Estimated A1c from average glucose: the formula and its limits

Short answer: estimated A1c (%) = (average glucose in mg/dL + 46.7) ÷ 28.7. An average of 154 mg/dL works out to about 7.0%. It is a reference number, not a lab result.

1. Where the formula comes from

The A1c-Derived Average Glucose (ADAG) study (Nathan et al., Diabetes Care, 2008) compared A1c with measured average glucose and found the linear relationship average glucose = 28.7 × A1c − 46.7. Turning it around gives the estimate above.

2. Quick conversion table

126 mg/dL (7.0 mmol/L) → about 6.0%
154 mg/dL (8.6 mmol/L) → about 7.0%
183 mg/dL (10.2 mmol/L) → about 8.0%
212 mg/dL (11.8 mmol/L) → about 9.0%

Using mmol/L? Multiply by 18 to get mg/dL first (8.6 × 18 ≈ 155).

3. Why it can differ from your lab A1c

A1c reflects roughly the last 3 months, weighted toward the most recent weeks. A finger-stick log only sees the moments you tested, so testing mostly fasting can pull the average down, and testing mostly after meals can push it up. Conditions such as anemia, pregnancy or kidney disease can also change how A1c relates to glucose.

4. Use it as a check-in, not a target to chase

The estimate is useful for seeing whether your logged readings are moving in the same direction as your last lab A1c. Bring both numbers to your appointment and let your care team interpret them.

How Dropvel helps

Dropvel calculates an estimated A1c from the readings you logged with the ADAG formula and labels it as a reference estimate. It appears in the Trends tab for 7, 14, 30 or 90 days once there are at least 5 readings, next to average glucose and time in target, and it is included in the PDF report for your doctor.

This guide is general information, not medical advice. Your targets, medication and insulin doses are decided with your care team.

FAQ

Can an estimated A1c replace my lab test?

No. It is a reference number from the readings you logged. Your lab A1c is the value your care team uses.

How many readings do I need for a useful estimate?

More is better, and a spread of times (fasting, after meals, bedtime) matters more than the count. Dropvel only shows an estimate once a period has at least 5 readings, and a 90-day period gives the closest match to how A1c works.

Why is my estimate lower or higher than my lab A1c?

Finger-stick logs miss the times you didn't test, and things like anemia, pregnancy or kidney disease can change how A1c relates to glucose. Talk to your care team if the gap is large.

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