The 15 g protein bar at 120 calories wins on protein density. It gets 50% of its calories from protein. The 20 g bar at 250 calories gets 32%. If efficient protein is the goal, the first is the stronger choice. The larger protein number does not overturn the arithmetic.
Both examples are hypothetical, not FEERO product claims. They expose a simple problem with front labels. A bigger serving gets a bigger protein number surprisingly easily.
The 15 g and 20 g protein calculation
Protein provides about four calories per gram under the standard label calculation. [1]
For the first bar, 15 multiplied by four gives 60 protein calories. Divide 60 by 120 and multiply by 100. That gives 50%.
For the second, 20 multiplied by four gives 80 protein calories. Divide 80 by 250 and multiply by 100. That gives 32%.
Compare grams per 100 calories too. The first gives 12.5 g. The second gives 8 g. This puts both bars on the same energy budget, making the gap easier to see.
Do not mix up weight and energy. A bar that weighs 50 g and contains 15 g of protein is 30% protein by weight. That is a different measure from the percentage of calories supplied by protein.
If you are trying to lose fat
The 120 calorie option leaves more room for the rest of your food. That is useful if your meals already provide enough energy and you mainly need a protein top up.
There is a catch. You need to compare what you actually eat. If you eat another snack straight after the smaller bar, count that food too. The final total matters more than winning the first comparison. A wrapper cannot predict your whole afternoon.
Research on higher protein diets during weight loss found modest benefits for retaining lean mass on average. It does not show that choosing the highest protein percentage always produces the best result. [2]
If you are trying to gain muscle
The 250 calorie bar supplies more energy in one serving, which is useful when you struggle to eat enough. It supplies five extra grams of protein and 130 extra calories in this example. In that situation, the extra calories have a clear job.
But extra energy alone does not build muscle. You still need effective training and enough protein over the day. Protein supplementation studies test the wider training diet, not a contest between two front labels. [3]
At maintenance, fit the serving to your remaining needs. Use hunger, meal timing, budget and taste to make that choice.
Compare equal amounts when needed
To get 20 g of protein from the first bar's ratio, you would need about one and a third bars. That would provide 160 calories. The difference at equal protein is therefore 90 calories, not 130.
This is a useful calculation if you are choosing between products specifically to fill a 20 g gap. It also shows why the comparison must match your actual question.
Check fibre, saturated fat, allergens and price before buying. None of those appears in the protein percentage. Our two bars with the same protein guide explains what else can differ.
For 15 g at 120 calories versus 20 g at 250 calories, the protein density winner is settled. Choose the first when efficient protein is your priority. Choose extra calories when you need extra food, rather than mistaking them for extra protein quality.