By Julie J. -
This is a great question!
The reason Swim.com does not have calorie tracking is that calories burned are extremely difficult to estimate for each individual person and it is only with an accurate heart rate monitor combined in calculation with individual body statistics (height, weight, gender, age, BMI, etc) that you would get a fairly accurate statistic.
Many of the swim tracking devices that come with a heart rate monitor, such as the Garmin Fenix or Forerunner series, are sold with a chest strap that transmits on Bluetooth frequencies that do not work in water. Many wrist-based HR watches have also turned off HR monitoring while doing a lap swim workout because the water can greatly obscure the measurement taken by the optical hardware. There are a few devices out there that will take HR while swimming, such as the Swimovate Poolmate HR, which has a chest strap that transmits on a 122KHz frequency. Garmin also has a HRM-SWIM strap that stores HR data inside the unit and then syncs it to your workout later. These devices, however, are currently niche products and combine for a very small share of the lap swim tracking device market. For this reason, our small development team has decided not to pursue adding data from these devices at this time.
One thing we have done though, is added heart rate support for the Apple Watch 2 only. We were able to do this because Apple did not disable the HR data collection in lap swim recording and because you do not need an additional device, such as a strap, to record this data. One thing to note is that we have come to find through many rounds of development processes that this HR data is more of a ballpark figure and not extremely accurate, and therefore will likely not produce accurate calorie expenditure data.
In short, because of the current market conditions, inaccuracies in data, difficulties involved with the data collection, and because it is not a metric that enhances the friendly competition aspect of our product (leaderboard), we have not yet incorporated calorie data into Swim.com.