The new BBC calculator can tell you if you are a healthy weight for your height and how you rank against others globally.
Using your age, sex, nationality, height and weight, it will come up with a number representing your Body Mass Index or BMI.
This is a measure doctors use to gauge obesity.
The app plots your BMI and shows how you compare to people from your own and other nations.
Researchers see global weight gain as a bigger threat to mankind than population growth.
As well as the health implications, experts are also concerned about the environmental impact.
The adult human population has a combined weight of 287,000,000 tonnes, researchers say.
Increasing obesity could have the same impact on global resources as an extra billion people, they believe.
We asked five people on the streets of London to give the app a go.
App rating: Most like average person from Morocco
"I was happy to hear I am in the normal range. I kind of knew that. I try and keep fit and eat healthily. I have sort of heard of the BMI, but didn't really pay much attention to it. But now I know the ranges, its really useful information to know.
"Now I will be more mindful of my weight and keep an eye on BMI.
"It's a really cool thing, I guess. Will I use this site? Not sure. Now I know my BMI and it's OK, would I want to use it again? Well, I will check in from time to time, I guess.
"This information is not going to change my eating habits drastically. But this information is good to know and I will share this with my friends and get them to use the site."
App rating: Most like an average person from Tonga
"Wow, I was shocked to see how high my BMI was. I'm aware I'm overweight. I guess I hadn't given it that much thought and thought about it globally. Its interesting that I am like someone from Tonga. Goodness, imagine that.
"I've heard about BMI and its good to know what mine is. I should lose weight and when I do I will use this site to see how I stand globally. I think that's the most interesting part.
"I will change my diet now I know my BMI, but I will not be thinking of it while I'm in London on vacation.
"Yes, I will tell my friends about this site and my daughter especially."
App rating: Most like an average person from Austria
"I am really happy to know what my BMI is and I'm like an Austrian. They are chic, aren't they? I am surprised I have a BMI of 23 as I always want to be 7lb lighter.
"I know about BMI as when I was younger I was considered overweight.
"It's good for me to know I'm not overweight. It makes me feel happy and I want to maintain this healthy weight. I want to remain healthy.
"This is good and useful information. I have used similar ones before on various health websites when I've tried to lose weight. I will not change my eating habits a huge amount considering I'm average, but knowing this information makes me conscious of my health and still want to lose 7lb."
App rating: Most like an average person from Albania
"I didn't think I was really overweight. I've put on 5kg recently. I have a very physical job – I carry a lot of heavy stuff so I feel I am very muscular. I have a lot of muscle and I know that weighs a lot.
"I guess I am a bit overweight anyway. This tells me I am and I need to take care of myself. I have to take care of what I eat. I eat a lot I think. But I eat a lot of fruit and vegetables as well.
"This is very interesting and is good to know how you are, so you can take care of your body and not have heart attacks and high blood pressure.
"I will change the way I eat a bit and I will tell people about this site. It's made me think a little about my health."
App rating: Most like an average person from Croatia
"I know I was a little overweight, but I thought it would be a larger figure. I was so surprised that I was like someone from Croatia. Yes, it makes sense to know your BMI. It makes sense to know about your weight.
"It's a useful tool and it will make me think about losing a bit of weight in the future. Yes definitely, I need to go to the gym. I am a bit alarmed at my BMI.
"In my country, you have to go to specialist areas to get this sort of information. It's great that anyone can get this sort of information by just tapping in weight, height info.
"I will change my eating habits and think about exercise a bit more now. I will tell my friends back home about this and encourage them to do it."
App rating: Most like an average person from Vietnam
"I'm really surprised. I was eating normally and healthily and doing the right exercise.
"I don't feel the BMI says anything, as long as I feel healthy. I have done the BMI test previously. I feel healthy. I don't look at my health by looking at my BMI.
"I think the interesting bit is where you are placed globally. I was told I was petite. I am like someone from Vietnam. I am really surprised.
"I will use this site again and tell my friends about it. I will put it on Facebook as it's something people will like to do. I know my friends will use it. It's a really useful tool."
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