Personally I use a heart rate monitor, although I don't have a power meter so it doesn't have all of the metrics for really, really close accuracy, but it still makes it easier. My recommendation would be a HRM with cadence and speed sensor (GPS optional) but if you don't want to go that route take into account not just your distance, speed and/or time (which are the factors that those calculators typically use) but also your perceived effort.

For example, I rode 8.7 miles pretty hard the other day. It was pretty windy out though, and my average speed was only 12 miles per hour (max 18) and it took just under 44 minutes. My Garmin says I burned 360 calories. Using a calculator such as this http://prod.bicycling.com/training-n...ned-calculator I burned somewhere between 296 and 395 (12mph is right on the dividing cusp between light and moderate speed). If I didn't have my HRM I would take into account my effort and go with the higher number, which would put me relatively close to what my Garmin got. If I was right on the cusp and the wind was behind me the whole time or I was coasting a lot I would probably go with the lower number or an average of the two.

Of course this means you need to know your speed, or at least your distance and time so you can get an average.

In the past, I've bought most of my HRMs and equipment on Ebay at a steep discount.