What the Daily Value is, and what it is not
Food labels in the United States carry a column headed "%DV" - percent Daily Value. The Daily Value behind that percentage is a reference amount for a nutrient, not a target and not a requirement. The US Food and Drug Administration publishes the reference values used on the Nutrition and Supplement Facts labels, and several of our calculators report the same percentage so that a portion of food can be read against the same scale a label uses.
Two different tables sit behind the numbers. For vitamins and minerals, the reference values are the Reference Daily Intakes (RDIs), which were derived from the historical Recommended Dietary Allowances of the National Academies. For macronutrients such as total fat, saturated fat, carbohydrate, fiber, and sodium, the Daily Values are independent reference amounts chosen for label use. Both tables were designed for adults and children over four years of age, on a 2,000-calorie reference diet, and both are what "%DV" on any US label means.
The formula
Percent Daily Value is one division and one multiplication: take the amount of the nutrient in your portion, divide by the Daily Value for that nutrient, and multiply by 100. If a portion supplies 390 mg of potassium and the Daily Value for potassium is 4,700 mg, the result is 390 ÷ 4,700 × 100 = 8.3%DV. The Nutrition Facts label rounds to the nearest whole percent and prints "8%"; calculators can carry the decimal, which is why our mango potassium calculator and similar tools show values like 8.3% rather than 8%.
The same arithmetic applies to any nutrient with a published Daily Value: iron 18 mg, calcium 1,300 mg, vitamin C 90 mg, total carbohydrate 275 g, dietary fiber 28 g, and so on. Our bread iron calculator, crab vitamin C calculator, and walnut fiber calculator each compute this percentage from the FoodData Central nutrient content of the portion you enter.
A worked example
Take a 150 g portion of cooked crab. FoodData Central lists about 2.7 mg of vitamin C per 100 g of crab, so the portion supplies roughly 4.05 mg. With a vitamin C Daily Value of 90 mg, the portion contributes 4.05 ÷ 90 × 100 = 4.5%DV. The number is small, and that is the point of the scale: it puts a single food next to the whole day's reference amount without pretending the food is a recommendation. The crab vitamin C calculator runs this calculation for any portion size.
What the percentages do not mean
A food that supplies 25%DV of a nutrient is not "25% of what you need", and a food at 5%DV is not deficient. The reference diet is a labeling convention built on a 2,000-calorie intake; individual needs differ with body size, life stage, and medical circumstances, and the FDA itself states that Daily Values are not recommended intakes for individuals. Percentages from different foods also do not simply add up to a day's requirement, because absorption and meal context vary. Read the percentage as a position on a standard ruler, not a score.
Where the reference values change over time
The Daily Value tables are updated by regulation when the underlying science review concludes. Fiber, for example, moved from 25 g to 28 g, and sodium's reference intake was raised to 2,300 mg in the 2016 label revision. Our calculators state which reference table edition they use on each tool page, and the methodology page lists the values in force for the macronutrients and micronutrients our tools report. If a regulation changes a reference value, the affected tool pages are updated to match.
Summary
The Daily Value is a fixed reference amount per nutrient; %DV is your portion divided by that amount. Our calculators automate the lookup and the division for specific foods. For the underlying data sources see the methodology page, and for the standing limits of general nutritional information see the disclaimer.
Nutrients to get "less of" use the ceiling, not the floor
The label groups nutrients into two kinds. For nutrients like dietary fiber, vitamin D, calcium, iron, and potassium, a higher percentage moves you toward the reference amount. For saturated fat, sodium, and added sugars, the Daily Value works in the opposite direction: the percentage tells you how much of a ceiling your portion consumes, and lower is closer to the limit. A portion at 20%DV of sodium has used a fifth of the reference ceiling of 2,300 mg. Confusing the two directions is the most common misreading of a label, and it is why our calculators always name the direction in the result line rather than printing a bare percentage. When a calculator reports a percentage for sodium or saturated fat, it is a share of a ceiling; when it reports fiber or potassium, it is a share of a reference floor.