The term FTP stands for Functional Threshold Power and refers to the amount of power you can produce for a maximal effort of 1 hour. Now, going as hard as you can for 1 hour isn’t that much fun unless you have some sweet climbs in your area and you like climbing. If you like time trialing, a 40km or 25 mile Time trial will produce a great FTP effort but those types of efforts are not really fun to make on a weekly or monthly basis but they are good to gauge improvement.
Another way to determine your FTP is with a shorter 20 minute all out effort. These efforts can be done on a trainer or outside but you should try to keep consistency with your efforts. This means do your efforts on the same road at the same time of day with the same wind conditions or inside on the trainer with the same build up to the effort. By staying consistent you can compare past results as well as future results and know that you have done everything to make them consistent. A 20 minute effort will produce a number that is higher than what you will do for a 1 hour effort so multiply this number by 95% to get an estimated but very close FTP based on power.
To take it one step further you can compare yourself to other riders FTP by looking at the watts/Kilogram you produce. To do the math you must first convert your weight in pounds to kilograms which is done by dividing your weight in pounds by 2.2lbs per 1kg to get your weight in Kilos. Now take this number and divide your FTP power number from your effort by your weight in kilograms and you will get your watts/kg number.
Example: Rider A weighs 145lbs and produced 350 watts for 20 minutes
FTP Calculation: 350 x .95 = 332.50 watts as estimated FTP
Weight from lbs to kgs Calculation: 145lbs/2.2kg =65.91kgs
watts/kg Calculation: 332.5/65.91kgs =5.04watts/kg
Why does watts/kg matter?? Generally bigger riders weigh more than smaller riders and produce more raw power, but raw power isn’t everything unless you are doing Paris Roubaix or blasting into a headwind on a flat road in Nebraska. As soon as the road starts to tilt upwards gravity starts to work and bigger riders need more power to overcome the pull of gravity, smaller riders need less power to go the same speed. Now the rider with the higher watts/kg will be able to climb faster and recover quicker.