How this instrument works
A ketogenic macro split works from a fixed-ratio starting point: fat, protein, and carbohydrate are each assigned a percentage of total calories before the arithmetic runs, and none of the three is left to fall out as a remainder. A common starting ratio sets fat at 75%, protein at 20%, and lets carbohydrate take whatever share is left — typically 5%, occasionally less.
Carbohydrate is pushed that low for a specific physiological reason. The body stores a limited supply of glycogen and draws it down first for fuel. Restrict carbohydrate intake to roughly 20 to 50 grams a day, the range the clinical literature on low-carbohydrate diets typically cites, and glycogen depletes quickly enough that the liver switches toward breaking down fat for energy, producing ketone bodies — acetoacetate, acetone, and beta-hydroxybutyrate — that muscle and brain tissue can burn in glucose's place. That metabolic shift, not the raw fat percentage, is the actual goal; ketosis does not occur at moderate carbohydrate intakes no matter how high fat climbs.
The diet's clinical history runs through epilepsy treatment, not weight management. Ketogenic eating was formalized in the 1920s as a therapy for seizures resistant to medication, decades before it became a popular approach to fat loss, and it is still used clinically today for epilepsy that does not respond to drugs. The percentages this calculator defaults to — 75/20/5 — are one common preset rather than a fixed law; clinical ketogenic diets for epilepsy often run fat considerably higher and protein lower still.
- Enter Total Calories — the day's calorie budget the split is built from.
- Set Fat Percent (default 75) — fat's share of total calories.
- Set Protein Percent (default 20) — protein's share; carbohydrate takes whatever percentage remains.
- Read Fat, Protein, and Carb grams; the working block shows each conversion from percentage to grams.
Worked example — 2000 kcal, 75% fat, 20% protein
Fat claims 75% of the day's 2000 kcal, which is 1500 kcal; divide by 9 kcal per gram of fat and that is about 166.7 g. Protein takes 20%, or 400 kcal, and at 4 kcal per gram that is 100 g. The two together account for 95% of calories, leaving 5% — 100 kcal — for carbohydrate, which at 4 kcal per gram works out to 25 g: less carbohydrate than a single medium banana.
Shift the ratio to 70% fat and 25% protein on an 1800 kcal budget and the numbers move accordingly: fat calories are 1260, or 140 g; protein calories are 450, or 112.5 g; the remaining 5%, 90 kcal, is 22.5 g of carbohydrate. The carbohydrate percentage barely moves across common presets — it is fat and protein that trade share against each other, while carbohydrate stays pinned near the floor a ketogenic split needs.
Questions
Why is carbohydrate pushed so low on a ketogenic diet?
To deplete glycogen fast enough that the body has no better option than burning fat. Glycogen is the body's stored form of glucose, and at moderate carbohydrate intake it gets replenished daily, so fat oxidation never has to ramp up much. Cut carbohydrate down toward 20 to 50 grams a day and glycogen stores run low within days, pushing the liver to convert fat into ketone bodies as an alternative fuel — the state called ketosis. It is not gradual as fat percentage rises; it behaves like a threshold tied to how little carbohydrate is coming in.
How is a keto split different from IIFYM?
In how many macros get fixed. This calculator, and the ketogenic approach generally, pins all three macros — fat, protein, and carbohydrate — as percentages of calories before any grams are computed. This site's separate IIFYM calculator only fixes two: protein by body weight and fat by percentage, then lets carbohydrate absorb whatever calories are left. A keto plan never allows carbohydrate to float upward the way an IIFYM plan can.
Will hitting this exact carbohydrate gram figure guarantee ketosis?
No — treat it as a starting estimate, not a guarantee. Individual tolerance for carbohydrate while staying in ketosis varies meaningfully: some people stay in ketosis up near 50 grams a day, others need to sit closer to 20. Activity level, insulin sensitivity, and how consistently total calories are held all shift the real threshold. The gram figure here is the arithmetic result of the chosen percentages, not a measured guarantee that ketone production will follow.
What are ketone bodies, and where do they come from?
They are the alternative fuel the liver produces once carbohydrate is scarce and fat becomes the dominant energy source. Free fatty acids released from fat tissue travel to the liver, where they are broken down and converted into three ketone bodies — acetoacetate, acetone, and beta-hydroxybutyrate — which circulate in blood and get burned by muscle and, notably, the brain, which cannot run on fat directly but adapts well to ketones.
Why was the ketogenic diet invented in the first place?
As a treatment for epilepsy. Physicians in the 1920s observed that fasting reduced seizure frequency and built a diet that mimicked fasting's metabolic effects — high fat, very low carbohydrate — as something patients could sustain longer than an actual fast. It stayed a niche epilepsy treatment for decades before its fat-loss effects made it popular well outside neurology, and it is still prescribed clinically for seizures that resist medication.
Is 75% fat and 20% protein the only valid keto ratio?
No, it is one common default. Clinical ketogenic diets used for epilepsy often run fat higher still, 80% to 90% of calories, with protein correspondingly lower, while looser versions aimed at general use sometimes allow fat closer to 60% or 70%. What stays consistent across versions is the low carbohydrate ceiling; fat and protein are the two figures that actually move between one keto plan and another.
References
- Daley SF et al. — The Ketogenic Diet: Clinical Applications, StatPearls (NCBI Bookshelf)
- Oh R, Gilani B, Uppaluri KR — Low-Carbohydrate Diet, StatPearls (NCBI Bookshelf)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.