The 15% Rule and the Not So Famous Bowman’s 7 1/2% Rule

The 15% Rule and the Not So Famous Bowman’s 7 1/2% Rule

I believe the 15% rule was figured out in the mid 50’s. Of course this was back in the days of film/screen, but it still holds true today in the digital world.

The “Rule” stated that if you increase any given technique by adding 15% more kV the following film would have twice the density/opacity (which we will now just call opacity) as the original film. And that’s all there is to the rule.

Now what most people have done with it is cut the mAs in half after they increased the kV 15%, but that’s now an addition to the 15% Rule (which we will now call the 15% Rule with mAs compensation). And a great addition it is. It was a way to change the technique but still end up in with basically the same opacity that you started with. This is because when you increase the kV 15% the film was double the perfect opacity from what you wanted, but then by cutting the mAs in half you also cut the opacity in half thereby ending up right where you began but with a new technique.

What must be mentioned here is from the many experiments I performed on both the 15 and 7 1/2% Rules (I’ll get to that one), the Exposure Index (EI) number stayed the same or was very close. This means that after changing the techniques with either or both rules your EI number will still be correct!!

There are really 2 main reasons why someone would do the 15% Rule with mAs compensation. The first is to cut the patient dose. To read more about this, please refer to Blog #12 from 6/15/2013 (How Low Can You Go?) where I explain that by increasing 15% more kV and cutting the mAs in half you can save your patient almost 33% of the Entrance Dose!!

The second reason would be to cut the time for the exposure. This is really only needed on portable machines that have a built in 100 mA station (which is most portables on the market). 100 mA means if your technique has 200 mAs then your exposure time will be 2 seconds, a 50 mAs exposure would be 1/2 a second and so on. This usually happens when doing portable abdomens on a patient who is unable to follow breathing instructions and your exposure time is over 1/4 of a second long. I’ve always taught (even back in the film days) that an image taken out of the optimum kV range which is possibly or even definitely longer scale (greyer) than normal is always better than an image with motion. In my opinion the only thing worse than motion is actually cutting off the anatomy.

So now let’s quickly discuss the not so famous (yet!!) Bowman’s 7 1/2% Rule. As you will see it is simple and easy to do and quite often the perfect thing to use when changing techniques. In a nutshell, all we are going to do is use half of the 15% Rule. Since it’s easier to see it with actual techniques, let’s start with:

80 kV @ 40 mAs
92 kV @ 20 mAs = 15% Rule with mAs compensation
86 kV @ 30 mAs = 7 1/2% Rule and cutting out a quarter of the mAs

All that you need to do is first figure out what 15% of the kV is and what half the mAs would be and then just use half of each. As you can see from the above example; 86 is exactly between 80 and 92 and 30 is exactly between 40 and 20. With film all three of these techniques would have had the same basic opacity. In the digital world (both CR and DR) the EI number will be the same.

So why would use the 7 1/2% Rule? If you are still using low kV/high mAs techniques and now are willing to change, sometime doing the 15% Rule with mAs compensation will still not increase the kV high enough. But if you did it all again the kV would now be too high, but just doing the 7 1/2% Rule will be perfect. Using the same example from before we see:

80 kV @ 40 mAs
92 kV @ 20 mAs = 15% Rule with mAs compensation
106 kV @ 10 mAs = 15% Rule with mAs compensation
99 kV @ 15 mAs = 7 1/2% Rule and cutting out a quarter of the mAs

So if you decide that you are unwilling to go above 100 kV for a particular shot, then using the 7 1/2% Rule would be perfect as it takes you to 99 kV. Of course this works with any starting kV or mAs and is applicable anytime you don’t want to increase your kV a full15%.

If you have problems figuring out 15% of a number, I have a chart already made that shows 15% from 50-120 (see below). To download this chart, go to: There also is a column that shows how much more opacity would be added if just 1 kV was added. I have a whole discussion on that column in my full day lectures, but since that would take another page, for now I’ll just call it a day.

1 kV =

Leave a Reply

Your email address will not be published. Required fields are marked *