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NNT Calculator

NNT Calculator

Your Outcomes:

Patient Years:

Control Group:


Experimental Group:


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The NNT calculator estimates the effectiveness of a medicine or a treatment in a certain population range of patients.

What Is NNT?

The NNT(Number Needed to Treat) is the number of patients you need to cure or treat to prevent an additional bad outcome(death, stroke). The NNT statistics are done to find the productivity of a medicine or a therapy in a given population of patients. 

How to Calculate NNT?

The formula to calculate the NNT is:

$$ NNT = \frac{1}{ARR} $$

To calculate the NTN, you need to estimate the Absolute Risk of Reduction (ARR). 

$$ ARR = (Control\;event\;rate) – (Experimental\;event\;rate) $$

Example 1:

Find the NNT of group patients if the values of the Control and Experimental are 15% and 10% respectively.


Control group = 15%

Experimental group = 10%


You can find the ARR by the formula:

$$ ARR = (Control\;event\;rate) – (Experimental\;event\;rate) $$

$$ ARR = 0.15 – 0.1 = 0.05 $$

The Absolute Risk Reduction is 0.05

$$ NNT = \frac{1}{ARR} = \frac{1}{0.05} = 20 $$

The NNT is equal to 20

It means you need to treat 20 patients in order to avoid the result in one patient. If you are finding difficulty in estimating the NNT of medicine use a number needed to treat calculator to make your estimation precise, calculate number needed to treat and find the effectiveness of the medicine.

Example 2:

Calculate the NNT of medicine if the 300 patients are treated for 7 years and the control group is 20% and the experimental group 10%.


For calculating the number needed to treat, we first estimate the Ro and R1.

The number needed to treat calculation is:

$$R_0 = 1 – e^(\frac{-Control\;Events}{Time})$$

$$R_1 = 1 – e^(\frac{-Experimental\;Events}{Time})$$

$$ARR = R_0 – R_1 = 0.18221841717416$$

Absolute Risk Reduction (ARR)=0.18

Th number needed to treat formula is:

$$NNT = \frac{1}{ARR} = \frac{1}{0.18221841717416} = 5.4879194732782$$

Number Needed to Treat (NNT)=5.49

The number needed to treat the calculation reflects that you need to treat 5.49 patients in order to avoid harm resulting in one patient.

Working of NNT Calculator:

To use the NNT calculator, follow the directions below:


  • Choose the percentage or the patient years from the drop-down menu.
  • Enter patient-years, control group, and experimental groups
  • Hit the calculate Button


  • ARR and NNT 


What Is Meant by NNH?

The (NNH) means the number needed to harm, it reflects the number of patients needed to treat to show the harmful effects on one patient. For estimating the NNH we recommend using the online NNH calculator for the effectiveness of a medicine on a certain data set.

Is a Low NNT Better than a High NNT?

The lower NNT is better as the higher number indicates the medicine is less effective.


From the source of Wikipedia: Number needed to treat, How to calculate NNT?

From the source of NNT, NNH, Number Needed to Harm Formula