Among those who eat out most frequently, that figure rises to around 30%.
Researchers found much of the food consumed outside the home is purchased “on the go” from convenience stores and supermarkets, while cafés and coffee shops were also among the most commonly used outlets.
The report highlights concerns about the wider food environment, including the prevalence of high-calorie options, meal deals and the promotion of foods high in fat, sugar and salt.
Fast food outlets were found to serve some of the highest-calorie meals, with nearly half of takeaway meals analysed containing more than 1,200 calories — more than half the recommended daily intake for an adult.
Food Standards Scotland is calling for action from government and the food industry to help improve dietary health.
Recommendations include making nutritional information more accessible, reducing portion sizes, promoting healthier menu options and reformulating recipes to improve their nutritional value.
The findings come as almost one in three adults in Scotland are living with obesity, while nearly one in five children are considered at risk.
Laura Wilson, head of public health nutrition at Food Standards Scotland, said: “Eating out of home is now a routine part of everyday life in Scotland, not just something we do on special occasions.
“While it can be convenient and enjoyable, the food on offer often makes it difficult to eat well.
“When more than a fifth of our daily calories come from out of home, the options that are available, promoted and how they are priced really matter for our health.”