CGM to A1C Converter

Estimate your A1C (Glucose Management Indicator) from your continuous glucose monitor's average reading. Convert CGM data to estimated lab A1C values.

Average Glucose

From your CGM app

Glucose Management Indicator

GMI (Glucose Management Indicator) is the estimated A1C calculated from continuous glucose monitor (CGM) data. It uses the internationally standardized formula: GMI = 3.31 + (0.02392 × average glucose in mg/dL), developed from the same ADAG study used for A1C conversion.

How to Use This Calculator

Select your glucose unit (mg/dL or mmol/L)
Enter your CGM average glucose from your CGM app
Use 7-day, 14-day, or 30-day average for best accuracy
Click Calculate to see your estimated A1C (GMI)
Review the estimated Time in Range

FAQ

GMI (Glucose Management Indicator) is the standardized term for estimated A1C derived from CGM data. It uses the same mathematical relationship between average glucose and A1C that lab tests use, but calculates it from your CGM's continuous readings rather than a blood sample. Both measure the same thing - average glucose over time - but GMI reflects the CGM monitoring period (typically 7-30 days) while lab A1C reflects 2-3 months.

GMI and lab A1C can differ by ±0.5% or more for several reasons: (1) Time frame - GMI reflects recent weeks while A1C reflects 2-3 months, (2) Individual variation in red blood cell lifespan affects how glucose binds to hemoglobin, (3) CGM accuracy variations, especially during sensor warm-up or if the sensor is near expiration, (4) Recent changes in glucose control may show in GMI before appearing in lab A1C. Both are valid measures; consistent trends matter more than exact agreement.

Time in Range (TIR) is the percentage of time your glucose stays between 70-180 mg/dL (3.9-10.0 mmol/L). The target is >70% in range. TIR is increasingly recognized as equally important to A1C/GMI because it shows glucose variability - two people can have the same A1C but very different TIR. Higher TIR correlates with lower risk of complications. TIR also captures hypoglycemia (below 70) and hyperglycemia (above 180) that A1C alone doesn't show.

For the most accurate GMI estimation, use a 14-day or 30-day average if available. Longer averages smooth out daily variations and better represent your typical glucose control, similar to how lab A1C reflects 2-3 months. Use a 7-day average only if that's all your CGM provides or if you want to see how recent changes are affecting your control. Avoid using averages during periods with sensor issues, illness, or major diet/medication changes, as these won't represent your typical control.

GMI is a valuable tool for frequent monitoring between lab tests, but it shouldn't completely replace lab A1C testing. Lab A1C remains the gold standard for diagnosis and treatment decisions because it's directly measured rather than estimated. Use GMI to track trends and guide daily management, but continue getting lab A1C tests at intervals recommended by your healthcare provider (typically every 3-6 months for people with diabetes). GMI is particularly useful for catching trends early and adjusting treatment before your next scheduled lab test.

Calculator Limitations

Estimation only: GMI estimates A1C from CGM data - may differ from lab A1C by ±0.5% or more
CGM accuracy: Results depend on CGM sensor accuracy and proper calibration
Time period: Reflects recent monitoring period (7-30 days), not the 2-3 months that lab A1C measures
Individual variation: Red blood cell lifespan and glucose binding vary between individuals
Not diagnostic: Should not replace lab A1C for diagnosis or major treatment decisions

Medical Disclaimer: This calculator is for educational purposes only. GMI is an estimate and should not replace lab A1C testing for diagnosis or treatment decisions. Always consult your healthcare provider for proper interpretation of glucose monitoring data and diabetes management.