A ratio is a comparison between two different values. A ratio expresses a proportion, not an absolute value. There are a few different ways of writing a ratio.

Colon Notation

The colon is pronounced as the word "to". A ratio of A:B is read as "A to B." A ratio in the form of A:B translates to a quantity of A/(A+B) and B/(A+B). For example, a ratio 1:3 means a comparison between the values 1/4 and 3/4.

Decimal Notation

Decimal notation describes a ratio in the form of a single number where the second proportional values is not explicit, but implied. A decimal number in the form of A/C implies a B value of C-A. A:B translates to A:(C-A) or the two values A/(A+C-A) and B/(A+C-A), or A/C and B/C respectively. For example, 0.25 or 1/4 is a 1 to 3 comparison, as 4 - 1 = 3. The value of 3 is implied.

Calcium:Phosphorus Ratio

Ratios are often spoken of in the context of Ca:P, or Calcium to Phosphorus. The generally accepted value for nutrition is 2 Calcium to 1 Phosphorus, or 2:1. Unfortunately, Ca:P ratios can be misleading as it is a comparative and not an absolute value. For example, if a food item has 1800 mg of calcium and 1000 mg of phosphorus, it would have a ratio of 1.8:1. Consider a food item with 5mg of Calcium and 1mg of Phosphorus. This would have a ratio of 5:1. In terms of nutrition, the food item with a Ca:P ratio of 1.8:1 contains far more Calcium than the second food item, and is more nutritious, even with a substandard Ca:P ratio.

In summary, ratios alone can not used as a method of determining if a food source provides a significant source of Calcium, since ratio values can be misleading without knowing the absolute values of specific items.