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Sample Size Calculator

Enter a confidence level, margin of error, and expected proportion to get the sample size your survey needs, with and without a finite population correction.

Confidence level

How far the result may be from the truth, plus or minus.

Leave at 50% when unsure for the safest, largest sample.

The total size of the group you are studying. Everything is calculated in your browser. Nothing is uploaded.

Required sample size
Responses neededfor a large population
385

At 95% confidence with a 5% margin of error and a 50% expected proportion.

385
Without correction
—
With correction
1.96
Confidence z-score
Not set
Population
How it is calculated

The squared z-score for your confidence is multiplied by the proportion times one minus the proportion, then divided by the squared margin of error, and rounded up to a whole number of respondents.

z-score (95%)
1.96
Expected proportion (p)
0.5
Margin of error (e)
0.05
z squared times p(1 - p)
0.9604
e squared
0.0025
Sample size before rounding
384.16

How to calculate sample size

  1. Set your confidence level

    Choose 90%, 95%, or 99% confidence for your survey or poll.

  2. Enter margin of error and proportion

    Type the margin of error you can accept and the expected proportion, or leave it at 50% when you are unsure.

  3. Add a population size

    Enter the total group size to apply the finite population correction, or leave it blank for a large or unknown population.

  4. Read the required sample size

    The number of responses you need appears instantly, shown with and without the correction.

Why use this tool

Confidence, margin, and proportion together

The three levers that decide sample size sit in one place and recalculate the moment you change any of them.

Finite population correction built in

Add a population size and the tool reports the required sample both with and without the correction, so small groups get a realistic, lower number.

Safe 50% default

When you do not know the proportion in advance, 50% gives the largest and therefore safest sample size for any margin of error.

Clear formula breakdown

The z-score, proportion, margin, and the unrounded figure are all shown, so the final number is never a black box.

Runs entirely in your browser

Everything is calculated on your device as you type. Nothing is uploaded, stored, or logged.

About this tool

Before you run a survey or a poll, you need to know how many responses will make the result trustworthy. This calculator answers that from three inputs: the confidence level you want, the margin of error you are willing to accept, and the proportion you expect to measure. It multiplies the squared z-score for your confidence by the proportion times one minus the proportion, divides by the squared margin of error, and rounds up to a whole number of respondents.

Confidence and margin of error are the two levers that move the number most. A higher confidence level or a tighter margin of error both push the sample size up, sometimes steeply. When you do not know the proportion in advance, leave it at 50%, which produces the largest and therefore safest sample size for any margin. If your total group is small, add a population size to apply the finite population correction, which lowers the required sample because you are drawing from a limited pool.

The tool shows the required sample size with and without the correction, along with the z-score, proportion, and margin it used, so the result is never a mystery. Everything is calculated in your browser as you type and nothing is uploaded. For summarizing the data you collect afterward, the standard deviation calculator reports spread and averages, and the percentage calculator handles quick shares and changes.

Frequently asked questions

How does the sample size calculator work?
It uses the standard formula n = z squared times p(1 - p) divided by e squared, where z is the critical value for your confidence level, p is the expected proportion, and e is the margin of error as a decimal. The result is rounded up to a whole number of respondents. If you enter a population size, the finite population correction is applied on top.
What confidence level and margin of error should I use?
Most surveys use a 95% confidence level with a 5% margin of error, which is a good balance of rigor and cost. A 99% confidence level or a tighter margin such as 3% both increase the sample size, sometimes sharply. Pick the confidence and margin your decision actually needs rather than the strictest possible values.
Why is 50% the default expected proportion?
The term p(1 - p) is largest when p is 50%, so using 50% yields the biggest sample size for a given confidence and margin. When you have no prior estimate of the proportion, 50% is the safe choice because it guarantees your sample is large enough no matter how the results turn out.
When do I need to enter a population size?
Enter a population size when your total group is small enough that sampling a large share of it is realistic, for example the employees at one company or the members of a club. The finite population correction then lowers the required sample. For a large or unknown population you can leave the field blank and the standard formula applies.
Is my data uploaded anywhere?
No. Every calculation runs in your browser as you type. Your inputs are never sent to a server, stored, or logged.

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