What is the sample size formula?

Cochran's formula: n0 = (Z² × p × (1-p)) / E². Where Z is the z-score for desired confidence level (1.96 for 95%), p is the expected proportion (0.5 = most conservative), E is the margin of error as a decimal (0.05 = ±5%). For finite populations adjust: n = n0 / (1 + (n0 − 1)/N).

What sample size do I need for a survey with 10,000 people?

For 95% confidence and ±5% margin of error with maximum variability (p=0.5), you need approximately 370 respondents from a population of 10,000.

What confidence level should I use?

95% confidence is the default in social and market research. 90% is acceptable for directional decisions; 99% for high-stakes decisions. Higher confidence requires larger samples.

SpaceForms / Tools / Sample Size Calculator

Sample Size Calculator (Free, 2026)

Calculate the minimum number of survey respondents you need for statistically valid results. Uses Cochran's formula with finite-population correction.

Population size

Total people you could survey

Confidence level

Higher = larger sample

Margin of error (%)

Tighter (lower) = larger sample

Expected response proportion (%)

Use 50% if unknown (most conservative)

Minimum sample size needed

370

respondents at 95% confidence with ±5% margin of error

Cochran's formula: n₀ = (Z² × p × (1−p)) / E²
Finite-population correction: n = n₀ / (1 + (n₀−1)/N)

Quick reference (95% confidence, ±5% margin)

Population	Sample size needed
100	80
500	217
1,000	278
5,000	357
10,000	370
50,000	381
100,000+	~384

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Sample Size — full definition
Margin of Error
Confidence Interval
NPS Calculator
CSAT Calculator

Quick reference (95% confidence, ±5% margin)

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