One of the traditional ways of measuring heart rate while you're running when you don't have a heart-rate monitor is to stop and measure your pulse for 10 seconds and then multiply that value by 6. This method, however, is not very accurate, because there will be a small error in the 10-second value -- as soon as you stop, your heart-rate begins to decrease, and that small error is multiplied by 6. For example, a one-beat error in your measurement would mean that your resulting heart-rate could be anywhere in the range from 6 beats too low up to 6 beats too high.
Usually, when we're running, we aren't interested in a precise value, just in an approximate number. For example, you might want your heart-rate during a comfortable or resting run to be 70% of maximum. Rather than use the inaccurate method described above, let's use a more accurate procedure by reversing the procedure given above.
1. Calculate your maximum heart-rate as 220 - your age. I'm 71, so my max is 149.
2. Multiply your maximum by the %max as a decimal value between 0 and 1. For 70% I would multiply by 0.7 and get 104.
3. Divide that heart-rate value by 6. I would divide 104 by 6 and get 17.
That value is my 10-second HR for 70% of my maximum. Now, I can stop running and measure my 10-second heart-rate. If I'm close to that value I would know I was running close to my 70% goal. This method eliminates the X6 factor and gives an approximation or "ball park" value, which generally is good enough for recreational runners. If you are more serious in your running, you might consider getting a heart-rate monitor.