THE NUMBERS, EXPLAINED
How to use this calculator.
Simulates month-end deposits and monthly interest. Stops after 1,200 months and reports an unreachable or longer-horizon target.
Start with your inputs
Enter starting balance (£), target (£), monthly deposit (£), annual rate (%). Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Starting balance (£) | 0 |
| Target (£) | 1200 |
| Monthly deposit (£) | 100 |
| Annual rate (%) | 0 |
A worked example
With the example inputs above, months to target is 12. Change the inputs to explore your own scenario.
| Result | Example answer |
|---|---|
| Months to target | 12 |
How this is calculated
Months to target
monthsToGoal(a,b,c,d)
Symbols and units
- a
- Starting balance (£)
- b
- Target (£)
- c
- Monthly deposit (£)
- d
- Annual rate (%)
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Calculation steps and notation
Starting with the opening balance, apply balance × (1 + annual percentage/1200) + monthly deposit. Repeat until the target is reached, counting months. Return zero if already funded; report not reached if it takes more than 1,200 months.
Assumptions and limits
Simulates month-end deposits and monthly interest. Stops after 1,200 months and reports an unreachable or longer-horizon target.
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 target. 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: Simulates month-end deposits and monthly interest. Stops after 1,200 months and reports an unreachable or longer-horizon target.
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.
Further reading
Investor.gov: financial planning tools ↗
Background reading; this does not imply endorsement or independent review of SumFizz.
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 →