Why this tool is useful
Inspect a transparent large-sample interval for a proportion and see every assumption used.
Everyday Calculators · runs locally
Proportion Confidence Interval Calculator
Calculate a simple Wald confidence interval for a sample proportion from a z-score and sample size.
Ready. Your working data stays in this browser.
How it works
It applies p ± z × sqrt(p(1-p)/n). For small samples or extreme proportions, use a more appropriate interval method.
Practical example
A 50% proportion from 400 observations at z=1.96 gives about 45.1%–54.9%.
Validation
The calculation is deterministic and explains validation errors instead of silently changing invalid input.
Privacy boundary
Input and output remain in this tab. Copy and download happen through browser APIs without a server upload.
Result passport
Every run reports its tool mode, timestamp and input/output size so transformed data can be audited.