GKI Calculator

What Is the Glucose Ketone Index (GKI)?

The Glucose Ketone Index is a single number that captures the relationship between your blood glucose and blood ketone levels. Instead of looking at each value in isolation, GKI gives you one metric that reflects your overall metabolic state — making it easier to assess your overall metabolic state.

The formula is straightforward:

GKI = Glucose (mmol/L) ÷ Ketones (mmol/L)

If your meter reads glucose in mg/dL, divide by 18.016 first to convert to mmol/L — the calculator above handles this automatically.

Where GKI Comes From: The Research

Seyfried's key insight: glucose and ketones aren't just independent numbers — their ratio is what determines the metabolic environment your cells actually operate in.

GKI was introduced by Dr. Thomas Seyfried and his team at Boston College. In their 2015 paper published in Nutrition & Metabolism, Seyfried proposed GKI as a clinical biomarker for tracking the therapeutic efficacy of ketogenic metabolic therapy (KMT), particularly in the management of brain cancer and other conditions where suppressing glucose-driven growth pathways matters.

The original research examined how maintaining a low GKI correlated with better outcomes in preclinical glioblastoma models.

Reference: Meidenbauer JJ, Mukherjee P, Seyfried TN. "The glucose ketone index calculator: a simple tool to monitor therapeutic efficacy for metabolic management of brain cancer." Nutrition & Metabolism. 2015;12:12. (full text)

GKI Zones Explained

Your GKI score falls into one of four zones. Here's what each one means and when you might see it:

Zone	GKI Value	What It Means
Therapeutic	< 1.0	Deep ketosis. The threshold studied in Seyfried's clinical research on metabolic therapy for brain cancer.
Moderate Ketosis	1.0 – 3.0	Strong ketosis with well-controlled glucose.
Mild Ketosis	3.1 – 6.0	Light ketosis. Common in the early days of a fast or low-carb diet.
Out of Ketosis	> 6.0	Minimal to no ketosis. Body is primarily using glucose as fuel.

The < 1.0 therapeutic threshold is drawn from Meidenbauer, Mukherjee & Seyfried (2015), Nutrition & Metabolism. The remaining zones are practical guidelines widely used in the keto community and are not defined in the original research.

Why Fasting Enthusiasts Track GKI

If you practice intermittent fasting, extended fasting, or time-restricted eating, GKI is one of the most actionable numbers you can track. Here's why:

It answers "is my fast actually working?"

Plenty of people hit 16, 24, even 48 hours fasted and wonder whether they're truly in ketosis or still running on glycogen stores. Glucose alone doesn't tell you — it can stay in a normal range even when your body has fully switched to fat-burning. Ketones alone don't tell you either, because elevated glucose alongside elevated ketones means your body is still in a mixed-fuel state. GKI captures both sides of the equation in one number.

It shows you your personal carb threshold

Everyone's metabolism is different. Some people can eat 40g of carbs and maintain a GKI under 6. Others spike out of ketosis at 20g. By testing before and after meals over a few days, you can map exactly where your threshold sits — no generic macro calculator required.

It tracks your metabolic flexibility over time

As your body becomes more fat-adapted, you'll notice your GKI drops faster after meals and stays lower during fasts. Tracking the trend over weeks and months is one way to see whether your metabolic flexibility is actually changing.

It helps you optimize your refeeding window

For people doing protocols like OMAD (one meal a day) or 5:2 fasting, knowing your GKI before you break your fast lets you make smarter choices about what to eat first. If your GKI is borderline, you might choose to extend the fast another hour or two rather than break it with something that could spike you out.

Glucose Zones Reference

Zone	Range (mg/dL)
Normal	< 100
Elevated	100 – 125
High	126 – 199
Very High	≥ 200

Ketone Zones Reference

Zone	Range (mmol/L)
Trace	< 0.5
Light Ketosis	0.5 – 0.99
Nutritional Ketosis	1.0 – 2.99
Deep Ketosis	≥ 3.0

How to Test Your Glucose & Ketones

To calculate GKI you need two readings taken at roughly the same time:

1. Blood glucose — Use any standard blood glucose meter. Finger-prick meters are inexpensive and widely available. CGMs (continuous glucose monitors) work too but report interstitial glucose, which can lag behind blood glucose by 10–15 minutes.
2. Blood ketones — You need a meter that reads blood beta-hydroxybutyrate (BHB). Urine strips measure acetoacetate and are not accurate for GKI. Breath meters measure acetone and can be directionally useful, but blood BHB is the gold standard.

For the most consistent results, test first thing in the morning before eating or drinking anything other than water. This gives you a fasted baseline. If you want to see how food affects your GKI, test again 2–3 hours after a meal.

Frequently Asked Questions

Can I use a CGM for the glucose reading?

Yes. CGMs report glucose in the same units (mg/dL or mmol/L). Just be aware that CGM readings can lag behind finger-prick readings by 10–15 minutes, so for the most accurate GKI snapshot, a finger prick is slightly better.

Why does my GKI show "High" even though my glucose is normal?

GKI is a ratio. If your glucose is normal (say 85 mg/dL) but your ketones are very low (say 0.2 mmol/L), the math gives you a GKI of 23.6 — which is high. This just means you're running primarily on glucose, which is normal for someone eating a standard diet.

What's a "good" GKI?

It depends on your goals. For general wellness and moderate fat-burning, the Moderate Ketosis zone (1–3) is the sweet spot most people aim for. The Therapeutic zone (below 1) is intentionally restrictive and primarily used in clinical protocols. There's nothing wrong with being in the Mild Ketosis or Out of Ketosis zones — it just means your body is using more glucose relative to ketones at that moment.

How often should I test?

Once or twice a day is enough for most people. If you're experimenting with fasting lengths or dialing in your carb tolerance, test more frequently for a week or two to build your personal data set, then pull back to maintenance testing.