How to use: Calculate required sample size for statistically valid surveys and studies. Enter Population (total population size - use large number like 100,000 for unlimited), select Confidence Level (90%, 95%, or 99% - how sure you want to be), and enter Margin of Error (acceptable error %, typically 3-5%). Results show minimum sample size needed. Higher confidence and lower margin require larger samples. Example: Population 10,000, 95% confidence, 5% margin ≈ 370 samples. 99% confidence needs larger sample. Applications: market research surveys, political polling, clinical trials, quality control testing, A/B testing. 95% confidence with 5% margin is industry standard for most surveys. Essential for research design and statistical validity.