How to log a meal you didn't weigh

If you've used a calorie app before, you may remember the kitchen scale coming out at every meal, and the week you stopped bothering. Weighing gives the most precise figure, but plenty of meals happen where a scale can't come along, at work or at a friend's table. You can still log those meals. A careful estimate is usually close enough to be useful, as long as you know roughly how far off it might be and where the misses tend to hide.

Start with numbers someone has already worked out

Packaged food is the easy case. The label gives energy per 100 g and often per portion, so if you ate half a 400 g tub of soup, you can halve the figure for the pack. Even a label is an estimate. European Commission guidance for the authorities that check food labels accepts that foods can't always contain exactly what the label says, because of natural variation, production and storage. For fat, protein, carbohydrate or sugars at 10 to 40 g per 100 g, it allows a margin of 20% either side of the declared value (European Commission).

For home cooking, treat a batch as one thing. Add up what went into the pot, say the pack of mince, the tin of tomatoes and the oil, then divide by the number of portions it made. If a chilli filled four bowls and you had one, you ate a quarter of the pot. You do the sums once, and every leftover portion is already worked out.

Restaurant meals often come with numbers now. In England, food businesses with 250 or more employees have had to show calories on their menus since April 2022, next to each dish, along with the statement that adults need around 2,000 kcal a day (Department of Health and Social Care). In the US, chains with 20 or more locations must list calories on menus and give fuller nutrition information on request (FDA). Those figures are fairly good on average. When researchers measured 269 restaurant items in a laboratory, stated and measured energy were about 10 kcal per portion apart overall. Still, 19% of the items contained at least 100 kcal more than stated, and dishes with lower stated values tended to contain more than stated (Urban et al.).

Use your hands and the plate

When there's no label, compare the food with something whose size you know. The British Dietetic Association's portion guide does this with everyday objects (BDA):

The comparisons work in reverse too. A piece of chicken the size of two decks of cards is roughly 180 g.

Hands are less exact than they feel. In a study where 67 people estimated 42 weighed foods and drinks, a hand technique that uses finger widths as a ruler put 80% of estimates for regularly shaped foods within 25% of the true weight, and 13% within 10%. Cups and spoons did worse, with 29% of estimates within 25% (Gibson et al.). The authors only vouched for foods that closely resemble a geometric shape, so a heap of rice or a ladle of stew is likely to be rougher.

For mixed plates, think in fractions: half the plate pasta, a quarter salad, a spoonful of pesto on top. If you photograph a meal, a fork or your hand in the shot shows how big the plate is.

Where the misses tend to hide

The errors in a rough log aren't spread evenly. In one study, young adults wore a camera that took a picture every 30 seconds for three days, and researchers compared the pictures with what the participants had recorded. Snacks were frequently missed both in an app and in daily interviews. Sauces and condiments, fats and oils, sugary extras and alcohol were missed more often in the app than in the interviews (Chan et al.). The same few things come up again and again.

• Oil and butter. A tablespoon of olive oil is about 120 kcal (USDA), and once it's cooked into food you can't see it. The camera study's authors point out that photos share the problem, since added fats and condiments are hard to make out in an image.
• Sauces and dressings. Savoury sauces and condiments were the most common thing missing from the app records in the same study.
• Drinks. A 175 ml glass of 12% wine can be up to 158 kcal, and a pint of 5% beer up to 222 kcal (NHS). Milk was the drink most often left out of the interviews in the camera study.
• Nuts and nibbles. The BDA portion of nuts is 30 g, one handful, and a bowl you pick at while talking is hard to count in handfuls.
• Seconds. A second helping is easy to fold into the first in your memory. Give it its own entry, even if it's "about half again".

How far off is a rough log?

Researchers check self-reported eating against doubly labelled water, a method that measures how much energy the body uses during everyday life. A review of 59 such studies found that most reported significant under-reporting: by 8 to 30% with 24-hour recalls, and by 11 to 41% with food diaries kept as people went along (Burrows et al.). Smartphone photo methods were not exempt, coming in 20 to 37% low in one of the studies the review included.

Put together, a careful estimate of a single food might land within a quarter of the truth, and a whole day's log, with its forgotten oil and drinks, tends to run low. Knowing the direction of the error is useful in itself. If a day's total ever looks lower than seems plausible, check the oil, sauces, drinks and snacks before you doubt the main meals.

A rough log you keep is worth more

Logging tends to get quicker with practice. In a 24-week online programme, participants spent about 23 minutes a day logging in the first month and under 15 minutes by the sixth. Among those still logging at six months, the time they spent wasn't linked to their results, while logging in more often through the day was (Harvey et al.). That programme was about weight loss, but it suggests that keeping at it matters more than how long each entry takes.

Consistency also makes your errors easier to live with. If you estimate your usual porridge the same way every morning, any error repeats each day, and the log still shows you whether this week looked like last week, which a precise log abandoned after ten days can't.

Imoonk is built around this kind of logging. You describe a meal in a voice note, a photo or a line of text, review the estimate, and correct it in plain words, such as "actually it was two slices". Foods you eat often can be saved, and a recipe keeps one batch with nutrition worked out per portion, which is the divide-the-pot method from earlier without the sums.

If you have a medical condition, are pregnant, or have a history of disordered eating, a GP or registered dietitian is a better guide than any app on whether and how to track what you eat.

Sources

1. Validity of Dietary Assessment Methods When Compared to the Method of Doubly Labeled Water: A Systematic Review in Adults — Frontiers in Endocrinology, 2019.
2. Using Wearable Cameras to Assess Foods and Beverages Omitted in 24 Hour Dietary Recalls and a Text Entry Food Record App — Nutrients, 2021.
3. Accuracy of hands v. household measures as portion size estimation aids — Journal of Nutritional Science, 2016.
4. Portion sizes (Food Fact Sheet) — British Dietetic Association, 2021.
5. Calorie labelling in the out of home sector: implementation guidance — Department of Health and Social Care, 2021.
6. Menu Labeling Requirements — US Food and Drug Administration, 2023.
7. Accuracy of Stated Energy Contents of Restaurant Foods — JAMA, 2011.
8. Guidance document for competent authorities on the setting of tolerances for nutrient values declared on a label — European Commission, 2012.
9. Calories in alcohol — NHS, 2023.
10. Oil, olive, salad or cooking — USDA FoodData Central, 2019.
11. Log Often, Lose More: Electronic Dietary Self-Monitoring for Weight Loss — Obesity, 2019.