This number needed to treat calculator estimates the NNT value along with CER EER percents, absolute and relative risk reduction. There is in depth information about these medical study indicators below the form.

Control group total number:*
Control group interest cases:
Experimental group total number:*
Experimental group interest cases:*

How does this number needed to treat calculator work?

This is a health tool designed to offer you quick access to the most used formulas in regard to trial studies with control and experimental groups.

There are four types of information you need to input in the number needed to treat calculator: the total number of subjects in both types of groups and the number of subjects with the observed outcome in the studied groups.

You will be then offered a result comprising of the CER, EER, ARR, NNT, RRR and RR values both in numbers and percentages as resulted. The following lines describe the main specifications used.

Describing results of medical studies

Control event rate - percent of subjects from the control group that presented the study’s outcome of interest.

CER = Control group interest cases/Control group total number *100

Experimental event rate - percent of subjects from the experimental group that presented the study’s outcome of interest.

EER = Experimental group interest cases/Experimental group total number *100

Absolute risk reduction - absolute value of difference between CER and EER. Evidences the excess risk in relation to the control group action.


Number needed to treat: the subjects to be given the experimental treatment to prevent the study’s outcome.

NNT = 1/ |CER-EER| or NNT = 1/ARR

Relative risk reduction: dividing the absolute risk by the control event rate


Reported relative risk:

RR: 1-|CER-EER| / CER  or  RR: 1-RRR  

Example calculation

Control group total number: 125

Control group interest cases: 85

Experimental group total number: 110

Experimental group interest cases: 60


■ CER percent (Control event rate): 0.680 (68%)

■ EER percent (Experimental event rate): 0.545 (54.5%)

■ ARR (Absolute risk reduction): 0.135 (13.5%)

■ NNT (Number needed to treat): 7

■ RRR (Relative risk reduction): 0.198 (19.8%)

■ RR (Reported relative risk): 0.802 (80.2%)


1) Laupacis A, Sackett DL, Roberts RS. (1988) An assessment of clinically useful measures of the consequences of treatment. N Engl J Med; 318(26):1728-33.

2) Christensen PM, Kristiansen IS. (2006) Number-needed-to-treat (NNT)--needs treatment with care. Basic Clin Pharmacol Toxicol; 99(1):12-6

04 Apr, 2015