Statistics

Confidence Level Calculator

Convert between confidence level, alpha (α), tail area, and the z critical value (z*). Also estimate the implied confidence level from CI endpoints when you know the standard error.

Confidence level Alpha (α) Z critical (z*) One-tailed / Two-tailed Steps
This tool uses the standard normal (z) distribution. For small-sample mean intervals with unknown σ, a t critical value (t*) is often used instead.

Confidence Level, Alpha, and Z* Converter

Pick a mode and tail type, then calculate confidence level, α, tail probability, and z* instantly.

Example: 95 for 95% confidence.
Example: α=5 means 5% outside the interval (two-tailed splits it).
Use a positive z* (e.g., 1.96).
This mode estimates the implied confidence level of a two-sided interval when you know its endpoints and the standard error used to build it. It recovers z* = (Upper − Lower) / (2·SE) and then computes Confidence = 2·Φ(z*) − 1.
SE must be > 0. If you only know margin of error (ME), use z* = ME/SE.