Calculate mAs for radiography by multiplying tube current (mA) by exposure time in seconds, or solve for mA or time, with SID and 15% rule adjustments.

Fractions such as 1/20 s are accepted.

Optional settings

mAs Formula

The following formula is used to calculate milliampere-seconds (mAs).

mAs = mA × t
  • Where mAs is milliampere seconds
  • mA is tube current in milliamperes
  • t is total time (s)

To calculate milliampere seconds (mAs), multiply the milliamps by the time in seconds.

mAs Definition

What is mAs?

One milliampere-second equals 0.001 coulomb of tube charge. For constant current, mAs equals tube current in mA multiplied by exposure time in seconds. At otherwise fixed settings it is related to X-ray output, but mAs alone is not patient dose and does not establish a safe exposure.

Example Problem

How to calculate mAs?

  1. First, determine the total milliamps the x-ray is producing.

    For this example, the x-ray is emitting 300 milliamps (mA).

  2. Next, determine the total time the radiation is being emitted.

    In this case, the x-ray takes a total time of .03 seconds.

  3. Finally, calculate the mAs.

    Using the formula above, the mAs is calculated to be:
    mAs = mA * t
    mAs = 300 *.03
    mAs = 9