What Is the CGM to A1C Calculator?
The CGM to A1C Calculator is a free online tool designed for users who need quick, accurate calculations in the practical calculation space. By entering your convert, average glucose, a1c, you get instant results including estimated a1c, average glucose, category. No formulas to memorize, no spreadsheets to build — just enter your numbers and get the answer in seconds. Whether you're a beginner or experienced professional, this calculator saves you time and eliminates guesswork.
Why This Calculation Matters
Getting estimated a1c right can make the difference between success and costly mistakes. In practical calculation, small errors compound quickly. Manual calculations are error-prone and time-consuming, especially under pressure. This calculator applies proven formulas used by users worldwide, giving you confidence that your numbers are correct. Use it to get accurate results with precision and avoid common pitfalls that trip up beginners.
When Should You Use This Calculator?
This tool is most useful when you know your convert and need to find the right estimated a1c. It's also great for quick estimates before committing to a decision, and to double-check manual calculations or professional quotes, and when comparing different scenarios side by side. Bookmark this page and come back whenever you need a fast, reliable answer — the calculator is always free and requires no signup.
CGM to A1C Calculator
Average Glucose to A1C Conversion Table
Based on the ADAG study formula. Time-in-range (TIR) estimates assume standard glucose variability.
| Avg Glucose (mg/dL) | Est. A1C (%) | Category | Approx. TIR (70-180) | Risk Level |
|---|---|---|---|---|
| 90 | 4.8% | Normal | >95% | Low |
| 110 | 5.5% | Normal | >90% | Low |
| 126 | 6.0% | Prediabetes | ~85% | Moderate |
| 140 | 6.5% | Prediabetes/Diabetes | ~75% | Elevated |
| 170 | 7.5% | Diabetes | ~55% | High |
| 212 | 9.0% | Diabetes (uncontrolled) | ~30% | Very High |
How to Use This Calculator
- Enter Your Convert: Start by entering your convert — this is the primary input for the calculation.
- Fill In Additional Details: Complete the remaining fields: average glucose, a1c, time in range 70-180 mg/dl. Each value refines the calculation for greater accuracy.
- Click Calculate: Hit the Calculate button to run the numbers. Results appear instantly below.
- Review Your Results: Check your estimated a1c, average glucose, category. Use these figures to inform your next decision or compare against alternative scenarios.
How It Works
This calculator converts between CGM average glucose readings and estimated A1C (also called Glucose Management Indicator, or GMI) using the standard ADAG formula. It also provides context on time-in-range metrics.
The basic rule:
- A1C = (average glucose in mg/dL + 46.7) / 28.7 (ADAG study formula)
- Average glucose = (A1C × 28.7) − 46.7 (reverse formula)
- Normal: A1C below 5.7% (avg glucose below 117 mg/dL)
- Prediabetes: A1C 5.7-6.4% (avg glucose 117-137 mg/dL)
CGM-derived average glucose may differ from lab A1C because A1C reflects a 3-month weighted average of red blood cell glycation, while CGM captures a 14-30 day average. Certain conditions (anemia, hemoglobin variants) can affect A1C accuracy. Always discuss results with your healthcare provider.
Tips & Considerations
- Double-check your convert before calculating — even small input errors can significantly change your results.
- Run the calculator with different values to compare scenarios and find the optimal approach for your situation.
- Pay attention to both estimated a1c and average glucose — they work together to give you the full picture.
- Bookmark this page for quick access next time you need to get accurate results.
- If you're unsure about your time in range 70-180 mg/dl, start with a conservative estimate and adjust from there.
Frequently Asked Questions
How does CGM average glucose relate to A1C?
The ADAG study established the formula: A1C = (average glucose + 46.7) / 28.7. An average glucose of 120 mg/dL corresponds to approximately an A1C of 5.8%. CGM-derived estimates are now called GMI (Glucose Management Indicator) and may differ slightly from lab A1C.
What is a good time in range?
For most people with diabetes, the target is >70% time in range (70-180 mg/dL). For those without diabetes, >90% is typical. Less than 4% should be below 70 mg/dL (hypoglycemia). Each 5% improvement in time-in-range corresponds to roughly a 0.5% reduction in A1C.
Why might my CGM average differ from my lab A1C?
Lab A1C measures glycated hemoglobin over 2-3 months and is affected by red blood cell lifespan. CGM measures interstitial glucose over 14-30 days. Conditions like iron deficiency anemia, hemoglobin variants, kidney disease, or recent blood transfusions can cause lab A1C to be higher or lower than expected from glucose readings.
What is a normal average glucose?
For non-diabetic adults, average glucose is typically 80-100 mg/dL (A1C 4.6-5.4%). The prediabetes range is 117-137 mg/dL (A1C 5.7-6.4%). Optimal metabolic health advocates suggest targeting an average under 100 mg/dL with minimal spikes above 140 mg/dL.
What is GMI (Glucose Management Indicator)?
GMI is the term used for the A1C estimate derived from CGM data. It replaced the older term eA1C (estimated A1C) to clarify that it is a CGM-based calculation, not a lab measurement. GMI and lab A1C may differ by 0.3-0.5% in either direction, which is normal.
How often should I check my A1C?
For people with diabetes: every 3 months if adjusting treatment, or every 6 months if stable. For prediabetes: every 6-12 months. CGM users can track GMI continuously but should still get a lab A1C 2-4 times per year to confirm. The lab test captures information CGM does not.