We want to base it off of % and we are not sure if we should do it individually or with teams.

Teams would be better in my opinion... it'll help by creating a situation in which coworkers would have to work together and give one another positive feedback and encouragement. Not to mention, they can also coach each other and be "buddies" through diet and exercises

It might depend on how many people are in your office. Teams could produce a spirit of commeraderie and teamwork. Plus, the group buddy system has been proven to be an effective weight loss method.

I did a "biggest loser" contest at work 2 years in a row. We each put $1 in a box every week, and .25 for every pound gained that week. (Hopefully none).
At the end (we went for 16 weeks) the person with the highest percentage lost got 1/2 the money and the other 1/2 we went and had a few drinks on a night that worked for everybody. You could do it however you want-a 1st, and 2nd place, go out for lunch instead of drinks....
You take your starting weight and at the end take the lbs. lost and divide that by your starting weight. Example: you weighed 180 to start and lost 10 lbs. --- you percentage is 5.6.
Good luck!

Good Idea-
% is a good way-
I think that you should do it both individually and in teams.
The team could "design" the diet approach, encourage everyone as a team to reach certain goals & exercise together at lunch time.
Good Luck

Individually is better. But oh boy.....are you guys a bunch of fat lazy workers? Scary~!

We did one with teams of 4. Did a mileage thing. like 1 mile walked = 1 mile across our state. Whoever got across the state first won!!
1 mile walked or jogged = 1 mile across state

You can make it a team challenge and have a special prize for most weight loss by an individual.

We are doing a health challenge at my office now. We broke into teams. We have a very large company. Every team has a captain. The captain keeps track of their team's points. The members email the captains with their points every Friday. You get points for things like working out, losing pounds, pedometer steps, giving blood, seeing the doctor, fitness classes, etc. We also have a weight loss challenge going on that's individual. Someone weights everyone weekly that's part of challenge. You have to pay a dollar if you gain weight or if you don't lose anything. If you don't weight in for the week you're eliminated.

Do it individually and make it a combo contest. Combine pounds lost and say distance walked or stairs climbed. During the work day and run it for a month. Everyone who takes part should get a little something just for taking part. Then award prizes for most weight lost, distance walked or stairs climbed. Its a sneaky way to improve everyones fittness and many staff may just keep doing it.