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Effect Size Calculator

Four ways in — two groups, a t, an F, or a correlation r — and conversions between d, r, η² and the odds ratio. A p-value says whether an effect is there; these say how big it is, which is the number a reader actually needs.

Related tools

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Open Effect Size Calculator and you get an answer straight away, with no account to create. Its place is under Statistics & probability; Sample size calculator and ANOVA Calculator (One-Way) answer the questions closest to this one.

How to use it

  1. Open the tool — no signup or install needed.
  2. Enter your input or adjust the available options.
  3. Get your result instantly, then copy or download it.

Frequently asked questions

What is Effect Size Calculator?

Four ways in — two groups, a t, an F, or a correlation r — and conversions between d, r, η² and the odds ratio. A p-value says whether an effect is there; these say how big it is, which is the number a reader actually needs.

When would I actually use this?

Summarising a dataset before drawing conclusions from it, checking whether a difference between two groups is real, and putting an interval around an estimate.

What is the most common mistake?

Reading a p-value as the probability that the hypothesis is wrong. It is the probability of seeing data at least this extreme if the null were true — a different statement, and a much weaker one.

How is Effect Size Calculator different from Sample size calculator?

They sit next to each other but answer different questions: Sample size calculator is the one to open when you need it to find the survey sample size needed for a population, confidence level and margin of error. Pick whichever matches what you're starting from — both are free.

Is there a tool for the next step?

ANOVA Calculator (One-Way) is the closest one after this: Full one-way ANOVA table — SS, df, MS, F and p — plus η², ω² and Cohen's f, because a significant F on its own tells you nothing about how large the difference is. The equal-variance assumption is stated, not buried.

What else is worth having open alongside it?

Average Deviation Calculator (mean absolute deviation) and Bayes Theorem Calculator — they come up in the same task often enough to be worth a second tab.

Where do the figures come from?

Distributions and test statistics are computed from their standard definitions, not from a lookup table, so results carry more digits than a printed table would. The assumptions each test makes are stated with it.

Further reading

All guides →
ExplainerProbability Basics, With ExamplesProbability measures how likely something is, from 0 to 1. Here's the basic idea, how to combine events with AND and OR, and the complement.ExplainerA 1 % Drop Rate Does Not Mean One Hundred RunsAt 1 %, a hundred attempts give you 63.4 % — not certainty. Ninety per cent takes 230 attempts and ninety-nine takes 459, and more than a third of players are still empty-handed at a hundred.ExplainerWhat Is a Z-Score? Standard Deviations Above the MeanA z-score is z = (x − μ) / σ — how many standard deviations a value sits from the mean. Learn how to compute it, read the standard normal curve, and turn it into a percentile.ExplainerMean, Median and Mode: What's the Difference?Mean, median and mode all describe the 'middle' of your data — but answer different questions. Here's what each means, with examples and when to use which.ExplainerGeometric vs Arithmetic Mean: Which to UseThe arithmetic mean adds and divides; the geometric mean multiplies and takes a root. Learn which fits growth rates, ratios, and averages.How-toHow to Calculate Standard Deviation (Step by Step)Standard deviation measures how spread out your data is. Here's the formula, a worked example, and the difference between population and sample.