Max Heart Rate

http://www.brianmac.co.uk/maxhr.htm

if your style of training requires you to know this, then i suggest you get a Polar heart rate monitor/watch.
You can get the most basic and it will help you get the most out of your regime and give you real feedback to your fitness.
i use one and find it as good as having an instructor - it will tell you when to push it and when to ease back.......