Binomial Probability
Exact and cumulative binomial probabilities for n trials.
P(X = 3)
11.7188%
0.117188
P(X ≤ 3)
17.188%
P(X ≥ 3)
94.531%
Expected value
5
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How the Binomial Probability works
The Binomial Probability Calculator computes both exact and cumulative probabilities for a fixed number of independent trials, each with the same probability of success — making it an essential tool for students, statisticians, quality analysts, and anyone working with binary outcome experiments.
At its core, this calculator applies the binomial probability formula to determine the likelihood of observing exactly k successes in n independent trials, where each trial has a constant probability p of success. You supply three inputs: the number of trials (n), the number of successes you're interested in (k), and the probability of success on any single trial (p). The calculator then returns the exact probability P(X = k) as well as cumulative probabilities such as P(X ≤ k) and P(X ≥ k), giving you a complete picture of the distribution rather than just a single point estimate.
The binomial model is valid under four strict conditions: each trial must be independent, there must be a fixed number of trials, each trial must have exactly two outcomes (success or failure), and the probability of success must remain constant across all trials. Violating any of these — for example, sampling without replacement from a small population — renders the binomial model inappropriate and requires alternatives like the hypergeometric distribution. Recognizing when your scenario genuinely fits these assumptions is the most important analytical step before using this calculator.
Cumulative probabilities are where the real analytical power lies. Rather than just knowing 'what is the chance of exactly 7 heads in 10 flips,' researchers and quality engineers typically need to answer questions like 'what is the probability of 7 or fewer defects?' or 'what is the chance of at least 3 successes?' The calculator handles both the lower-tail cumulative (P(X ≤ k)) and upper-tail cumulative (P(X ≥ k)) automatically, saving you from summing individual probabilities by hand and reducing arithmetic errors.
A common mistake is confusing p with the observed sample proportion and treating it as fixed when it was actually estimated from data — doing so ignores estimation uncertainty and can lead to overconfident probability statements. Another frequent error is forgetting that 'success' is simply a label for the outcome you're tracking; it doesn't have to be a positive event. Defects, failures, and disease occurrences can all be coded as 'success' for calculation purposes, as long as you stay consistent. Finally, for very large n or extreme values of p, the normal or Poisson approximations are often more practical; the calculator's exact output helps you judge when those approximations are close enough.
Formula
P(X=k) = C(n,k) · p^k · (1−p)^(n−k)
Pro tips
- Always verify your four assumptions (independence, fixed n, binary outcomes, constant p) before inputting values — if sampling without replacement from fewer than ~20 population members, use the hypergeometric distribution instead.
- Use the cumulative P(X ≥ k) output for acceptance sampling and quality control decisions, where you need to know the probability of exceeding a defect threshold, not just hitting an exact number.
- When p is very small (< 0.05) and n is large (> 100), check whether the Poisson approximation with λ = n×p gives a close result; if it does, it simplifies further calculations in your analysis pipeline.
- For homework or exam problems, compute the expected value (n×p) and variance (n×p×(1−p)) first — they instantly tell you where the distribution is centered and how spread out it is, helping you sanity-check the calculated probabilities.
- If you are working with a two-sided question (e.g., 'probability of between 3 and 7 successes inclusive'), calculate P(X ≤ 7) minus P(X ≤ 2) using the cumulative outputs rather than summing five individual exact probabilities.
Key terms
- Number of Trials (n)
- — The total count of independent, identical experiments performed, such as the number of coin flips or product inspections in a batch.
- Number of Successes (k)
- — The specific count of favorable outcomes you want to evaluate the probability for within the n trials.
- Probability of Success (p)
- — The fixed probability, between 0 and 1, that any single trial results in the designated 'success' outcome.
- Binomial Coefficient C(n, k)
- — Also written as 'n choose k,' this counts the number of distinct ways k successes can be arranged among n trials and is the combinatorial heart of the formula.
- Cumulative Probability
- — The summed probability of observing at most (or at least) a certain number of successes, used for threshold-based decision making rather than point estimates.
- Expected Value (Mean)
- — The average number of successes you would expect over many repetitions of the experiment, calculated as n × p for a binomial distribution.