You enter age, weight, height and gender, but I presume it’s still guessing somewhat when calculating calories burnt?

So, how accurate are they?

It’s a Polar F4

Not at all.

Polar are generally reckoned to have the most accurate algorithms. I reckon you can half anything Garmin tells you and Endomondo just works it out on duration (so you’re still burning calories sitting outside the pub on your mid-ride break).

Edit: probably not too bad then

See it’s told me 1040 kcal for 1 hr constant uphill cycling, plus 10 mins back down again. That seems way too high.

It is weight dependent 🙂

Mmmm, I’m not skinny 🙂

They suck.. I’m pretty fit so register as obese on the BMI scale but I can run on the crosstrainer for 30 mins and burn over 600 cals (after adding in weight/height/age). I wasn’t sweating half as much as I should be for that figure..

Completely unscientific, but there are a few online calorie calculators that should give some alternative answers.
http://www.healthstatus.com/cgi-bin/calc/calculator.cgi
http://www.nutristrategy.com/fitness/cycling.htm

Cheers I’ll take a look