Every week, our nutrition team receive e-mails from frustrated dieters who just don’t seem to be able to lose weight no matter how hard they try. They focus on their weight loss, exercise regularly and stick rigidly to their meal plans. At least, they think they're sticking to the plan...

One of the first questions we ask is 'Do you weigh your food?' When we get the answer 'no' we need to ask, 'So how do you know if you are eating the right portion size?' The truth is that many of us don't and there is good reason why not.

We live in a society where restaurants serve huge portions. Drinks are served in ½ litres and litres, and you can super-size it a few extra pennies. With excessive portions becoming the norm, it's extremely difficult to discern what actually constitutes a normal portion and how much you are really eating.

Being able to estimate portion sizes is one of the keys to weight control and weight maintenance. Weight loss depends on energy balance which is the difference between calories eaten and calories used through daily activity. Understanding just how much you are actually eating is vital if you want to get a good idea of how much fat and calories you are taking in.

The following guidelines can help you estimate portion sizes. They are not intended to restrict how much you eat - you should be eating until you are satisfied - but they can be useful if you need to stop overeating.

100g of meat, poultry or fish is the same size as:
a) the palm of your hand
b) a deck of cards
c) a cassette tape

A medium potato is the same size as a computer mouse

40g of cheese is the same size as a small matchbox

1 medium piece of fruit is the same size as your fist

50g (2 oz) of spaghetti has a diameter of ¾ of an inch

A serving of crisps is around 25g or approximately 16 crisps (look out for 25g, as opposed to 40g, bags of crisps)

A serving of uncooked rice is half a cup

A serving of cake is usually 1/8, 1/12 or 1/16. Check the recipe or box for the recommended portion size and stick to just the one slice.

A serving of vegetables is about 80g but can be more because apart from starchier vegetables like potatoes, peas, parsnips and corn, we should add unlimited veg to our plates.

A serving of biscuits is usually 1-3 biscuits depending on the type. That could be 1 chocolate chip cookie, 2 jaffa cakes or 3 malted milks.

A teaspoon of butter or margarine is the size of the tip of your thumb.

The difference between the correct portion and a large portion can mean the difference between gaining weight and reaching your target. Yes, it's that important.

So get those portions right. Weigh or measure your portion once and remember how it looks. Use the same bowl, plate or cup to make things easier for yourself. See the difference on the food scale and you'll see the difference on the weighing scale too.

If you think these tips might help some of your friends and family, please feel free to share it with them. Email it, tweet it or post it to facebook and you could help others to lose weight, look great and feel amazing.