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Specificity Calculator

Table of contents

What is the equation for specificity?How do I interpret test specificity?How to use this specificity calculatorOmni tools related to test accuracyFAQs

Welcome to the specificity calculator! We've built it to help you quickly determine how well a diagnostic test can identify true negative cases. In the brief article below, we will answer the following questions:

  • What is the specificity of a test?
  • What is the equation for specificity?
  • How do you calculate specificity?
  • How do you use this calculator?


What is the equation for specificity?

The specificity equation reads as follows:

Specificity = TN / (FP + TN)


  • TN — Number of true negative cases; and
  • FP — Number of false positive cases.

That is, specificity describes the proportion of healthy people that tested negative to the number of healthy people regardless of their test result, i.e., among true negative cases and false positive cases. Therefore, specificity corresponds to the probability of having a negative test result, conditioned on being healthy.

How do I interpret test specificity?

If a test has high specificity, then there is a high probability that a healthy person will get a negative test outcome. In other words, a healthy person will rarely obtain a positive result.

Hence, if a test has high specificity and you get a positive result, you will very likely have the disease. However, if in a test with high specificity, you get a negative outcome, there is no reason to celebrate: specificity does not take into account false negatives!

⚠️ Extreme example: Imagine a test that always gives a negative result. Its specificity is 100% (all healthy people will get a negative result). However, all sick people will get a negative result as well! This test is completely useless despite having 100% specificity.

How to use this specificity calculator

Calculating specificity is very easy in theory, but actual computations may be time-consuming. Omni's specificity calculator will help! You only need to:

  1. Input data: the number of true negatives TN and false positives FP.

  2. The specificity of your test appears immediately!

  3. You can also compute sensitivity, provided that you have the necessary data.

  4. Even more coefficients are available in the More coefficients section of the calculator.


How do I calculate specificity?

To determine specificity, you need to:

  1. Know the number of true negative cases TN.

  2. Know the number of false positive cases FP.

  3. Determine the number of healthy people: TN + FP.

  4. Execute the specificity equation:

    Specificity = TN / (FP + TN)

Table for sensitivity and specificity.

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