P-Value Calculator · Practical guide
P Value from T Score Calculator
T-statistic plus degrees of freedom in — one- and two-tailed p-values out.
At a glance
The Short Answer
Enter your t-statistic and degrees of freedom above to get one- and two-tailed p-values. The reference point most students meet first: t = 2.228 with df = 10 gives a two-tailed p of about 0.05 — right at the significance line.
Interactive tool
P-Value Calculator
Enter your own values, then compare the result with the examples below.
P-Value Results
Why Degrees of Freedom Matter
The t-distribution has heavier tails than the normal distribution, so the same statistic yields a larger p-value than a z-score would. As degrees of freedom grow, the t-distribution converges to the normal and the gap shrinks.
Critical T Values for α = 0.05 (Two-Tailed)
| Degrees of Freedom | Critical t |
|---|---|
| 5 | 2.571 |
| 10 | 2.228 |
| 20 | 2.086 |
| 30 | 2.042 |
If your t-statistic exceeds the critical value for your degrees of freedom, your two-tailed p-value is below 0.05.
Worked Example
A one-sample t-test on 21 observations produces t = 2.00 with df = 20 (n − 1). That falls just short of the critical 2.086, so the two-tailed p-value is about 0.06 — close, but not significant at the 0.05 level. The calculator shows both tails instantly so you can check against the test type your study pre-registered.
Frequently Asked Questions
How do I find the degrees of freedom for my t-test?
For a one-sample t-test it is n − 1, where n is your sample size. A two-sample test uses a larger df based on both sample sizes.
Why is my t p-value larger than the z p-value for the same number?
The t-distribution has heavier tails to account for estimating the standard deviation from a small sample. The difference is large at low degrees of freedom and nearly vanishes above df = 30.
Is this calculator's t result exact?
It uses a close normal-based approximation that is accurate enough for checking significance. For publication-critical values at very low degrees of freedom, verify against statistical software.
What t value is significant with 30 degrees of freedom?
For a two-tailed test at alpha = 0.05, the critical value is about 2.042. Anything beyond that gives p below 0.05.