How it works
This is the plain-vanilla loan calculator: three inputs, one fixed monthly installment, and the full cost picture. Enter the amount you want to borrow, the annual percentage rate your lender advertises, and the repayment span in years. The engine returns the constant monthly installment, the cumulative interest charge over the life of the agreement, and a period-by-period amortization breakdown.
What makes the tool genuinely useful is side-by-side term comparison. Shortening the repayment window raises each monthly installment but compresses the total interest you pay. Extending it does the opposite — smaller installments, larger lifetime cost. The calculator lets you toggle between, say, a 36-month and a 60-month structure on the same principal so you can see exactly where the crossover sits.
This is an estimation tool, not professional financial advice. Lender disclosures may include fees, insurance, or rate structures not captured here.
The formula
M = P × [r(1+r)^n] / [(1+r)^n − 1]
Where P is the borrowed principal, r is the monthly rate (annual rate ÷ 12 ÷ 100), and n is the total number of monthly installments (years × 12). Total interest equals M × n − P.
Worked example
Take a $15,000 general-purpose personal loan at 9.5% APR. Below is the 5-year (60-payment) scenario, followed by the 3-year (36-payment) alternative on the same principal and rate.
5-year structure
Monthly rate: 9.5 ÷ 12 ÷ 100 = 0.0079167
M = 15,000 × [0.0079167 × (1.0079167)^60] / [(1.0079167)^60 − 1]
M = 15,000 × [0.0079167 × 1.6016] / [0.6016]
M = 15,000 × 0.021065 ≈ $315.98
Total repaid: $315.98 × 60 = $18,958.80
Total interest: $18,958.80 − $15,000 = $3,958.80
3-year structure (same $15,000 at 9.5% APR)
n = 3 × 12 = 36
M = 15,000 × [0.0079167 × (1.0079167)^36] / [(1.0079167)^36 − 1]
M = 15,000 × [0.0079167 × 1.3276] / [0.3276]
M = 15,000 × 0.032091 ≈ $481.36
Total repaid: $481.36 × 36 = $17,328.96
Total interest: $17,328.96 − $15,000 = $2,328.96
The two-year shorter window lifts the monthly installment by roughly $165 but cuts the lifetime interest by about $1,630. That is the core tradeoff this calculator exposes.
| Term | Monthly installment | Total interest | Total repaid |
|---|---|---|---|
| 3 years | $481.36 | $2,328.96 | $17,328.96 |
| 5 years | $315.98 | $3,958.80 | $18,958.80 |
Common mistakes
The most frequent slip is entering the annual percentage rate as a decimal (0.095) instead of a percentage (9.5). The tool divides by 100 internally; feeding it 0.095 produces a rate near zero and a dramatically understated installment. A second pitfall: quoting a "flat rate" rather than an APR. Some lenders advertise a flat add-on rate that ignores the declining balance — the resulting interest is materially higher than an amortizing APR of the same number. Always confirm you are entering the amortizing annual rate, not a flat charge. Finally, if an origination fee is deducted from the disbursement (you receive $14,250 on a $15,000 note with a 1% fee), calculate on the full $15,000 — that is the principal you repay, not the amount that hit your account.