Growth rate, or percent change, is a result of a change in size over a certain amount of time. This can be a relatively easy thing to calculate, provided you have all of the information needed to make a good calculation. The way that you calculate will also change based on whether you have two observations of weight/size over time, or if you have more. Three or more observations doesn’t mean more work, necessarily.

1. The simple formula for two observations looks like this: growth rate equals second size measurement minus first size measurement divided by length of time. So, if you have a baby who weighs fourteen pounds at six months and sixteen pounds at nine months, then the growth rate is 16 pounds-14 pounds/3 months. The rate of growth is 2/3 pounds, 8 ounces, or 0.66 pounds per month. To calculate the percent change, then you take a look at how big this answer, 0.66 pounds, is compared to fourteen pounds. So, 0.66 divided by 14 is 0.047. If you want to turn this into a percentage, then you multiply by 100. 0.047 multiplied by 100 turns out to be 4.7, so the percent of change for this baby is 4.7% of its body weight in three months.

2. If you have three or more measurements, then in most circumstances, you can ignore the mi. The formula becomes a little bit more complex here. Take the same baby at its twelve month doctor’s checkup. Now the baby weighs 23 pounds. You can still use the same formula, just skip the middle number, and use the last number minus the first. So, 23-14 is 9 pounds/6 months, or 1.5 pounds per month.

3. If you have many observations, sometimes it helps to calculate the growth rate with a graph. Plot all of the numbers on the graph, and you will get something that is called a best fit line. If this line is straight, then the rate of change will be simple, a number of pounds per month. But sometimes the line is curved, or will be straight for a bit and then curve almost straight up. When this happens, you have a more complicated rate of growth, which will turn into an algebraic equation like y=x squared. This is the way that bacteria populations grow, for example. These are not as easy to calculate out, but usually require using the equation to predict the growth at any point in time.