Ever felt the irritated sink of disappointment after opening a packet of chips and seeing half the bag is empty?

Well, there's a reason for the dead air, and it's not that snack companies are trying to rip you off.

The Herald on Sunday looked at 10 pre-packaged snack foods to see how much of a packet was empty space.

Eighty per cent were more empty than full, with only two items containing food which took up more than 50 per cent of the space inside.

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Food is sold by weight in New Zealand and all products included the weight they were supposed to be on the packaging.

We compared a variety of crackers, chips and chocolate.

To calculate our numbers, we measured the gap between the top of the bag or box and the point where the food inside began once the box or bag had been gently shaken to settle its contents, and worked out what percentage that was of the full length of the box.

Our testing method was explained to the producer's of the products.

Of the four large bags of chips tested all were about 60 per cent empty.

Of the three boxes of crackers we tested, two were also just over half empty, and one was 65 per cent empty.

Chocolate generally bucked the trend - a box of Cadbury Favourites and Cadbury Roses were each less than a quarter empty, but a bag of Lindt chocolates was 69 per cent dead air - the most of any of the products.

Massey University lecturer Michael Parker said he could understand customer consternation at half-empty chip bags, but explained having air in the packets was vital in protecting snacks during production and transportation.

"It's providing cushioning throughout the distribution chain," he said.

Parker, who lectures on food packaging, didn't think companies were trying to be misleading.

"The people I've had contact with, they're always trying to reduce the head space in the pack because consumers do look at it and think 'wow, I'm being ripped off'."

Companies had incentives to reduce the size of their packaging as much as possible so they could fit more units per crate, reducing transport costs, he said.

Food also settled in the bag over time, meaning when it reached customers it took up less space than it had during production.

When bagged food was on the production line, there needed to be ample space in each packet so the machinery clamping the ends shut didn't catch any of the product inside.

"If you have the bag too short then the jaws will chop some of the chips," Parker said.

Several snack food companies, including PepsiCo, Bluebird and Dorito's parent company, and Arnott's, agreed air was essential to protect product.

In some cases the 'air' was in fact nitrogen which kept product fresh, the companies' spokespeople said.

A Cadbury spokesman also cited settling and product protection as reasons for gaps and a Lindt spokeswoman said packet size was "always influenced by the packaging process".

Nigel Cranston of 180 Degrees, which make Lavosh crackers, echoed the above.

He said packet size was also influenced by the standard sizing of various pieces of manufactoring equipment.

Griffin's, which makes Snax and Eta potato chips, did not respond by deadline.

Michael Seaton, marketing director at Kiwi-owned corn chip company Mexicano, said crushed chips were no good for anyone.

"They [the chips] really take a hammering - there's just so much handling," he said.

"Consumers might complain about having enough product in the bag - but they go crazy if they get crumbs for chips."

The amount of complaints directed to Consumer New Zealand about the amount of chips in a packet were low, CNZ adviser Maggie Edwards said.

"But the weight is on the package, so that covers them."

The main concern Consumer NZ had was if companies were reducing the weight of their product and while keeping the size of the bag the same, but this did not happen too frequently, she said.