Survey sample size calculator

How to calculate survey sample size

1. Choose a confidence level: 95% is the most common choice. Higher confidence requires more responses.
2. Set the margin of error: How far the survey result may be from the true value, for example ±5%.
3. Enter the population size: The total number of people you want to understand. Leave it blank for very large or unknown populations.
4. Read your sample size: The calculator shows how many completed responses you need and how many people to invite at a 20% response rate.

The sample size formula

For a large population, the required sample size is n₀ = z² × p × (1 − p) ÷ e². At 95% confidence (z = 1.96), p = 0.5 and e = 0.05, that gives 1.96² × 0.25 ÷ 0.0025 ≈ 384.2, rounded up to 385.

When the population is small and known, apply the finite population correction: n = n₀ ÷ (1 + (n₀ − 1) ÷ N). For a company with 1,000 employees, 384.2 ÷ (1 + 383.2 ÷ 1,000) ≈ 278 responses.

Sample size reference table

Required completed responses at 95% confidence and p = 50%:

Margin of error formula

If you already know your sample size, the margin of error is e = z × √(p × (1 − p) ÷ n), multiplied by √((N − n) ÷ (N − 1)) for a finite population. With 400 responses from a large population at 95% confidence, e ≈ ±4.9%. Use the margin of error calculator above.

Tips for reaching your sample size

• Keep surveys short. Every extra step increases drop-off. Florm’s partial responses show where people stop.
• Use the right question types: closed questions such as single choice, rating scales and NPS are faster to answer than open text.
• Share widely: a direct link plus a QR code for offline audiences.
• Segment carefully: if you need results per subgroup (for example per department), each subgroup needs its own adequate sample.

Measuring NPS? NPS is a difference of two proportions and needs more responses for the same precision. Check it with the NPS calculator.