Probability Calculator

  • Percentile to Z-Score Calculator
    Convert any percentile (0–100) into a standard normal z-score using the inverse normal CDF, z = Φ⁻¹(p). Fast, accurate, and free.
  • P-Value from Z-Score Calculator
    Convert a Z-score to a P-value for one-tailed or two-tailed hypothesis tests. Uses the standard normal distribution to compute statistical significance.
  • Sample Size Calculator (Proportion)
    Calculate the required survey sample size for estimating a proportion at a given confidence level and margin of error using n = z²·p(1−p)/E².
  • Chi-Square Test of Independence Calculator (2x2)
    Run a 2x2 chi-square test of independence online. Enter your contingency table cells to get χ², degrees of freedom, p-value, and significance at α = 0.05.
  • P-Value from t-Score Calculator
    Convert a Student's t-score and degrees of freedom into a p-value. Supports one-tailed and two-tailed tests with exact incomplete-beta computation.
  • Complement Probability Calculator
    Calculate the complement probability P(A') = 1 − P(A) instantly. Enter the probability of an event and find the chance it does NOT happen.
  • Five-Number Summary Calculator
    Find the five-number summary (minimum, Q1, median, Q3, maximum) plus IQR for any data set. Free online statistics calculator with worked examples.
  • Cohen's d Effect Size Calculator
    Calculate Cohen's d effect size from two group means, standard deviations and sample sizes, with pooled SD and a small/medium/large interpretation.
  • Critical t Value Calculator
    Find the critical t value for any significance level (α) and degrees of freedom. Supports one-tailed and two-tailed tests using the t distribution.
  • Spearman's Rank Correlation Calculator
    Calculate Spearman's rank correlation coefficient (ρ) from paired X and Y data. Handles ties, shows Σd² and n. Free statistics tool.
  • Critical Z Value Calculator
    Find the critical z value (z*) from any confidence level using the inverse normal CDF. Supports two-tailed and one-tailed tests. Free and instant.
  • Sample Size Calculator for a Mean
    Calculate the required sample size to estimate a population mean within a target margin of error and confidence level using n = (z·σ/E)².
  • Combination Calculator (nCr)
    Calculate combinations without repetition (nCr) instantly. Enter n and r to find how many ways you can choose r items from n where order does not matter.
  • Sample Mean (Average) Calculator
    Calculate the sample mean (average) of any data set. Enter your numbers and get x̄ = Σxi / n with sum and count instantly.
  • Permutation (nPr) Calculator
    Calculate permutations nPr = n! / (n−r)! instantly. Find the number of ordered arrangements when choosing r items from n. Free, fast, and accurate.
  • Return Period & Exceedance Probability Calculator
    Calculate the probability that a storm or flood with return period T is exceeded at least once over n years. Free hydrology risk calculator.
  • Discrete Probability Distribution Calculator
    Compute the mean, variance and standard deviation of a discrete probability distribution from a table of values and probabilities. Free and instant.
  • Interquartile Range (IQR) Outlier Calculator
    Find Q1, Q3, IQR and detect outliers with the 1.5×IQR Tukey rule. Computes lower and upper fences from your data set instantly.
  • One-Proportion Z-Test Calculator
    Run a one-proportion z-test online. Enter successes, sample size, and hypothesized proportion to get the z statistic, standard error, and p-value.
  • Precision and Recall Calculator
    Calculate precision, recall and F1 score from true positives, false positives and false negatives. Free classification metrics calculator.
  • Binomial Mean & Standard Deviation Calculator
    Compute the mean, variance, and standard deviation of a binomial distribution from n trials and probability p using μ=np and σ=√(np(1−p)).
  • Z-Score to P-Value Calculator
    Convert a z-score to a p-value for one-tailed or two-tailed tests using the standard normal distribution. Fast, accurate statistical significance lookup.
  • Poker Outs to Odds Calculator
    Convert poker outs into your chance to hit using the Rule of 4 and 2. Get approximate percentage, exact probability, and odds against for one or two cards.
  • Sampling Distribution of the Sample Mean Calculator
    Compute the mean and standard error (σ/√n) of the sampling distribution of the sample mean from population mean, std dev, and sample size.

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