📊 Statistics
Confidence Interval Calculator
A confidence interval says how precise your estimate is, not how likely it is to be right in any single case. A 95% interval built from a sample of 100 is much wider than one built from a sample of 10,000 — that width is the whole message. This version uses the normal approximation, which is standard practice once samples get past roughly 30.
How it works
Sample mean
Sample standard deviation
Sample size
Confidence level
Lower bound
Upper bound
Interval width
Margin of error
Standard error
Critical value (z)
Frequently asked questions
How do I use the Confidence Interval Calculator?
Fill in the fields above — the result recalculates instantly as you type. Everything runs in your browser, so the values you enter are never collected or stored.
Is this calculator free?
Yes. It is completely free, requires no account, and works on phones, tablets and desktops.