“Glucose equation” is a general term for several formulas that convert glucose-related numbers into each other: A1C to estimated average glucose, mg/dL to mmol/L, and glycemic index to glycemic load. Knowing the equations lets you translate one number on your lab report or food label into something that matches your everyday glucose readings.
Equation 1: A1C to Estimated Average Glucose (eAG)
The A1C test reflects average blood glucose over the prior 2 to 3 months, but the percentage number is abstract. The 2008 A1C-Derived Average Glucose (ADAG) study gave us a formula that converts A1C to an estimated average glucose in the same units as your meter.
| Formula | Units |
|---|---|
| eAG (mg/dL) = 28.7 × A1C − 46.7 | A1C as percent; eAG as mg/dL |
| eAG (mmol/L) = 1.59 × A1C − 2.59 | A1C as percent; eAG as mmol/L |
Examples:
- A1C 5.0% → eAG 97 mg/dL (5.4 mmol/L)
- A1C 5.7% → eAG 117 mg/dL (6.5 mmol/L)
- A1C 6.5% → eAG 140 mg/dL (7.8 mmol/L) — the diabetes threshold
- A1C 7.0% → eAG 154 mg/dL (8.6 mmol/L)
- A1C 9.0% → eAG 212 mg/dL (11.8 mmol/L)
For deeper context on what different A1C values mean, visit our A1C levels hub.
Equation 2: mg/dL ↔ mmol/L
Glucose meters in the US report in milligrams per deciliter (mg/dL). Most other countries and scientific journals use millimoles per liter (mmol/L). The conversion factor is 18, because the molecular weight of glucose is 180 g/mol, and 1 dL = 0.1 L.
| From | To | Formula |
|---|---|---|
| mg/dL | mmol/L | mmol/L = mg/dL ÷ 18 |
| mmol/L | mg/dL | mg/dL = mmol/L × 18 |
Quick examples:
- 100 mg/dL = 5.6 mmol/L
- 126 mg/dL = 7.0 mmol/L (fasting diabetes threshold)
- 200 mg/dL = 11.1 mmol/L (2-hour OGTT or random diabetes threshold)
- 70 mg/dL = 3.9 mmol/L (common low-glucose alarm)
Equation 3: Glycemic Load (GL) from Glycemic Index (GI)
Glycemic index measures how quickly a food raises blood sugar on a 0-to-100 scale, but it does not account for serving size. Glycemic load combines both.
| Formula | Interpretation |
|---|---|
| GL = GI × grams of carbohydrate per serving ÷ 100 | Low = 10 or less; Medium = 11 to 19; High = 20 or more |
Worked examples:
- 1 cup watermelon: GI 72, 11 g carbs → GL = 72 × 11 ÷ 100 ≈ 8 (low)
- 1 medium banana: GI 51, 27 g carbs → GL = 51 × 27 ÷ 100 ≈ 14 (medium)
- 1 cup white rice: GI 73, 45 g carbs → GL = 73 × 45 ÷ 100 ≈ 33 (high)
- 1 medium apple: GI 36, 21 g carbs → GL = 36 × 21 ÷ 100 ≈ 8 (low)
Equation 4: Plasma Glucose vs Whole-Blood Glucose
Lab plasma glucose is about 10 to 11 percent higher than capillary whole-blood glucose from a fingerstick because plasma has less water than red blood cells. Modern meters correct for this and report a plasma-equivalent number, so you typically do not have to apply a conversion yourself — but know this if you compare an old meter to a new one.
Equation 5: Glucose Management Indicator (GMI)
For continuous glucose monitor users, GMI estimates what an A1C would be based on the CGM average over the last 14 days. The formula is:
GMI (%) = 3.31 + 0.02392 × mean glucose (mg/dL)
A 14-day average of 150 mg/dL translates to a GMI of roughly 6.9. GMI is close to A1C in most people but not identical; differences of 0.3 to 0.5 percentage points are common and usually reflect individual differences in red cell turnover.
When Equations Break Down
All of these formulas are population averages. Your individual numbers can drift from them for biological reasons:
- Faster red cell turnover (hemolysis, recent transfusion, chronic kidney disease) lowers measured A1C without lowering true glucose.
- Slower red cell turnover (iron deficiency, splenectomy) raises A1C without true hyperglycemia.
- Hemoglobin variants (sickle cell trait, HbC, HbE) can interfere with specific A1C assay methods.
- CGM sensor drift can produce GMI values that differ from lab A1C.
- Post-meal spikes vs fasting — A1C and eAG summarize an average and hide the peaks and valleys that a CGM traces.
Using the Equations in Real Life
- Convert your A1C to an average glucose number you can relate to your meter.
- Translate lab results from mg/dL to mmol/L when reading international research.
- Estimate the real-world impact of a food by calculating its glycemic load, not just its index.
- Compare your CGM’s GMI to your lab A1C to see if they agree and to talk with your doctor if they consistently do not.
If you are trying to understand where you fall on the prediabetes-to-diabetes spectrum, start with our prediabetes overview, then look up your specific A1C value in the A1C hub.
The Bottom Line
A handful of simple formulas connect A1C, average glucose, mg/dL, mmol/L, glycemic index, and glycemic load. Use eAG = 28.7 × A1C − 46.7 to make sense of your lab A1C, divide or multiply by 18 to move between mg/dL and mmol/L, and multiply GI by grams of carbs per serving and divide by 100 to get glycemic load. These equations are useful anchors, but always read them against your own pattern of readings and your doctor’s interpretation.
