More than four out of five don't realise that a supermarket ham and cheese sandwich contains more salt than a packet of ready salted crisps, the survey found.
And almost two-thirds don't know that the daily limit on salt is just one teaspoon, the 'Daily Mail' reported.
The shocking findings expose wide ignorance about which foods are healthiest, with three-quarters of those questioned getting low scores in a food Intelligence Quotient quiz of 2,000 adults.
Fewer than one in 100 people managed to answer all 12 questions correctly.
More than half (58 per cent) don't know a fat-free strawberry yoghurt has more sugar than a bowl of cornflakes or black coffee with two sugars.
Altogether 77 per cent got fewer than half the questions right, putting them low on the food IQ scale. One in ten could only answer four questions correctly.
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But the survey, commissioned by the Department of Health's Change4Life initiative, found 84 per cent of people want to be healthier.
The cost of food and the time needed to prepare fresh ingredients deterred many from having a healthier lifestyle.
"It's really important to be aware of what hidden nasties may be in your food," TV chef Ainsley Harriott, the Change4Life campaign ambassador, said.
"There are simple changes you can make which will help: try to prepare food at home, cut down on saturated fat, swap high sugar options for lower ones, watch out for hidden salt in foods and check the label," Harriott said.
The average daily salt intake in the UK is 8.6g, but official figures often don't include salt added at the table or in the takeaway.
The Food Standards Agency recommends a 6g limit for adults and much less for youngsters, the paper said.
Research suggests people who cut back salt by about 3g a day - the equivalent of six slices of bread - can reduce their chances of developing cardiovascular disease by a quarter.
The recommended daily limit for saturated fat is 30g for men and 20g for women.