Estimate your Glucose Management Indicator (GMI) and approximate HbA1c from your CGM average. GMI is the modern way to express CGM data in HbA1c-equivalent terms — a single number you can compare against your last lab HbA1c. Enter your average sensor glucose and the calculator returns the GMI plus your time-in-range targets.
Time-in-Range & GMI Estimator
What GMI is — and isn’t
The Glucose Management Indicator (GMI) is a CGM-derived number that looks like HbA1c but is calculated entirely from sensor glucose data. The formula was published by Bergenstal et al. in 2018:
GMI (%) = 3.31 + 0.02392 × mean glucose (mg/dL)
It’s not the same as a laboratory HbA1c — the two can differ by up to 0.5%. The reasons include the contribution of pre-CGM-era data baked into HbA1c, individual variability in red-blood-cell turnover, and any condition that distorts HbA1c (anaemia, transfusion, haemoglobinopathies, kidney disease, pregnancy).
Use both. If they consistently disagree, your team will want to know why.
The TIR targets used here
Targets come from the 2019 international consensus on time-in-range (Battelino et al., Diabetes Care 2019) and are now incorporated into NICE, ADA, and EASD guidance.
| Population | TIR (3.9–10 mmol/L) | Below range (<3.9) | Severe low (<3.0) |
|---|---|---|---|
| Type 1 / Type 2 adults | > 70% | < 4% | < 1% |
| Older / high-risk | > 50% | < 1% | < 1% |
| Pregnancy with T1D | > 70% (target 3.5–7.8 mmol/L) | < 4% | < 1% |
Each 5% improvement in TIR is associated with a meaningful reduction in long-term complication risk in observational data — though the strongest evidence still comes from HbA1c-based trials.
FAQ
Why does my GMI differ from my last HbA1c?
Several reasons: HbA1c reflects 8–12 weeks while GMI usually reflects the last 14 days; HbA1c is influenced by red-cell lifespan, anaemia, kidney disease, pregnancy, and some haemoglobin variants; and CGMs themselves have a small bias (typically ±5%) compared with venous plasma glucose. A persistent gap of more than 0.5% is worth discussing with your team.
How long should my CGM data be averaged over?
The international consensus recommends 14 days of data with at least 70% sensor wear time. Most pump and CGM apps show this directly in the AGP (Ambulatory Glucose Profile) report.
Is high TIR more important than low HbA1c?
They tell complementary stories. HbA1c is a single number tied to long-term complication trials. TIR shows where in the day glucose is escaping target — useful for treatment adjustment. Most clinicians now use both together.
Related
References: Bergenstal RM et al. Glucose Management Indicator. Diabetes Care 2018;41:2275-2280. Battelino T et al. Clinical targets for continuous glucose monitoring data interpretation: recommendations from the international consensus on time in range. Diabetes Care 2019;42:1593-1603. ADA Standards of Care 2026. Reviewed: May 2026.
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