What is the difference between bone age and a Tanner Stage?

Last updated on September 5, 2020



I just had a question I was hoping you could answer.  I’m 15 and got a score of 3.7 on the Tanner calculator. I recently got a bone age test done, and my bone is at 13 years old.  I read that boys grow at their peak at bone age 14, so this doesn’t make sense to me: the Tanner scale would put me past the peak of my growth while the bone age would have had me just start my growth spurt.  So I guess my question is about how long I will grow?

Thanks very much for your help clearing this up.


The Tanner Stage of Development Calculator gives an estimated stage number. It can only be an estimate because it is based on your answers to about your body. Since you aren’t a trained observer, your observations may be off. Plus I must describe what to look for and you might not understand exactly what I meant to say. Even then, Dr. Tanner only divided his scale into five numbers. That leaves a lot of leeway in placing someone on the scale. I added the tenths to remove complaints about whether 2.9 was really a stage two or close enough to a stage three. Beyond all of this, the Tanner Stages are based on secondary characteristics. Measuring the extent of your pubic hair or body shape is caused by your development, but they are not the development itself. Since it isn’t a direct measurement, it cannot be precise. Finally, there are actually several scales based on different features being observed. The calculator produces a blended answer to the various scales.

The bone age test is the current “gold standard” for measuring a person’s development, yet it has variations as well. Because it requires people to compare x-rays to an atlas of standard images, you can have two people come up with slightly different results. Most studied variations are within a year.1 But it can be greater because the atlases were developed using children from Britain or Cleveland, Ohio, depending on which method is used. Studies have shown variations when the same data is used with children from other national origins. I also found a study noting that bone age results get skewed with the radiologist knows the chronological age of the person being tested. In other words, no test is perfect, but the bone age test is the best one we have so far. Notice in the table below that the mean bone age is actually a range including up to a year above or below the age given. This results in a particular bone age crossing several Tanner stages. A bone age of 13.0 can put you anywhere from Tanner Stage 2 to Tanner Stage 4.

Bone Age for BoysTanner Stage
Based on Genitals Only
Tanner Stage
Based on Pubic Hair Only
11.4 ± 1.1Stage 2Stage 1
12.0 ± 1.0Stage 2Stage 2
12.9 ± 1.0Stage 3Stage 2
13.9 ± 1.0Stage 4Stage 3
14.4 ± 1.1Stage 4Stage 4
14.9 ± 1.1Stage 5Stage 4
15.2 ± 1.1Stage 5Stage 5

Basically both the bone age test and the Tanner Stage Calculator say you are in your period of rapid growth. The Tanner stages generally last about two years, so you probably have six months to a year of accelerated growth left. You will continue to grow in stage four, but at a slower pace, that will gradually decrease to zero.

One more point to remember, though a bone age value is expressed in years, it doesn’t map to actual years. It might take you two years to move one bone age year, or it might take you six months. This is because the bone age values are based on taking the mean of a large sample of children. It doesn’t directly map to an individual child.

  1. Zvi Zadik, M.D., “Age and Bone Age Determinations: Inaccurate at Their Best“, Journal of Pediatric Endocrinology & Metabolism, 22, 479-480 (2009).