Whole grain and fibre are related ideas, but they are not interchangeable. A cereal can contain whole grains, isolated fibre ingredients, both or neither.
Whole grain describes the grain ingredient
Whole-grain ingredients retain the major grain components in proportions defined by local standards. The exact wording and accepted definitions differ by market.
Fibre is a measured label value
Fibre values can come from whole grains, bran, nuts, seeds, fruit or added fibre ingredients. The ingredient list helps explain the number.
Compare on the same weight basis
A large serving can create a larger fibre number even when the product is not more concentrated. Per 100 g or equal-weight comparison avoids that error.
Increase gradually when appropriate
Sudden dietary changes can be uncomfortable for some people. Anyone with digestive conditions or medical concerns should use individualized professional guidance.