THE NUMBERS, EXPLAINED
How to use this calculator.
Assumes identical independent failures, immediate failure detection and retries only after failure. Waits exclude request execution time. Maximum attempts includes the first request; a retry policy can amplify an overloaded service, so this is only a stated-probability model.
Start with your inputs
Enter independent failure probability per attempt (%), maximum attempts including the first, waits before retries (ms; 0 for one attempt). Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Independent failure probability per attempt (%) | 50 |
| Maximum attempts including the first | 3 |
| Waits before retries (ms; 0 for one attempt) | 100,200 |
A worked example
With the example inputs above, expected attempts is 1.75. Change the inputs to explore your own scenario.
| Result | Example answer |
|---|---|
| Expected attempts | 1.75 |
| Success probability (%) | 87.5 |
| Expected total waiting (ms) | 100 |
| Maximum total waiting (ms) | 300 |
How this is calculated
Expected attempts
Add 1 + p + p² through p^(maximum attempts−1), with p the failure fraction.
Success probability (%)
100 × (1−p^maximum attempts).
Expected total waiting (ms)
Wait before retry i is incurred with probability p^i; sum probability × wait.
Maximum total waiting (ms)
Add all configured waits.
Symbols and units
- a
- Independent failure probability per attempt (%)
- b
- Maximum attempts including the first
- c
- Waits before retries (ms; 0 for one attempt)
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
Assumes identical independent failures, immediate failure detection and retries only after failure. Waits exclude request execution time. Maximum attempts includes the first request; a retry policy can amplify an overloaded service, so this is only a stated-probability model.
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?
Expected attempts; Success probability (%); Expected total waiting (ms); Maximum total waiting (ms). 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: Assumes identical independent failures, immediate failure detection and retries only after failure. Waits exclude request execution time. Maximum attempts includes the first request; a retry policy can amplify an overloaded service, so this is only a stated-probability model.
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
AWS: retry and backoff assumptions ↗
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 →