THE NUMBERS, EXPLAINED
How to use this calculator.
Extra payments start with the first regular payment. Interest accrues monthly. No prepayment penalty is included.
Start with your inputs
Enter principal (£), annual rate (%), original term (months), extra monthly payment (£). Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Principal (£) | 1200 |
| Annual rate (%) | 0 |
| Original term (months) | 12 |
| Extra monthly payment (£) | 100 |
A worked example
With the example inputs above, months to payoff is 6. Change the inputs to explore your own scenario.
| Result | Example answer |
|---|---|
| Months to payoff | 6 |
| Interest (£) | 0 |
How this is calculated
Months to payoff
Calculate the standard monthly payment with q = b/1200: a × q/(1−(1+q)^(−c)), or a/c at zero interest. Add d. Each month add balance × q interest, then subtract the payment capped at the amount owed. Count months until the balance is below £0.0000001, with a 1,200-month limit.
Interest (£)
Add the interest charged in every simulated month, including the final partial-payment month.
Symbols and units
- a
- Principal (£)
- b
- Annual rate (%)
- c
- Original term (months)
- d
- Extra monthly payment (£)
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Assumptions and limits
Extra payments start with the first regular payment. Interest accrues monthly. No prepayment penalty is included.
Calculations use the values you enter. Blank or invalid inputs show an error instead of silently substituting zero. Results are displayed to useful precision; calculations retain the underlying numeric precision.
Questions about this tool
What does the result tell me?
Months to payoff; Interest (£). The result applies to the method and inputs described above. Check the units and assumptions before using it in a decision.
Why might another calculator differ?
Differences in units, rounding, timing or model definitions can produce different answers. Compare the input definitions and the method, rather than only the final number. For this tool: Extra payments start with the first regular payment. Interest accrues monthly. No prepayment penalty is included.
Are my inputs stored or sent to AI?
Calculations on this page happen in your browser. The separate AI question finder sends its submitted conversation to OpenAI; these calculator fields are not sent. Favourites and recent tool names are stored locally. A shared link contains the inputs you choose to include.
Review status
Software example and browser checks are recorded in our quality report. No external expert review is claimed. Created with coding assistance; calculations run as ordinary browser code.
Read the quality report →