The short answer
Ignore the cups and compare grams. Convert both cereals to per-100g values for calories, added sugar, fiber, and sodium, then check the ingredient list for actual whole-grain content. Front-of-box claims in this category are calibrated to distract you from exactly these four numbers.
The serving-weight spread is enormous
Light puffed and air-popped cereals list servings around 27–32g — sometimes labeled "1 cup" or "1 1/4 cups." Flake cereals sit near 35–40g. Dense granolas, bran twigs, and biscuit-style cereals list 55g or more. The FDA lets manufacturers pick a serving that reflects a reference amount for that cereal's density, which is exactly why two "one cup" labels can describe almost double the food. A 30g serving at 110 calories and a 55g serving at 200 calories can be nutritionally similar products; per serving, one looks twice as indulgent. Per 100g is the fix: divide each value by the serving grams and multiply by 100.
"Made with whole grain" is a floor, not a feature
That front-of-box phrase only requires the product to contain some whole grain — a few grams qualify. The panel cannot tell you grams of whole grain, so use two proxies. First, the ingredient list: "whole grain oats" or "whole wheat flour" as the first ingredient means the base is whole grain; "enriched wheat flour" first with "whole wheat" fifth means it is not. Second, the fiber line: a genuinely whole-grain cereal typically shows 3g or more of fiber per serving, because the bran comes along with the grain. A "made with whole grain" cereal showing 1g of fiber is telling on itself.
Read the sugar line like a children's-category veteran
Cereals marketed to children commonly carry 10–12g or more of added sugar per roughly 30g serving — sugar approaching a third of the product by weight. Adult-positioned cereals span the full range from 0g added (plain shredded wheat, plain puffed grains) to dessert levels. The useful read is added sugar per 100g: under about 15g per 100g is moderate for this category; over 30g per 100g is candy territory regardless of what the mascot says. Frosting, honey, and marshmallow pieces all land in the same added-sugars line, so the panel already catches the tricks.
Worked example: a puffed cereal vs a dense one
Cereal A is a honey-flavored puffed cereal: 29g serving (1 1/4 cups), 110 calories, 12g added sugar, 1g fiber, 150mg sodium. Cereal B is a plain shredded-wheat style: 55g serving, 180 calories, 0g added sugar, 7g fiber, 0mg sodium. Per serving, A looks lighter — fewer calories, smaller everything. Per 100g, A is about 379 calories, 41g added sugar, and 3.4g fiber; B is about 327 calories, 0g added sugar, and 12.7g fiber. The normalization reveals what the serving sizes hid: B is the denser bowl but the leaner food, and A's sugar is over 40% of its weight. Whether B's plainness works for your household is a taste question — but now it is an informed one.
Do not skip sodium
Cereal is a quiet sodium source: many flake and puffed cereals carry 150–300mg per serving, and a realistic double pour doubles it. Per 100g, that is roughly 500–1000mg — comparable to bread, gram for gram. If you watch sodium, add the line to the comparison; it differs more between brands than most shoppers expect, and it is invisible from the front of the box.
Your bowl finishes the math
Everything above describes the dry cereal. Milk adds roughly 8g protein and 100–120 calories per cup, fruit adds sugar that is not on the box, and the second pour nobody admits to doubles everything. You do not need to model every breakfast — just decide whether you are comparing cereals or comparing breakfasts, and keep additions consistent across both options.
Price belongs at the end, per 100g
Family-size boxes, store brands, and bagged cereals scramble shelf prices. Unit pricing per ounce or per 100g is the honest comparison, and store brands frequently match name brands within rounding on sugar and fiber at two-thirds the price. Record the store and date if a promotion decided the winner; next week the same math may crown the other box.
What this comparison cannot tell you
Per-100g arithmetic cannot tell you which cereal your kids will eat, whether a fortified vitamin panel matters for your diet, or how a cereal fits a medical condition. Label values are rounded, and limited editions churn constantly — recheck the panel when a box design changes. For celiac disease or allergies, the gluten and allergen statements matter more than any macro comparison. The panel definitions are in the FDA's Nutrition Facts guidance and serving-size rules.
When two boxes are in the cart, AptBite can scan both labels, convert them to per 100g, and keep added sugar against fiber in one view.
