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Sample Size Calculator
Work out how many completed responses your survey needs before you send a single invitation. Enter your population size, pick a confidence level, set the margin of error you can live with, and the number updates as you type.
Your inputs
Everyone you want the result to speak for. Surveying your own company? That is your total headcount.
How sure you want to be that the real answer sits inside your margin of error. 95% is the usual choice.
The wobble you will accept, in percentage points. Tighten it and the sample size climbs fast.
Nothing you type here leaves your device.
Key terms
Population size
Total number of people in the population you are researching. If you survey your organization, population size is the total number of employees.
Confidence level
A percentage that indicates how sure you can be that a certain amount of the population will select an answer. A 95% confidence level, for example, means that the outcomes between x and y are 95% certain.
Margin of error
A percentage that tells you how much you would expect your survey results to represent overall population views. The lower the margin of error, the better the confidence.
This figure assumes a simple random sample and a 50/50 population proportion, the most cautious assumption you can make. Real accuracy also rests on who answers and how well they represent everyone else.
How many people need to take your survey?
Surveying everyone is slow and expensive, so researchers sample instead: question a smaller group, then read the answer back onto the whole population. The sample size is what makes that step defensible. Question too few people and you are reading noise; question far more than you need and you have paid for precision the decision never called for.
01
Size the population
Count the group you want the result to speak for. An estimate is fine above a few tens of thousands, where the number stops moving.
02
Set your tolerance
Choose a confidence level and a margin of error. 95% and 5% is the pairing most survey work is built on.
03
Plan the invites
The number above is completed responses. Divide it by the response rate you expect to get the size of your send list.
Free, private, and built for research
Most calculators of this kind want your email before they will show a number, and many run the arithmetic on someone else's server. This one does neither.
01
Runs in your browser
No API call sits behind this page. Your population size and study parameters stay on your machine.
02
No account, no email wall
Change an input, read the number, close the tab. Nothing is logged and nothing is asked of you.
03
Working you can check
The formula is printed above and the same inputs always return the same figure, so you can defend it in a method section.
The sample size formula
Nothing here is invented for this page. It is the textbook method for sampling a proportion, so you can check any number the calculator gives you.
- z is the z-score for your confidence level: 1.28 at 80%, 1.65 at 90%, 1.96 at 95%, 2.58 at 99%.
- p is the population proportion, held at 0.5 here because a 50/50 split demands the largest sample and never leaves you short.
- e is your margin of error written as a decimal, so 5% becomes 0.05.
- The finite population correction then divides that figure down using your population size N, which matters for small groups and fades away for large ones.
Who reaches for a sample size calculator
Anyone who has been asked "but is that enough people?" and wanted a number rather than an opinion.
01
Researchers
Size the sample before fieldwork starts and write the justification straight into your method section.
02
Product teams
Know how many users have to answer before a feature survey is worth taking to a roadmap meeting.
03
Marketing and CX
Back a brand tracker or satisfaction study with a sample you can defend instead of a round number.
04
HR and people teams
Work out how many employees make an engagement result representative of the whole company.
05
Students and academics
Produce the sample calculation an ethics submission or dissertation committee expects to see.
06
Agencies
Show a client in respondents, not adjectives, what a tighter margin of error actually costs.
You know the number. Now collect it privately.
BlockSurvey encrypts responses end to end, so the data behind your sample is never sold, mined, or used to train a model.
Work the other direction
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Sample size and margin of error are two readings of the same equation.
Frequently Asked Questions
- That depends on three things: how large the group you are studying is, how confident you want to be in the result, and how much error you can live with. Enter those three values above and the calculator returns the minimum number of completed responses you need. As a rough anchor, a population of 10,000 at a 95% confidence level and a 5% margin of error needs about 370 responses.
