Blood Sugar and A1C: How Daily Readings Convert to 3-Month Averages
The A1C-to-glucose formula is eAG = 28.7 x A1C - 46.7. A1C of 7.0% = average glucose of 154 mg/dL. Why CGM-derived GMI sometimes disagrees with lab A1C and what to do about it.
What Is A1C and How Does It Relate to Daily Blood Sugar?
A1C (glycated hemoglobin, HbA1c) is a blood test that measures the percentage of hemoglobin proteins in red blood cells that have glucose permanently attached. Because red blood cells have a lifespan of approximately 90 to 120 days, A1C reflects the average blood glucose concentration over the preceding 2 to 3 months. An A1C of 5.7% corresponds to an estimated average glucose (eAG) of approximately 117 mg/dL. An A1C of 7.0% corresponds to 154 mg/dL. An A1C of 10.0% corresponds to 240 mg/dL. The conversion formula, validated by the ADAG (A1C-Derived Average Glucose) study involving 507 participants from 10 international centers and published in Diabetes Care in 2008, is: eAG (mg/dL) = 28.7 x A1C - 46.7. This formula has a correlation coefficient (r) of 0.92, indicating strong but imperfect agreement. The ADA uses A1C as the primary diagnostic criterion for diabetes (6.5% or higher on two separate tests) and the primary management metric for assessing long-term glucose control. However, A1C has fundamental limitations that CGM technology now addresses.
The A1C-to-Blood Sugar Conversion Table
The following conversion values are used by clinicians and CGM software to translate between A1C percentages and average blood sugar in mg/dL. An A1C of 5.0% equals an estimated average glucose of 97 mg/dL. An A1C of 5.5% equals 111 mg/dL. An A1C of 5.7% equals 117 mg/dL — the prediabetes threshold. An A1C of 6.0% equals 126 mg/dL. An A1C of 6.5% equals 140 mg/dL — the diabetes diagnostic threshold. An A1C of 7.0% equals 154 mg/dL — the standard management target. An A1C of 7.5% equals 169 mg/dL. An A1C of 8.0% equals 183 mg/dL. An A1C of 8.5% equals 197 mg/dL. An A1C of 9.0% equals 212 mg/dL. An A1C of 9.5% equals 226 mg/dL. An A1C of 10.0% equals 240 mg/dL. An A1C of 11.0% equals 269 mg/dL. An A1C of 12.0% equals 298 mg/dL. These conversions apply to the standard NGSP (National Glycohemoglobin Standardization Program) A1C assay and are based on venous plasma glucose equivalents. Each 1% change in A1C corresponds to approximately a 28.7 mg/dL change in average glucose — a relationship that is linear across the clinically relevant range of 5% to 12% A1C.
Why A1C and CGM Averages Sometimes Disagree
A common source of confusion for CGM users is the discrepancy between their lab A1C and the CGM-derived average glucose or Glucose Management Indicator (GMI). A person with a lab A1C of 7.0% (expected average of 154 mg/dL) may see a CGM average of 140 mg/dL — a 14 mg/dL gap. This discrepancy occurs because A1C is not a pure measurement of average glucose. A1C reflects the rate of hemoglobin glycation, which varies between individuals based on red blood cell lifespan, hemoglobin variants, and glycation kinetics. Approximately 15% of the population has a higher-than-average glycation rate (they produce more glycated hemoglobin at the same glucose level), and 15% has a lower-than-average rate. A 2018 study by Bergenstal et al. in Diabetes Care analyzed 528 participants with simultaneous A1C and CGM data and found that the gap between lab A1C and GMI ranged from -1.0% to +1.0%, with a standard deviation of 0.36%. Conditions that affect red blood cell turnover — iron deficiency anemia (falsely elevates A1C), hemolytic anemia (falsely lowers A1C), recent blood transfusion, chronic kidney disease, and hemoglobin variants (HbS, HbC, HbE) — can skew A1C results by 0.5% to 2.0%. In these populations, CGM-derived GMI is a more reliable indicator of actual glucose control.
Glucose Management Indicator: CGM-Derived A1C
The Glucose Management Indicator (GMI) is a metric calculated from CGM data that estimates what a person's A1C would be based on their mean sensor glucose over 14 or more days with at least 70% sensor wear time. The GMI formula, developed by Bergenstal et al. and published in Diabetes Care in 2018, is: GMI (%) = 3.31 + (0.02392 x mean glucose in mg/dL). For a mean CGM glucose of 154 mg/dL, the calculated GMI is 7.0% — identical to the expected A1C. However, GMI and lab A1C will match exactly only if the individual has average hemoglobin glycation kinetics. The clinical advantage of GMI is that it updates in real time as CGM data accumulates, whereas lab A1C requires a quarterly blood draw. GMI also responds more quickly to glucose changes — a dramatic improvement in glucose control will be reflected in GMI within 2 weeks, but won't fully appear in lab A1C for 2 to 3 months because of the red blood cell lifespan. Every major CGM platform now displays GMI: Dexcom Clarity, LibreView, CareLink, and Tidepool all include GMI on their standard reports. The ADA 2024 Standards of Care recommend using GMI alongside lab A1C for a more complete assessment of glycemic status.
Time in Range vs A1C: Which Metric Matters More?
A1C provides a single number that averages glucose over 3 months, but this average conceals critical information about glucose variability. Two people can have identical A1C values of 7.0% (average glucose 154 mg/dL) with dramatically different glucose profiles: one may have stable glucose between 120 and 180 mg/dL with TIR of 80%, while the other swings between 50 and 300 mg/dL with TIR of 40%. The second person has far greater hypoglycemia risk and higher oxidative stress despite the same A1C. The 2019 International Consensus on Time in Range (Battelino et al., Diabetes Care) established that TIR provides complementary and, in many cases, superior clinical information to A1C. The consensus demonstrated that each 10% increase in TIR (2.4 additional hours per day in range) corresponds to approximately 0.5% reduction in A1C. A TIR of 70% correlates with an A1C of approximately 7.0%. Most importantly, TIR has been directly associated with diabetic complications: a 2019 analysis of the DCCT dataset by Beck et al. in Diabetes Care found that a 10% lower TIR was associated with a 64% increased risk of retinopathy progression and a 40% increased risk of microalbuminuria. The clinical community is increasingly viewing A1C as necessary but insufficient, with TIR, TBR, and glucose variability providing the complete picture that guides modern diabetes management.