Total Interest Paid Lifetime

What this calculator does

Carrying multiple debts at the same time makes the cumulative interest cost hard to see. This calculator takes up to three debts (each with its balance and rate) plus a single combined monthly payment, computes the weighted-average rate across the three balances, and runs that as a single amortising loan to project the total interest paid before the combined balance clears. The result surfaces lifetime interest, total amount paid, months to clear, and the weighted-average rate used.

How the math works

The calculator computes the weighted-average annual rate across the three balances: weighted rate = Σ(balance_i × rate_i) ÷ Σ(balance_i). It then treats the sum of balances as a single loan at that weighted rate and the entered monthly payment, and applies the closed-form amortisation: n = −ln(1 − rB ÷ M) ÷ ln(1 + r), where r is the monthly rate, B is the total balance, and M is the monthly payment. Total paid is M × n; total interest is total paid minus B. This single-loan simplification is a common approximation; the actual interest under three separate amortisations depends on which debt receives the marginal payment each month, so the calculator's figure should be read as a planning estimate rather than an exact projection.

Worked example

Three debts: 10,000 at 18%, 15,000 at 12%, and 5,000 at 24%. Combined monthly payment of 800. Total balance: 30,000. Weighted rate: (10,000 × 18 + 15,000 × 12 + 5,000 × 24) ÷ 30,000 = 16%. The closed-form amortisation produces approximately 52.33 months (53 when rounded up to the next whole month), total paid of about 41,865.48, and total interest of about 11,865.48. Raise the monthly payment to 1,000 on the same balances and the simulation gives about 38.57 months, total paid of about 38,566.77, and total interest of about 8,566.77 — saving roughly 3,299 in interest by adding 200 per month.

What moves the result most

Three levers shape the outcome. The combined monthly payment changes the rate of principal reduction directly: a higher monthly figure cuts more principal per month, which reduces the months exposed to interest. The weighted rate determines the monthly interest charge on the remaining balance — higher rates mean a larger interest portion of each payment and a slower drop in principal. The total balance scales the absolute interest figure linearly. Of the three, the monthly payment is the most actionable lever for a borrower who wants to reduce lifetime interest without changing the underlying loans.

What this calculation does not capture

The single-weighted-rate model assumes the combined monthly payment is applied across the three debts in proportion to the weighted rate. In practice, borrowers may direct extra payments toward a specific debt — for example, the highest-rate debt to minimise total interest, or the smallest-balance debt to clear lines from the list. Different allocation choices produce different total-interest figures than the weighted-rate single-loan simplification. The calculator also does not model rate changes during the payoff period, missed payments, fees added to principal, balance transfers, or promotional rates. The figure is best used as a starting baseline against which actual progress can be compared.

Reading the output

The headline figure is the projected lifetime interest under the entered schedule. The secondary details show the total balance, total paid, months to clear, and the computed weighted rate so the working is transparent. The months-to-clear figure is rounded up to the next whole month, which is why the totals (paid, interest) reflect the continuous closed-form result rather than month × monthly-payment exactly.