Sample Size Calculator:
How Many Participants Do I Need?
Planning a study and wondering how many participants you need? This Sample Size Calculator is designed to help students and researchers estimate the sample size required for common statistical tests. If you have been asking yourself, “How many participants do I need?” or “How do I calculate sample size for my study?”, this tool provides a clear and practical starting point.
The calculator guides you through the key decisions involved in sample size planning, including the statistical test you expect to use, the effect size, the significance level, and the level of power recommended in your discipline. It is designed for common quantitative analyses and gives you a simple step-by-step way to estimate sample size before you begin collecting data.
It can help you plan sample size for common procedures such as t-tests, ANOVA, correlation, regression, chi-square, and other widely used statistical tests. Use the result as a guide for planning your study, and always consider the conventions in your research area and the advice of your supervisor, research adviser, or statistician where appropriate.
Why sample size matters
Choosing an appropriate sample size is an important part of study design. If your sample is too small, you may miss meaningful results. If it is unnecessarily large, you may spend more time and resources than needed. Estimating sample size early helps you design a stronger and more efficient study.
A thoughtful sample size estimate can also strengthen the credibility of your study. It shows that you have considered the relationship between your research question, your planned analysis, and the size of the effect you expect to find. For students and researchers planning a quantitative project, this is an important step in designing a stronger and more efficient study.
What this tool does
Provides a simple starting point for estimating sample size for common statistical tests.
Who is it for
Useful for students, researchers, and anyone planning a basic quantitative study.
How it helps
Guides you step by step so you can make decisions more clearly and confidently.
Before you begin
To estimate sample size, it helps to have a rough idea of the analysis you plan to use and the assumptions you will make about effect size, significance level, and statistical power. You do not need to know everything in advance, but thinking through these issues before using the calculator will give you a more meaningful estimate.
Statistical test
Identify the main analysis you plan to use. If unsure, start with the statistical test selection tool on this website. This will help you identify the tests that will address your research question.
Effect size
Review research in your area and identify the effect size reported for similar outcome measures. If this is not available, estimate whether you expect a small, medium, or large effect. Use the Effect Size Calculator on this website if you need help.
Significance level
Choose the alpha level you plan to use. This is usually .05 unless you are a medical researcher in which case you might need to use .01.
Power
Check the conventions in your discipline. Some fields use .80, while others recommend .90. Use the value used in your research area.
This calculator is intended as a planning guide only. The estimate should always be considered in light of your study design, the assumptions you have made, and the conventions in your discipline. For more complex analyses or formal power analysis, use a specialised program such as G*Power or seek statistical advice.
If sample size planning feels confusing at first, that is completely normal.
This calculator is here to give you a practical starting point and help you move forward with more confidence.
Sample Size Calculator
Follow the step-by-step instructions to select your planned analysis, enter the required values,
and view the estimated sample size. Need help choosing an effect size value? Use the Effect Size Calculator available on this website to calculate, convert, or extract an effect size from previous research.
These tools are intended as a guide only. They do not replace careful consideration of your study design, your discipline-specific literature, or professional statistical advice. Where appropriate, please consult a qualified statistician, supervisor, or research adviser before making final decisions.
Need a more advanced option?
This calculator is designed as a practical guide for common statistical tests. If your study involves a more complex design, you may need a more specialised power analysis tool. One widely used option is G*Power. It is particularly useful for more advanced designs or when you need greater flexibility than this calculator provides.
Sample Size Calculator FAQ
Question 1: What sample size so I need for my study?
The answer depends on the statistical test you plan to use, the effect size you expect, the significance level you choose, and the level of power you want to achieve. A study expecting a small effect usually needs a larger sample than a study expecting a large effect. The statistical test also matters, which is why sample size should always be considered alongside your research question and the selection of the statistical test. This calculator helps you work through those decisions and estimate the sample size needed for many common statistical tests.
Question 2: What effect size should I use?
Where possible, review previous studies that used similar measures or designs and see what effect sizes were reported. If that is not available, make a reasoned judgement about whether you expect a small, medium, or large effect. Your choice of effect size will have a strong impact on the estimated sample size. If you need help choosing an effect size value, use the Effect Size Calculator on this website to calculate, convert, or extract an effect size from previous research.
Question 3: What power level should I choose?
Many researchers use .80 as a minimum standard, but some disciplines or study contexts recommend .90. The best choice is usually the one most commonly used in your research area, unless your supervisor, adviser, or discipline recommends something different.
Question 4: Can this calculator be used for all study designs?
This calculator is designed for common statistical tests and basic quantitative study planning. More complex designs, unusual models, or specialist analyses may require formal power analysis in a program such as G*Power.
Question 5: When should I use G*Power instead?
Use G*Power when your study design is more complex, when you need a formal power analysis for a proposal or ethics application, or when the test you need is not covered by this calculator.
Question 6: What if I am not sure which statistical test to use?
Start with the Choose a Statistical Test page, then return to this calculator once you know your planned analysis.
Resources
Not sure which test to use? Visit the Choose a Statistical Test page.
Need help getting your data ready? See the Preparation page.
Looking for guidance on how to conduct a statistical test? Explore the SPSS Survival Manual page.
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