Sensitivity & Specificity Calculator

How it is calculated

You enter the four cells of a 2×2 table of counts: TP (true positives), FN (false negatives), TN (true negatives) and FP (false positives). The tool then calculates sensitivity = TP ÷ (TP + FN), specificity = TN ÷ (TN + FP) and accuracy = (TP + TN) ÷ (TP + FN + TN + FP), each multiplied by 100 and shown as a percentage with two decimals. The inputs are numbers of people or cases, not percentages. There is no result if a count is negative, if there are no actual positives (TP + FN = 0) or if there are no actual negatives (TN + FP = 0). The tool does not give predictive values; those depend on how common the condition is in the group tested.

Common mistakes

• Putting the cells in the wrong places. Swapping FN and FP is the most common error, and it turns a test that misses cases into one that raises false alarms.
• Typing percentages instead of counts. If you type 90, 10, 80 and 20 from a published sensitivity and specificity, the tool treats them as 200 people split evenly, so the accuracy is wrong whenever the real groups differ in size.
• Reading accuracy as the whole story. When the condition is rare, a test can have high accuracy and still miss most real cases, so look at sensitivity and specificity separately.
• Assuming sensitivity tells you the chance that a positive result is correct. That is the positive predictive value, which this tool does not calculate and which depends on prevalence.