers.

Is monthly payment the right comparison?

No — total cost over the loan is what matters. A lower payment over a much longer term often costs vastly more. Always look at total interest.

How do I model variable rates?

Default is fixed rate. For variable, run the calculator at the initial rate; once the rate adjusts, recompute with the then-outstanding balance and remaining term.

Are closing costs included?

Not by default. Add them to principal if you want a worst-case "total" or compare offers separately.

How are payments allocated to principal vs interest?

Each month: interest = balance × monthly rate. Principal = payment − interest. New balance = old balance − principal. Early-life payments are mostly interest; that flips later.

Should I always take the longest term for the lowest payment?

No — longer terms cost dramatically more total interest. Pick the shortest term you can afford comfortably; aggressive principal reduction beats payment minimisation.

Tips​

  • Compare total interest, not monthly payments.
  • Run the prepayment scenario — a small monthly extra often saves shocking amounts of total interest.
  • For mortgages, consider a 15-year vs 30-year — the rate is usually lower and the total cost dramatically less.
  • Always include closing costs / fees in the comparison — APR captures these.
  • Refinance break-even: divide closing costs by monthly savings — that is months to break even.

Try it now​

The full loan-calculator runs in your browser at https://ztools.zaions.com/loan-calculator — no signup, no upload, no data leaves your device.

Open the tool ↗

Last updated: 2026-05-05 · Author: Ahsan Mahmood · Edit this page on GitHub