Margin of Error Calculator

Enter the confidence level, sample size, sample proportion, and population size to find the margin of error using the calculator below.

Proportion of the sample with the characteristic of interest.
%
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Margin of Error:

 

Steps to Solve

Margin of Error Formula

graphic showing the margin of error formula where the MOE is equal to the confidence level z-score times the square root of the sample proportion times 1 minus the sample proportion, divided by the sample size

Step One: Find the Z-Score

z-Score = ?

Step Two: Find the Margin of Error

MOE = z · (p̂ · (1 - p̂)) ÷ n



How to Find the Margin of Error

In statistics, the margin of error is the amount of error in random sampling surveys. Because surveys are often done on a sample of a larger population, there will inevitably be some amount of error.

A lower margin of error produces higher confidence levels in a survey sample. Thus, the margin of error is a good indicator of the confidence you can have in the results of the sample.

The margin of error is expressed as a percentage indicating the degree that the sample results differ from the total population, for instance when calculating a confidence interval.

Margin of Error Formula

The margin of error is equal to the z-score for the confidence level times the margin of error. You can find the margin of error using the following formula:

graphic showing the margin of error formula where the MOE is equal to the confidence level z-score times the square root of the variance divided by the sample size

Where:
z = z-score for the confidence level
σ² = variance
n = sample size

Thus, the margin of error is equal to the z-score z times the square root of the variance σ² divided by the sample size n. You can find the z-score for your desired confidence level in the table below.

While you can calculate variance using a calculator, you can also use the sample proportion to calculate it. The sample proportion is denoted and represents the percentage of the sample that has the characteristic you’re interested in.

The variance σ² can be expressed with the formula σ² = p̂ · (1 – p̂), where is the sample proportion.

By substituting the formula for variance using the sample size into the formula above, the margin of error formula becomes:

graphic showing the margin of error formula where the MOE is equal to the confidence level z-score times the square root of the sample proportion times 1 minus the sample proportion, divided by the sample size

Where:
z = z-score for the confidence level
p̂ = sample proportion
n = sample size

Thus, the margin of error is equal to the z-score z times the standard error, which is the square root of the sample proportion times 1 minus the sample proportion , divided by the sample size n.

Example

Let’s find the margin of error for a survey asking people what their favorite color is. The survey sampled 250 people, and 40% of respondents answered that their favorite color was red. We want a 95% confidence level in the margin of error.

Start by assigning values to the variables needed for the formula.

z = 1.96 (95% confidence = 1.96 [table])
p̂ = 0.4 40% ÷ 100 = 0.4
n = 250 sample size

Next, substitute the variables in the formula and solve.

MOE = 1.96 · √(0.4 · (1 – 0.4) ÷ 250)
MOE = 1.96 · √(0.4 · 0.6 ÷ 250)
MOE = 1.96 · √0.00096
MOE = 1.96 · 0.03098
MOE = 0.06073

Then, multiply the margin of error by 100 to convert it to a percentage.

MOE = 0.06073 × 100 = 6.073%

Thus, the margin of error is equal to ±6.073%. Or rather, We are 95% confident that 40% of the total population has the favorite color of blue, plus or minus 6.073%.

Z-Scores

The table below shows the z-scores for various confidence levels.

Table showing the z-scores for various confidence levels.
Confidence Level Z-Score
70% 1.04
75% 1.15
80% 1.28
85% 1.44
90% 1.645
91% 1.7
92% 1.75
93% 1.81
94% 1.88
95% 1.96
96% 2.05
97% 2.17
98% 2.33
99% 2.576
99.9% 3.29