Why use this calculator
Most keto calculators impose a fixed ratio such as 5 per cent carbs, 25 per cent protein and 70 per cent fat, and that ratio quietly breaks at both ends. On a low intake it hands you too little protein to protect muscle, and on a high intake it hands you more carbohydrate than ketosis tolerates, because a percentage of a big number is a big number.
This calculator works the way a dietitian would set it up instead. Net carbohydrate is a fixed gram ceiling, since ketosis responds to grams rather than to percentages. Protein is scaled to your body weight, because the requirement follows your mass and training, not your calorie total.
Fat then fills whatever energy is left, which is exactly what fat is for on a ketogenic diet: an energy buffer, not a target to chase with butter in coffee. The resulting percentages are shown, but they are an output rather than an input.
How to use it
- Choose metric or imperial, then enter your weight measured in the morning before eating and your height measured against a wall without shoes.
- Enter your age in whole years and choose your sex, both of which feed the Mifflin-St Jeor equation.
- Choose the activity level that describes a normal week, taking the lower option if you are between two.
- Set your calorie deficit as a percentage of maintenance: 15 to 20 per cent suits most people, and anything above 25 is hard to sustain and costs muscle.
- Set your net carb limit, which is total carbohydrate minus fibre; 20 grams is the standard starting point and many people hold ketosis at 30 or more once adapted.
- Finally choose a protein target scaled to your body weight, using the higher option if you lift weights or are dieting hard.
What each result means
- Maintenance (TDEE)
- Calories that would hold your weight steady at the activity level selected. The deficit is taken from this figure.
- Target Calories
- Your daily energy target after the deficit. If it falls below your resting metabolic rate, reduce the deficit rather than pressing on.
- Daily Deficit
- How many calories a day below maintenance you are eating. This, not ketosis itself, is what drives fat loss.
- Net Carbs
- Total carbohydrate minus fibre, the ceiling that keeps you in ketosis. Grams matter here, not the percentage of calories.
- Protein
- Grams of protein scaled to your body weight. Adequate protein is what preserves muscle while you are in a deficit.
- Fat
- Grams of fat filling the remaining energy. It is a ceiling and a buffer, not a quota you must eat your way up to.
- Macro Split (carbs / protein / fat)
- The percentage split that falls out of the grams above, shown for comparison with the ratios other keto tools impose.
- Predicted Weekly Fat Loss
- What the energy arithmetic predicts each week. The first week is always faster because glycogen and its bound water leave.
The formula
Resting metabolic rate uses Mifflin-St Jeor: BMR equals 10 x weight in kilograms plus 6.25 x height in centimetres minus 5 x age in years, plus 5 for men or minus 161 for women. Maintenance is BMR multiplied by an activity factor of 1.2 to 1.9. Target calories are maintenance multiplied by one minus the deficit percentage divided by 100. Net carbohydrate is the gram limit you set, costing 4 kcal per gram. Protein in grams is body weight in kilograms multiplied by 1.2, 1.6 or 2.0, also at 4 kcal per gram. Fat in grams is whatever energy remains, target calories minus carb calories minus protein calories, divided by 9 kcal per gram, and is floored at zero. The percentages shown are derived from those grams rather than set in advance.
How this number is used in practice
The first fortnight on a ketogenic diet is dominated by water, not fat. Each gram of stored glycogen holds roughly three grams of water, so emptying glycogen stores commonly drops two to four kilograms in the first week. It comes straight back the moment carbohydrate returns, which is why people who cycle on and off keto experience dramatic swings that have nothing to do with body composition.
The same shift causes the keto flu: sodium, potassium and magnesium follow the water out, and the headaches, fatigue, cramps and irritability most beginners report are largely an electrolyte problem. Adding three to five grams of sodium a day, plus potassium and magnesium from food or supplements, resolves most of it within days.
Beyond that, the evidence is fairly consistent that ketogenic diets perform about as well as other diets once protein and calories are matched, and that adherence is what separates the people who succeed from those who do not. Keto suits people who find it genuinely easier to eat well without carbohydrate, and it has established clinical uses in drug-resistant epilepsy and a growing role in type 2 diabetes management under supervision.
It is a poor fit for others, particularly endurance and team-sport athletes who need high-intensity output. Anyone taking insulin or sulfonylureas must involve their doctor before starting, because medication doses often need reducing quickly to avoid hypoglycaemia. The same applies to anyone who is pregnant or breastfeeding, under 18, has kidney or liver disease, pancreatitis, gallbladder disease, or a history of disordered eating. Like every energy figure here, the calorie target is a population estimate carrying roughly ten per cent individual error.
Worked examples
30 year old male, 170 lb, 70 in, sedentary, 20 per cent deficit, 20 g net carbs, moderate protein
1,668 kcal a day with 20 g net carbs, 123 g protein and 122 g fat, a 417 kcal daily deficit against maintenance of 2,085.
35 year old male, 85 kg, 175 cm, lightly active, 15 per cent deficit, 25 g net carbs, higher protein
2,073 kcal a day with 25 g net carbs, 170 g protein and 144 g fat, a 366 kcal daily deficit.
45 year old female, 70 kg, 163 cm, moderately active, 20 per cent deficit, 20 g net carbs, moderate protein
1,653 kcal a day with 20 g net carbs, 112 g protein and 125 g fat, a 413 kcal daily deficit.
When people use it
- Setting starting macros before a first attempt at a ketogenic diet
- Checking whether a fixed-ratio keto plan has left your protein target too low
- Adjusting the net carb ceiling upward once you are fat adapted and want more vegetables
- Recalculating after 5 kg of weight loss, when maintenance calories have fallen
- Planning how to split a tight carb allowance across two, three or four meals
Tips
- Keep net carbs under your limit every day for the first three weeks. Ketosis responds to grams, and a single high-carb meal resets the adaptation.
- Do not chase the fat number. Fat is a ceiling that balances energy, and eating up to it when you are not hungry simply removes the deficit.
- Add sodium deliberately, roughly 3 to 5 g a day, plus potassium and magnesium. Most keto flu is an electrolyte problem, not a carbohydrate withdrawal.
- Hit protein every day. The old warning that protein will knock you out of ketosis does not hold up at the intakes shown here.
- Read labels for net carbs rather than total. Fibre and most sugar alcohols are subtracted, but maltitol behaves much more like sugar than the label suggests.
- Expect a 2 to 4 kg drop in the first week and do not read it as fat. It is glycogen and the water bound to it, and it returns with carbohydrate.
- Talk to your doctor first if you take insulin or sulfonylureas, as doses frequently need lowering within days to avoid hypoglycaemia.
What it assumes, and where it stops being accurate
- The Mifflin-St Jeor equation estimates resting metabolism to within about 10 per cent for most adults, and is less reliable for very muscular, very lean or severely obese people.
- It assumes a net carb ceiling produces ketosis, which varies between individuals: some hold ketosis at 50 g while others need under 20 g.
- Protein is scaled from total body weight, which overestimates the requirement for people carrying a high body fat percentage. Lean mass is the better basis if you know it.
- Predicted loss uses 7,700 kcal per kilogram of fat and ignores the large water shift in the first week and the fall in expenditure as weight drops.
- It is not valid during pregnancy or breastfeeding, for anyone under 18, or for anyone with kidney or liver disease, pancreatitis, gallbladder disease or a history of disordered eating.
- It does not adjust for medication. People taking insulin, sulfonylureas, diuretics or SGLT2 inhibitors need medical supervision before cutting carbohydrate sharply.
Terms used on this page
- Ketosis
- A metabolic state in which the liver produces ketone bodies from fat, typically at blood ketones of 0.5 to 3.0 mmol/L.
- Net carbohydrate
- Total carbohydrate minus fibre and most sugar alcohols: the figure that determines whether ketosis holds.
- Keto flu
- Headache, fatigue, cramps and irritability in the first days of a ketogenic diet, mostly caused by sodium and fluid loss.
- Gluconeogenesis
- The liver making glucose from protein and other substrates, driven by demand rather than by how much protein you eat.
- Fat adaptation
- The two to four week process by which muscle improves at oxidising fat and ketones during exercise.
Questions people ask
- How long does it take to get into ketosis?
- Most people reach nutritional ketosis within two to four days of holding net carbohydrate under about 20 to 50 grams a day, and blood ketones typically sit between 0.5 and 3.0 millimolar. Feeling adapted, with energy and training performance recovered, usually takes another two to four weeks as the body improves at using fat and ketones.
- What are net carbs and do they matter?
- Net carbs are total carbohydrate minus fibre, and in most places minus most sugar alcohols too, since those contribute little usable energy. It is the figure that matters for ketosis. Be cautious with maltitol, which raises blood glucose far more than its label implies, and with products marketed as keto that rely on it.
- Can I eat too much protein on keto?
- This worry is largely overstated. Gluconeogenesis is driven by demand rather than by protein supply, so the intakes here, between 1.2 and 2.0 grams per kilogram, do not knock ordinary people out of ketosis. Too little protein is the real risk, because it costs muscle during a deficit and leaves you hungrier.
- Why do I feel terrible in the first week?
- That is the keto flu, and it is mostly electrolytes rather than carbohydrate withdrawal. Falling insulin makes the kidneys excrete sodium, and potassium and magnesium follow. Adding 3 to 5 grams of sodium a day plus potassium and magnesium usually clears the headaches, fatigue and cramps within a few days.
- Is keto better than other diets for fat loss?
- Not inherently. Controlled trials that match calories and protein generally find similar fat loss across diets, and adherence is what actually separates results. Keto suits people who find appetite easier to manage without carbohydrate. It suits endurance and team-sport athletes poorly, because high-intensity work depends on glycogen.
- Should I eat fat to hit my fat number?
- No. Fat is the balancing macro, so the gram figure is a ceiling that keeps you at your calorie target, not a quota. If you are full at fewer calories that is simply a larger deficit, which is the point when you are losing fat. Eating extra fat you do not want is the most common reason keto stalls.
Use this with
Keto Macro Calculator answers one part of the question. These pick up where it stops: Macronutrient Calculator, TDEE Calculator, Protein Intake Calculator and Intermittent Fasting Calculator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- World Health Organization: healthy diet. Population guidance on fats, sugars and fibre
- Dietary Guidelines for Americans. Acceptable macronutrient distribution ranges and nutrient adequacy
- NHS: healthy weight. Safe rates of weight loss and calorie reference intakes
Keto Macro Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.