Several years ago, Bicycling magazine published the following data:

MPH	KPH	Calories / lb-min
8	12.9	0.0295
10	16.1	0.0355
12	19.3	0.0426
14	22.5	0.0512
15	24.1	0.0561
16	25.8	0.0615
17	27.4	0.0675
18	29.0	0.0740
19	30.6	0.0811
20	32.2	0.0891
21	33.8	0.0975
23	37.0	0.1173
25	40.2	0.1441

Add 22 calories/hour for each 100 ft of elevation

To translate this into a calorie burn rate, multiply the calories/lb-hr number by your weight in pounds and by the number of minutes you rode. For a 150 lb rider near sea level, cruising at 25 km/hour burns about 500 calories per hour. Riding at 35 km/hour burns about 900 calories per hour under the same conditions.

Your body's carbohydrate reserves amount to two or three thousand calories, and your fat reserves are more like 70,000 calories. For a typical rider, about 21% of your energy comes from fat, on average, and the rest from carbohydrates; with more carbohydrates being burned at higher levels of exertion. caffeine will increase the percentage of fat consumed for energy to 40% for the same exertion. When you run out of carbohydrates as an energy source, you "bonk" and have to slow right down unless you replace the energy as you use it.

This is just a rough estimate; variables such as the bike, terrain, hills, the aerodynamic efficiency of your tuck, and how efficient your body is will all impact the amount of calories burned quite substantially.

Assuming an upright position on flat terrain, with no wind:

Rider Weight	12 mph	14	15	16	17	18	19
110	293	348	404	448	509	586	662
120	315	375	437	484	550	634	718
130	338	402	469	521	592	683	773
140	360	430	502	557	633	731	828
150	383	457	534	593	675	779	883
160	405	485	567	629	717	828	938
170	427	512	599	666	758	876	993
180	450	540	632	702	800	925	1048
190	472	567	664	738	841	973	1104
200	495	595	697	774	883	1021	1159