Measuring your hand span
An easy way to measure your hand span is to use a ruler or measuring tape. Lay this on a horizontal surface and your left hand flat over the ruler with fingers outstretched like a starfish. Align the outside tip of your little finger (pinky) with the zero marking and stretch out your thumb along the ruler until it starts to hurt. Read off the measurement on the outside edge of your thumb, and this will give your active 1-5 span. Repeat the process for your right hand; this time the outside of your thumb will align with the zero mark and your little finger will stretch out along the ruler. To measure your active 2-5 span, repeat the above process using your index and little fingers.
Another way is to use the hand gauge developed by David Steinbuhler to measure pianists’ hands as described above under Earlier hand span studies and shown in the illustration below. You can download this chart here: https://paskpiano.org/wp-content/uploads/2021/09/Hand-guage.pdf but when printing this out, you will need to ensure that the scale is accurate after printing by checking with a ruler.
Also, the stretch between fingers two and five is very important. You can measure this in the same way as shown below.
When comparing hand span data from different studies, it is important to consider measurement techniques. In particular, note that some other academic studies, including those by Wagner (see page: Earlier hand span studies) and Chi et al (see: Comparative studies using alternatively sized keyboards) measured hand spans from the mid-points of the thumb and other fingers, not from the outside edges as in the recent Australian and some US studies. When measured from mid-points, hand span measurements will be smaller overall.