You can use the **CONFIDENCE.NORM** function in Excel to calculate the half-width of a confidence interval for a population mean.

This function uses the following syntax:

**CONFIDENCE.NORM(alpha, standard_dev, size)**

where:

**alpha**: The significance level to use (e.g. for a 95% confidence interval you would use .05)**standard_dev**: The population standard deviation, assumed to be known**size**: The sample size

The following example shows how to use this function in practice.

**Example: How to Use CONFIDENCE.NORM Function in Excel**

Suppose we would like to calculate a 95% confidence interval for the mean height of a particular species of plant.

Suppose we collect a simple random sample of **50** individual plants and find that the sample mean height is **14.5** inches.

Suppose it is also known that the population standard deviation (σ) of height among this species of plant is **3.1** inches.

We can type the following formula into a cell in Excel to calculate the half-width of this 95% confidence interval:

=CONFIDENCE.NORM(0.05, 3.1, 50)

The following screenshot shows how to use this formula in practice:

The half-width for this 95% confidence interval is **0.85926**.

To actually construct the confidence interval, we must add and subtract this value to the sample mean.

Thus, the 95% confidence interval for the mean height of this particular species of plant is:

- 95% Confidence Interval: 14.5 ± 0.85926
- 95% Confidence Interval: (13.64074, 15.35926)

The 95% confidence interval for the mean height is between **13.64074 inches** and **15.35926 inches**.

One interesting thing to note is that the higher the confidence level, the wider the confidence interval will be.

For example, if we wanted to calculate a 99% confidence interval then we would use alpha = 0.01 for the first argument in the **CONFIDENCE.NORM** function:

=CONFIDENCE.NORM(0.01, 3.1, 50)

The following screenshot shows how to use this formula in practice:

The half-width for this 99% confidence interval is **1.12926**.

Thus, the 99% confidence interval for the mean height of this particular species of plant is:

- 99% Confidence Interval: 14.5 ± 1.12926
- 99% Confidence Interval: (13.37074, 15.62926)

Notice that this confidence interval is wider than the 95% confidence interval.

**Additional Resources**

The following tutorials explain how to perform other common operations in Excel:

How to Calculate Confidence Interval for Regression Coefficient in Excel

How to Plot Confidence Intervals in Excel

How to Perform Hypothesis Testing in Excel