Unit Converters · Free tool
P-Value Calculator
Convert any z- or t-statistic into a one- or two-tailed p-value instantly. Significance flagged at α = 0.10, 0.05, 0.01. Free online tool, no sign-up needed.
p-value (two-tailed)
p = 0.0455
Significant at α = 0.05 — reject the null hypothesis (with the usual caveats about effect size, replication, and multiple comparisons).
Z-test vs t-test — pick the right one
Z-test: use when the population standard deviation σ is known and n ≥ 30 (Central Limit Theorem). Rare in practice — most real data needs t.
t-test: use when σ is estimated from the sample (the usual case). The extra uncertainty from estimating σ widens the tails of the distribution; the t-table reflects this. As df → ∞, t → z.
Two-tailed tests "is the parameter different from the null?". One-tailed tests "is it specifically greater/less than?". Choose one-tailed only if your hypothesis was directional before seeing data.
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What it does
Convert a z- or t-statistic into a p-value (one- or two-tailed) and see significance at α = 0.10, 0.05, and 0.01 immediately. Uses the Abramowitz & Stegun normal CDF and the standard incomplete-beta t-CDF — accurate to roughly 1e-6.
To compute the underlying t-stat from a correlation, use the correlation calculator. To size a study before collecting data, see the sample size calculator.
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Paste this snippet into any page. Loads on-demand (lazy), no tracking scripts, and sized to most dashboards. Replace the height to fit your layout.
<iframe src="https://freetoolarena.com/embed/p-value-calculator" width="100%" height="720" frameborder="0" loading="lazy" title="P-Value Calculator" style="border:1px solid #e2e8f0;border-radius:12px;max-width:720px;"></iframe>How to use it
- Choose z-test (large sample, σ known) or t-test (σ estimated, the usual case).
- Enter the test statistic.
- For t-tests, enter degrees of freedom (df = n − 1 for a one-sample test).
- Read the two-tailed p, or switch to one-tailed if your hypothesis was directional before data.
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