About Financial Calculator
The figure that matters on a loan is rarely the monthly payment — it is the total interest, which is the part lenders quote least prominently. A £200,000 mortgage at 5% over 25 years costs about £150,000 in interest, and seeing that number changes how a rate comparison feels.
Compound interest is the same arithmetic pointed the other way, and it is worth seeing on a chart rather than as a single total. The gap between what you contributed and what the balance became is the whole argument for starting early.
Margin and markup get their own mode because they are constantly confused, and the confusion costs money. A 50% markup is a 33.3% margin — a business that targets a 50% margin and applies a 50% markup is systematically underpricing.
How to use the financial calculator
Choose a calculation
Loan repayments, compound interest, margin, break-even or ROI.
Enter your figures
Amount, rate and term. Results update as you type.
Read the totals
Not just the monthly payment, but the total paid and the total interest.
Open the schedule
For a loan, see how each payment splits between interest and principal.
Why early loan payments are almost all interest
Interest is charged on the outstanding balance, which is at its largest at the start. On a 25-year mortgage the first payment can be 80% interest, and it takes years before the split reverses.
This is why overpaying early has a disproportionate effect: every pound of extra principal removes all the interest that pound would have accrued for the remaining term. The amortisation schedule makes the shape of this obvious.
Margin and markup are not the same
- Markup is profit as a share of cost: an item costing £100 sold at £150 has a 50% markup.
- Margin is profit as a share of the selling price: the same item has a 33.3% margin.
- To hit a target margin, divide the cost by (1 − margin). For a 40% margin on a £60 cost, the price is £100, not £84.