CGM to HbA1c Calculator
Glucose Management Indicator (GMI) — For UK CGM Users
Enter Your Average CGM Reading
Find your average glucose in your CGM app (7, 14, 30 or 90-day average). The 90-day average gives the closest match to a lab HbA1c.
This calculator uses the standard Glucose Management Indicator (GMI) formula to estimate your HbA1c from your CGM average glucose reading.
GMI (mmol/mol) = 12.68 + (4.71 × your average glucose in mmol/L)
Your estimated GMI may differ from your NHS lab HbA1c by 0–5 mmol/mol. This is completely normal and happens because they measure slightly different things. Track the trend rather than the exact number.
Quick Reference Table
| Avg Glucose (mmol/L) | Est. HbA1c (mmol/mol) | Est. HbA1c (%) | Zone |
|---|---|---|---|
| 4.7 | 35 | 5.3 | Normal |
| 5.0 | 36 | 5.4 | Normal |
| 5.4 | 38 | 5.6 | Normal |
| 5.7 | 40 | 5.7 | Normal |
| 6.0 | 41 | 5.9 | Borderline |
| 6.5 | 43 | 6.1 | Pre-diabetic |
| 7.0 | 46 | 6.3 | Good T2 control |
| 7.2 | 47 | 6.4 | Good T2 control |
| 7.8 | 49 | 6.6 | At target |
| 8.5 | 53 | 7.0 | NHS T2 target |
| 10.0 | 60 | 7.7 | Needs review |
| 12.0 | 69 | 8.6 | Urgent |
Important Things to Know
Why does my CGM estimate differ from my lab HbA1c?
They typically differ by 0–5 mmol/mol. Your lab HbA1c measures glucose stuck to red blood cells over 3 months. Your CGM estimates from glucose readings (GMI). They measure related but different things.
Which number should I trust?
Your lab HbA1c is the official number your GP uses. But your CGM data is more useful day-to-day for spotting patterns and trends. Use both together.
What is Time in Range (TIR)?
The percentage of time your glucose stays between 3.9–10.0 mmol/L. Aim for over 70%. Many specialists now consider TIR as important as HbA1c.
Which average period is most accurate?
Your 90-day average gives the closest match to a lab HbA1c, since HbA1c reflects about 3 months of glucose levels. Shorter periods are useful for spotting recent trends.