You follow the rules: you eat your greens, you skip the junk.

So why is that waistband a little snugger than you'd like? It may not be what's on your plate, but how much.

For when it comes to portion size, it seems we've lost all sense of, well, proportion.

'Most people don't know what an appropriate portion should look like,' says Sian Porter, a consultant dietitian and spokesperson for the British Dietetic Association.

'But even healthy food contains calories. You can make really healthy choices and still eat too much.'

When it comes to portion size, it seems we've lost all sense of proportion - with many of us clueless as to what constitutes the correct serving size

Apart from the fact that you should have five 80g servings of fruit and veg a day, there are currently no official UK guidelines on portion sizes - or rules for food manufacturers to tell you what counts as a serving.

Indeed, a 2013 report from the British Heart Foundation which looked at how portion sizes had changed over 20 years found that ready-meal portions for dishes such as lasagne had increased by as much as 50 per cent.

Meanwhile the size of a typical digestive biscuit has gone up by 17 per cent - so eating just one biscuit a day now, compared with in 1993, would add 3,330 calories to your diet each year.

'Plates and wine glasses keep getting bigger, too,' adds Sian Porter.

Research has shown repeatedly that we are no good at working out how much food should be on our plate - study participants frequently over-estimate serving size, under-estimate calorie content, and fail to compensate for large helpings at subsequent meals.

It's not entirely our fault. The guidance we're given is pretty hopeless, suggests a 2012 paper published in the journal Proceedings of the Nutrition Society, which reviewed scientific studies into portion size and all the official advice.

For instance, we're told to eat two portions of oily fish a week, but not how much that actually is.

Even the NHS's 'Eat Well plate' - a visual tool showing a plate divided up according to food groups - doesn't specify exact amounts, only rough proportions.

It says we should eat 'plenty' of vegetables, 'plenty' of starchy carbs such as pasta and rice, 'some' protein such as chicken and fish, 'some' milk and dairy and 'just a small amount' of fat and sugar.

And while fruit or vegetable portion sizes are specified - 80g is one of your five a day, says the NHS - even if you could work out what 80g of broccoli would look like, what about 80g of spinach or 80g of blueberries

Apart from the fact that you should have five 80g servings of fruit and veg a day, there are currently no official UK guidelines on portion sizes - or rules for food manufacturers to tell us what counts as a serving

What's more, nutritional content and serving size on packaging is listed in grams, yet few of us bother weighing out food.

So how can you work out how much to eat, without calorie-crunching or taking scales to the supermarket?

Good Health has the answer at hand - literally. We asked Sian Porter to work out what an appropriate portion of basic foods should be and how this looked relative to the size of your hand.

For example, a serving of carbohydrates should be the size of your fist.

'The obvious advantage of using your hands is that you always have them with you,' says Sian Porter.

'Plus it's proportional. If you're a bigger person, you'll need a bigger portion, but your hands will be bigger so the portion is adapted automatically.

'Likewise, children need child-size portions, the size of their hands.'

MEAT: PALM OF THE HAND

The steak pictured is about 100g and the thickness of a deck of cards

A serving of any meat should be the size of the palm of your hand (but not your fingers).

The steak pictured is about 100g and the thickness of a deck of cards.

'Aim to have a portion of protein this size at every meal - you should spread protein throughout the day as we process it better in smaller, regular amounts,' says Sian Porter.

'But don't have more than 500g of red meat in a week. 'Choose other protein such as fish, beans, or pulses.'

WHITE FISH: WHOLE HAND

For white fish, the portion can be the size of your hand when laid flat, including your fingers

White fish such as cod, haddock or pollock is very low in fat and calories so the portion can be the size of your hand when laid flat, including your fingers (about 150g and 100 calories).

'White fish is great, because its protein is naturally low in fat,' says Sian Porter.

'It has only a small amount of omega-3s, but is a good source of selenium, important for the immune system and healthy hair and nails.'

UNCOOKED SPINACH: TWO DOUBLE HANDFULS

You should have vegetables with every meal and, as the picture shows, not just a couple of slices of lettuce

This is how much raw spinach you need for one of your five a day (80g) - practically a whole bag - and the same serving size applies to any salad leaves.

'You should have vegetables with every meal and, as the picture shows, a couple of slices of lettuce in a sandwich won't cut it,' says Sian Porter.

'So buy a pot of salad to have on the side.'

SMALL FRUITS: TWO CUPPED PALMS

A packet of blueberries is about 250g, which is three portions - so you don't have to eat the entire punnet

An 80g five-a-day portion of small fruit such as berries (or larger fruit cut up in a fruit salad) is roughly what you can fit in your cupped hands.

'A packet of blueberries is about 250g, which is three portions - so you don't have to eat the entire punnet,' says Sian Porter.

'There'd be no harm eating this much (it would give you around 90 calories), though grapes would have more sugar and 161 calories.'

VEGETABLES: CLENCHED FIST

Twice this amount of broccoli would technically count as two of your five a day, though variety is key

To count as one of your five a day (80g) a serving of veg needs to be at least the size of your fist.

'Twice this amount of broccoli would technically count as two of your five a day, though variety is key - aim for a rainbow selection of different coloured veg ' says Sian Porter.

'Have several portions of veg - they should fill half a plate.'

UNCOOKED PASTA: CLENCHED FIST

Carbs, for energy and fibre, should make up just a quarter of your plate

This might look small, but pasta doubles in weight once cooked, as it absorbs water. There's 75g here, giving 219 calories.

A portion of uncooked rice is also the size of your fist.

Carbs, for energy and fibre, should make up just a quarter of your plate (protein should make up another quarter, the rest should be veg).

More than this will pile on calories from extra sauce, too.

NUTS: ONE PALM

A good portion is what you can hold in a cupped palm. 'Try to eat nuts and seeds one by one'

'Nuts and seeds are a great snack, they're filling and contain heart-healthy unsaturated fats, but they are calorific,' says Sian Porter.

A good portion is what you can hold in a cupped palm.

'Try to eat nuts and seeds one by one, spaced out, rather than a few at once,' she advises.

POTATO: CLENCHED FIST

The potato here is 180g giving 175 calories, but baking potatoes can be twice as big - so think about sharing

'A portion of carbs should be around 200 calories (250 for a man),' says Sian Porter.

'The potato here is 180g giving 175 calories, but baking potatoes can be twice as big - so think about sharing one between two.'

It's the same for sweet potatoes - but unlike white potatoes these would count as one of your five-a-day.

OILY FISH: PALM OF HAND

One portion a week would give you enough heart-healthy omega-3 fatty acids in your diet

Like meat, a serving of oily fish such as salmon, mackerel or sardines should be the size of your palm.

The fillet here weighs about 100g and would provide around 200 calories - one portion a week would give you enough heart-healthy omega-3 fatty acids in your diet.

'Unless you're trying to lose weight, a slightly bigger portion won't do you any harm,' says Sian Porter.

BUTTER: THUMB TIP Any fat - butter, oil, and spreads such as peanut butter (shown here) - should be a serving no bigger than a teaspoon, or the size of the end of your thumb, from the knuckle to the tip of the nail, and no more than two or three portions a day. CHOCOLATE: INDEX FINGER A piece of chocolate the size of your index finger works out at around 100 calories (or about 20g - if you're a bigger person you'd get slightly more), and this would be an appropriate treat.

Any fat - butter, oil, and spreads such as peanut butter (shown here) - should be a serving no bigger than a teaspoon (LEFT), a piece of chocolate the size of your index finger works out at around 100 calories (RIGHT)

CHEESE: TWO THUMBS Cheese should be around 30g, the length and depth of both thumbs. There are around 125 calories here, giving a third of your daily calcium. 'This shows you could easily eat 100 calories without thinking,' says Sian Porter. The same amount grated will go further, making a heap the size of your fist. CAKE: TWO FINGERS A piece of cake should be the length and width of two fingers. (One end can be a bit fatter than two fingers if you're cutting it in a wedge). This makes it around 185 calories (200 for a bigger person) - fine as a treat or snack.