Protein Calculator

Protein is the one macro where a percentage is the wrong question

This tool asks you to type a protein percentage, works out your daily calories, and hands back the grams. It does that for five scenarios at once: maintaining, losing half a kilo a week, losing a kilo, gaining half a kilo, gaining a kilo.

Before you type anything, there is something worth understanding, and this calculator happens to demonstrate it better than any explanation could.

Percentages work fine for carbohydrate and fat. Those are fuel, and how much fuel you need genuinely does scale with how many calories you are eating. Protein is not fuel. Protein is building material, and the amount your body needs is set by how much body there is to maintain, not by what is on your plate. Your muscles do not know what you ate. They need what they need.

So expressing protein as a share of calories ties it to the wrong variable. And the consequence is not academic. It is sitting in the tool's own output.

Watch what happens when you diet

Take a 30 year old man, 175 cm, 75 kg, training 3 to 5 times a week. His maintenance works out at roughly 2,633 calories. Type 25 percent protein, a perfectly sensible figure, and here is what the tool returns across its five rows:

Read that table from the bottom up and it is quietly absurd.

The same man, the same 75 kg of body, the same 25 percent. And yet the harder he diets, the less protein he is told to eat. From 165 g down to 102 g, a cut of nearly 40 percent. Meanwhile the scenario where he is told to eat the most protein, a colossal 227 g, is the one where he is deliberately eating in surplus and has the least need to protect anything.

It is exactly backwards. When you are in a calorie deficit your body starts looking around for things to break down for energy, and your muscle is on the list. Protein is the thing that argues against that. Every serious source says the same: a deficit is when protein matters most, and the harder the deficit, the more it matters. Yet a fixed percentage quietly cuts your protein precisely when you are most exposed.

Nothing is broken. The tool is doing the arithmetic correctly. The unit is just the wrong one for the job.

So what percentage should I type?

Work backwards. Decide the grams first, then reverse the arithmetic into a percentage.

Protein carries 4 calories per gram, so:

Percentage to type = (your target grams × 4) ÷ your daily calories × 100

Our man wants to keep his 165 g while losing a kilogram a week on 1,633 calories. So 165 × 4 = 660 calories of protein, divided by 1,633, which is 40.4 percent. He needs to type 40, not 25.

That is the practical trick with this tool, and it is why the percentage box is best treated as an output rather than an input. Pick your grams, do the sum, type the answer. The percentage will and should climb as your calories fall. If that produces a number in the high thirties or low forties while you are cutting hard, nothing has gone wrong: that is just what holding protein steady looks like when the rest of the plate shrinks.

The number to actually hold on to

Which raises the obvious question: what should the grams be?

Use your body weight, not your dinner. The official floor for a healthy adult is 0.8 grams per kilogram of bodyweight, which for our man is 60 g. But that figure is a minimum designed to stop you becoming deficient, not a target for someone training, and our Protein Intake Calculator explains why it is so often misread. The joint position of the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine puts active people at 1.2 to 2.0 g/kg, and the upper end of that is where you want to be if you are lifting and cutting at the same time.

For our 75 kg man that is 90 g to 150 g, and around 150 g is the sensible target while he diets. Note what that means: it is a fixed anchor. It does not move when his calories move, which is the entire point.

One thing to be careful with

An honest warning about the two weight-loss rows.

Those rows work by subtracting a flat 500 or 1,000 calories from your maintenance, and for a small or sedentary person that subtraction goes somewhere absurd. A 50 kg woman with a sedentary job has a maintenance of roughly 1,419 calories. Subtract 1,000 and you are at 419, which is not a diet, and no calculator should imply otherwise.

So the tool now stops. The 2013 obesity guideline from the American Heart Association, the American College of Cardiology and the Obesity Society prescribes 1,200 to 1,500 calories a day for women and 1,500 to 1,800 for men when a deficit is the goal. Those numbers are the bottom of a prescribed range rather than a safety line, and this tool will not build a protein figure on anything below them. If a row would have gone lower, it is raised to the floor and labelled.

Take the label seriously rather than as a formality. Being clamped means the target you asked for is outside what the guideline contemplates for someone your size, and the honest read is that the goal is too aggressive rather than that the number needs working around. If you are small, or lightly active, or both, the mild option is the one to use. Our Calorie Intake Calculator works the same problem from the calorie side.

Questions people ask

What percentage do most people type?

Somewhere between 20 and 30. That is a reasonable maintenance figure. The point of this page is that it should not stay there when your calories drop.

Can I eat too much protein?

For a healthy person with healthy kidneys, high intakes have been studied and look safe. The real cost is opportunity: calories spent on protein are calories not available for the carbohydrate that fuels training or the fat that supports your hormones.

Should I use my total weight or my lean weight?

Total bodyweight is the usual basis and it is what the position stands use. If you carry a lot of fat, using lean mass gives a more sensible figure, and our Lean Body Mass Calculator will work it out.

How is this different from your other protein tools?

This one asks you for a percentage. Our Protein Intake Calculator gives you three authorities' answers in grams per kilogram, and our Protein Requirement Calculator handles illness and injury.

References

Where the figures come from. The 0.8 grams per kilogram Recommended Dietary Allowance is from the Institute of Medicine's Dietary Reference Intakes. The 1.2 to 2.0 g/kg range for active people is the joint position of the Academy of Nutrition and Dietetics, Dietitians of Canada and the American College of Sports Medicine. The point that the RDA is adequate to prevent deficiency but not to support training, and that intakes should sit at the upper end during energy restriction, is set out in a recent review in Nutrients. The resting equation the tool runs is Mifflin-St Jeor.

1. Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: nutrition and athletic performance. Journal of the Academy of Nutrition and Dietetics. 2016;116(3):501-528.
2. Current perspectives on protein supplementation in athletes: general guidance and special considerations for diabetes. Nutrients. 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC12655512/
3. National Academies of Sciences, Engineering, and Medicine. Dietary Reference Intakes for Energy. Washington DC: The National Academies Press. https://www.ncbi.nlm.nih.gov/books/NBK591020/
4. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition. 1990;51(2):241-247. https://doi.org/10.1093/ajcn/51.2.241