https://www.omnicalculator.com/sports/calories-burned
Enter your weight, duration, and activity and it calculates it.
I use this for yearly calorie burn for all my activities. This allows me to adjust my weight and speed year-to-year as a more accurate judge of expenditure.
I used to average 19.7mph on the bike, now I’m in more 18.8mph. In my IM years I weighed 165-ish now I’m 180-ish. Biking the same distance at the new weight and time is 20% more calories burned by that calculator.