To know how much weight you are losing (you are losing weight aren't you?) you have to know how many calories you consume and how many you expend. To state the obvious, you take in calories by eating and drinking and you get rid of them by moving. But it isn't always obvious which is greater. Unless you record them. Which is where a fitness tracker comes in. There are many, all varying in different ways - and costs.
I like Fitbit for several reasons. They have excellent customer support and a very large and active user community, which means that anything you want to know or share is out there for anyone to give and take.
I favour the Charge 2 purely because it fits my needs. It has removable straps: some available from Fitbit, and some, like the steel one illustrated, from elsewhere. It's not waterproof (though it is sweat proof), but that's OK because I don't swim. It has a heart monitor, which is important because I need to know how hard I am working. Everyone has a (theoretical) maximum heart rate which can be calculated by deducting your age from 220. I'm 65 so my maximum heart rate is 155. The Charge 2 allows you to see in real time what your heart rate is, and records it to see in the app. Working hard should see you hit the 80% mark.
The most basic way of telling how hard you are exercising is how much you are sweating, how out of breath you are and just generally how knackered you feel. But if you want to be a bit more scientific then the Charge 2 is ideal. You can either choose from a variety of work outs, from spinning to running on a treadmill, from an elliptical to just plain walking. If you don't pre set it, it can (somehow) know which exercise you are doing and record it accordingly.
The other very valuable part of the Fitbit experience is the app that comes with it. That's where all your efforts are recorded. It works out how many calories you have used up, the distance you have moved in steps and kilometres (or miles), your heart rate, your activity minutes and so much more.
You can also input what you have eaten, but I prefer an Under Armour app with the rather cheesey name of MyFitnessPal which has the the rather remarkable ability of being able to recognise almost any food, and to list its calories and nutrients.
So it (or they if you are also using MyFitnessPal) can tell you how many calories you have used versus how many you have taken in. We are fortunate enough to lose a certain number of calories each day just by existing, which varies depending on your size and age. This is known as your BMR. Use this calculator to find out how many for you: http://www.bmi-calculator.net/bmr-calculator/ Add those to those you have lost through exercise and it should, hopefully, amount to a fair chunk. To put flesh on to the bones (so to speak) my BMR is about 1800 calories a day. I burn about 1200 calories through exercise, and I consume about 1800 calories, so I lose about 1200 calories. If you reduce the calories you eat, or increase the calories you burn by 500 each day, this would approximate to 1 pound loss for the week. So my weight loss is about 2-3lbs per week.