To Manually Calculate a QTc

By Peter Bonadonna, EMT-P







Example:  A patient has a heart rate of 80 with a measured QT of 0.36 seconds


Step one:  If the patient has a heart rate of 80 bpm you should find that the number of small boxes between QRS complexes is 18.75  (to mathematically get there  (1500 divided by HR = small boxes)

Step two: To find RR in seconds, multiply small boxes between RR (in this case 18.75) times 0.04 seconds.

18.75 times 0.04 = 0.75 seconds  so the RR interval is 0.75 seconds  (or your averaged value)



Step three: Find the square root of the RR interval .  Use a calculator !  In this case the square root of 0.75 is 0.8660254



Step four:  Divide the Measured QT by the Square root value so if, for example, the measured QT is 0.36 seconds you would find the answer to be 0.4156921  or simply round to 0.42 seconds (a normal QTc value)



To double check your work, you can visit and visit the reference page. Here you will find an online calculator and graphics to double check your work.   Please note that the answers on this site are rounded to the one hundredths location so your number on the calculator will vary slightly.