The simplest way to convert blood sugar to A1C is the ADAG formula: A1C (%) = (average glucose in mg/dL + 46.7) / 28.7. Every 1 percentage point increase in A1C corresponds to about 28.7 mg/dL higher average blood glucose. The conversion is a useful estimate but not a substitute for lab testing.
The Conversion Formula
Derived from the A1C-Derived Average Glucose (ADAG) study by Nathan and colleagues (Diabetes Care, 2008), which followed 507 participants for 3 months with continuous and frequent self-monitoring:
- mg/dL to A1C: A1C (%) = (avg glucose + 46.7) / 28.7
- A1C to mg/dL: avg glucose = (28.7 × A1C) − 46.7
- mmol/L to A1C: A1C (%) = (1.59 × avg mmol/L) + 2.59
Conversion Chart
| A1C % | Average Glucose (mg/dL) | Average Glucose (mmol/L) | Category |
|---|---|---|---|
| 5.0 | 97 | 5.4 | Normal |
| 5.5 | 111 | 6.2 | Normal |
| 5.7 | 117 | 6.5 | Prediabetes |
| 6.0 | 126 | 7.0 | Prediabetes |
| 6.3 | 134 | 7.5 | Prediabetes |
| 6.5 | 140 | 7.8 | Diabetes |
| 7.0 | 154 | 8.6 | Diabetes |
| 7.5 | 169 | 9.4 | Diabetes |
| 8.0 | 183 | 10.2 | Diabetes |
| 9.0 | 212 | 11.8 | Diabetes |
| 10.0 | 240 | 13.4 | Diabetes |
Worked Examples
Example 1: Fingerstick Average
Your meter shows a 30-day average of 135 mg/dL. Using the formula:
A1C = (135 + 46.7) / 28.7 = 181.7 / 28.7 = 6.33%
This places you in the upper prediabetes range. Learn more on our A1C levels hub.
Example 2: Working Backwards from A1C
Your lab A1C is 7.2%. To estimate average glucose:
avg glucose = (28.7 × 7.2) − 46.7 = 206.6 − 46.7 = 160 mg/dL
Example 3: CGM in mmol/L
Your CGM shows an average glucose of 8.0 mmol/L. Using the mmol/L formula:
A1C = (1.59 × 8.0) + 2.59 = 12.72 + 2.59 = ~7.3%
eAG vs. GMI: What’s the Difference?
eAG (estimated average glucose) is calculated from a lab A1C using the ADAG formula. It tells you what your average glucose likely was to produce that A1C.
GMI (glucose management indicator) is calculated from CGM data using Bergenstal’s 2018 formula:
GMI = 3.31 + (0.02392 × mean CGM glucose in mg/dL)
GMI and A1C can differ by 0.3-0.5 points because red-blood-cell biology varies between people. The ADA now recommends reporting both in CGM users.
When the Calculator and A1C Disagree
If your meter average suggests one A1C but your lab shows another, consider:
- Sampling bias: Meter averages miss readings you didn’t take, especially post-meal highs or overnight lows.
- Iron deficiency: Raises A1C 0.3-0.5 points independently of glucose.
- Anemia: Shortens red-cell lifespan and may lower A1C artificially.
- Hemoglobin variants: Can interfere with certain A1C assays.
- Individual glycation rate: Some people’s red cells pick up glucose faster than others.
If the gap exceeds 0.5 points, a CGM wear is often the clearest next step. See the prediabetes hub for background.
Limits of the Calculator
The ADAG formula has a prediction interval of about ±0.5 percentage points for any individual. That means a calculated 6.5% could represent a true A1C anywhere from 6.0% to 7.0%. For clinical decisions — diagnosis, medication changes — trust a lab-measured A1C over any calculator.
Practical Uses
- Check whether your meter average aligns with lab A1C before the next appointment.
- Estimate what A1C a current glucose trend will produce in 3 months.
- Set a realistic glucose target for a target A1C (e.g., to hit 6.5%, aim for daily average ~140 mg/dL).
- Translate CGM GMI to an A1C estimate for non-diabetes conversations.
The Bottom Line
A blood sugar to A1C calculator is a fast way to translate between daily readings and long-term average. Use the ADAG formula for rough conversion, but remember results have a ±0.5 point margin and that lab A1C stays the clinical gold standard. Large gaps between calculated and measured values point to factors worth investigating with your clinician.
