Guidelines say that meals should not exceed 600 calories, but in this study they averaged 1,033 in restaurants and 751 in fast-food chains.
Dr Eric Robinson, lead researcher from Liverpool's department of psychological science, said to the BBC that the results were "shocking" but probably underestimated the calories consumed in restaurants.
"We don't know about energy intake but 'plate clearing' is a common behaviour.
"Our analysis did not include drinks, starters, desserts or side orders.
"It's really clear what the food industry need to do. They need to act more responsibly and reduce the number of calories that they're serving."
University of Liverpool researchers analysed meals from places like Hungry Horse and McDonald's.
The researchers looked at more than 13,500 meals on the menus of 21 sit-down restaurants and six fast-food chains.
By using online company information on calorie content, only one in 10 meals was classed as ‘healthy’ or fewer than 600kcal, as recommended by Public Health England.
Nearly half of the meals contained 1,000kcal or more.
It found that sit-down restaurants were five times more likely to offer high-calorie meals of 1,000kcal or more than fast-food restaurants.
Hungry Horse restaurants had the highest average meal calorie content of 1,358kcal.
Chains including Flaming Grill, Stone House and Sizzling Pubs has an average of 1,200kcal per meal.
KFC topped the fast-food list with an average of 987 calories per meal offered. Burger King, McDonald's and Subway were around 700kcal.
Even when the study compared similar meals, the energy content in restaurant meals was greater.
Burger meals in restaurants contained an average of 414kcal more energy than burger meals in fast-food chains, while salad meals in restaurants had 142kcal more energy than fast food salads.
The government is currently consulting on a plan to introduce mandatory labelling in restaurants, takeaways and cafes, which is likely to finish in the new year.
Kate Nicholls, chief executive of UKHospitality, said restaurants, pubs and other hospitality businesses were already taking action to reduce calories and offer healthier dishes - but there were costs to consider too.
"Proposals to shrink the size of dishes or cap calories would be yet another burden for hard-pressed operators to absorb, resulting in prices going up and investment in businesses going down; inevitably negatively impacting the overall customer experience."