how high must the heart rate be to be able to lose fat?

That's not a question that can be answered in a vacuum. Your maximum heart rate, physical condition, age and other personal limitations define the answer.

ANY activity burns additional calories. Think of food as fuel. Anything you do requires fuel. If you take in less fuel than you need in a day, your body turns to stored fat to make up the difference.

Fuel requires oxygen to "burn". Oxygen is transported via your cardiovascular system. A higher heart rate during exercise indicates you are burning more fuel, and thus requiring more oxygen.

To burn more calories, exercise harder for a longer period of time.

If you go hard enough, you will exceed your heart and lungs ability to transport oxygen. This is known as lactate threshold. At this point, you are living on borrowed time. Before very long, the lactic acid buildup will end your workout, race, or flight from the police when your muscles cramp up.

Fat burning zone, which you are alluding to, is kind of a misconception. There is a point, around 60-70% of your max heartrate where you are burning the most fat (triglycerides) relative to total calories burned. However, if you go harder than that you WILL burn even more fat, even though its percent of total calories burned will be lower.

Bottom line is, go as hard as you can without "blowing up" before you finish whatever your workout is.

If you're really interested in heart rate related workouts, get a monitor and Sally Edwards' Heart Rate Monitor Book. Be prepared to spend several months training with the monitor before you figure out what all the numbers mean. The irony is, by the time you've put in enough workouts to do that, you will no longer be fat and won't care what the 'fat burning zone' is.

To lose fat your heart rate should be between 60 and 70%. The percentage is based on your age and fitness level. For someone in their teens it will be almost 30 beats for ten seconds. If you are in your sixties it will be around 12 beats for ten seconds.

3 beats per second

about 3 beats pur second.