What is 5 g of tolerance worth in calories?

Five grams of protein is 20 kcal, five grams of carbs is 20 kcal and five grams of fat is 45 kcal. If a day misses all three by the full 5 g in the same direction, the day is off by 85 kcal.

The conversion

The Food and Agriculture Organization of the United Nations explains the Atwater general factor system in its chapter on the energy content of foods. The system gives each energy yielding component one value regardless of the food it comes from: 17 kJ (4.0 kcal) per gram of protein, 37 kJ (9.0 kcal) per gram of fat and 17 kJ (4.0 kcal) per gram of carbohydrate. The same chapter lists a general factor of 29 kJ (7.0 kcal) per gram for alcohol.

OnMacros uses the three macro factors to turn grams into calories. They are the reason a plan can describe a day in grams and still know how many calories it holds.

Apply them to the 5 g window:

• Protein: 5 g times 4 kcal is 20 kcal
• Carbs: 5 g times 4 kcal is 20 kcal
• Fat: 5 g times 9 kcal is 45 kcal

The best and worst case in a day

A macro perfect day needs protein, carbs and fat each inside 5 g of target. That is a limit on each macro separately, not a limit on calories. The most the calories can drift is the sum of the three limits.

• All three macros 5 g over: 20 plus 20 plus 45 is 85 kcal over target.
• All three macros 5 g under: 85 kcal under target.
• Opposite directions partly cancel. Fat 5 g over and carbs 5 g under is 45 minus 20, or 25 kcal over.

Take the example target used across OnMacros, 140 g protein, 220 g carbs and 55 g fat, which is 1,935 kcal. The window around it spans 1,850 to 2,020 kcal at the extreme corners. In practice most hit days sit well inside that range, because landing exactly on the edge of all three limits at once is unlikely.

What share of the day that is

How big 85 kcal feels depends on how large the day is. The extreme corner as a share of the calorie target:

• 1,500 kcal: 5.7 percent
• 1,600 kcal: 5.3 percent
• 1,935 kcal: 4.4 percent
• 2,000 kcal: 4.3 percent
• 2,500 kcal: 3.4 percent
• 3,000 kcal: 2.8 percent

So the same 5 g window is proportionally tighter on a bigger day. For a smaller day it allows a larger relative swing, which is one reason the window is stated in grams and not as a percentage.

Why grams and not calories

A calorie only limit would let a day pass while the mix was wrong. A day could land on 1,935 kcal with 40 g too little protein, because the 160 kcal of missing protein can be made up by about 18 g of extra fat. Counting each macro on its own closes that loophole. It also matches how people read food: labels and databases show grams per macro first.

Fat deserves a note. Because a gram of fat carries 9 kcal, a small gram error on fat moves the calories more than the same error on carbs or protein. A 5 g miss on fat is worth more than twice a 5 g miss on carbs. If you plan to the edge of the window, fat is the macro whose misses cost the most energy, and it is also the easiest to overshoot with oil, nuts or butter.

The measurement floor

A tolerance only makes sense above the noise in the measurements underneath it. Three sources of noise matter.

• Scales. A kitchen scale has its own resolution and error, and some food always clings to the bowl.
• Labels. United States rules for the Nutrition Facts label state that calories per serving are expressed to the nearest 5 calories up to and including 50 calories, and to the nearest 10 calories above 50. A serving rounded at the label can differ from the true value by up to 5 calories.
• Allowed variance. The same regulation treats a food with a label declaration of calories, total sugars, total fat, saturated fat, trans fat or sodium as misbranded if the composite exceeds the declared value by more than 20 percent, with a carve out for the variability of the analytical method. That is a legal ceiling on overstatement, not a promise of precision.

Put these together and a window tighter than a few grams would be asking for accuracy that no label can supply. Five grams gives the arithmetic room to be honest.

How to use the window

Think of the 5 g as a landing strip rather than a bullseye. When you plan a last meal, the goal is not to hit 55.0 g of fat. It is to end the day somewhere between 50 and 60 g. A plan that lands at 54.8 g, like the worked day in the first article of this series, is comfortably inside, and a plan that lands at 58 g is still a hit.

When you miss, check which macro moved and by how much. A miss on carbs and protein of a few grams each is small in calories. A miss on fat of the same size costs more. Knowing the exchange rate lets you decide whether a stray spoonful of oil needs a note or a correction.

Summary

• Each gram of protein or carbs is 4 kcal and each gram of fat is 9 kcal.
• The 5 g window is 20 kcal for protein, 20 kcal for carbs and 45 kcal for fat.
• At the extreme corner all three limits add up to 85 kcal.
• The window sits above the noise from scales and labels, which is why it is 5 g and not 1 g.