Time in Range Calculator

The consensus targets

These come from the 2019 international consensus on time in range, and they are what most clinics now work to for adults with type 1 or type 2 diabetes. Pregnancy, older age and high hypoglycaemia risk all have separate targets — if any of those apply to you, your team's numbers take priority over these.

Why this became the number people talk about

HbA1c gives one figure for roughly three months. Two people can share an HbA1c of 7.0% while one runs steadily near target and the other swings between hypos and highs that average out to the same place. Their risk profiles are not the same, and their days do not feel the same. Time in Range separates them, which is why it has become the measure people actually discuss with each other.

Time below range matters more than time above

It is tempting to chase the headline percentage, but the low bands carry the immediate danger. The targets are deliberately strict: under 4% below 3.9 mmol/L, and under 1% below 3.0 mmol/L. One percent of a day is about fifteen minutes. A rise in Time in Range bought by running low more often is not an improvement, and any honest reading of your own data starts at the bottom of the chart rather than the middle.

Five points is a real change

A 5 percentage point improvement in Time in Range is considered clinically meaningful — roughly 72 minutes more of each day spent in range. That is a useful frame, because it makes the goal concrete and achievable instead of a vague push toward a perfect number that nobody sustains.

How much data you need

Fourteen days of CGM data with at least 70% sensor wear is the usual standard for a figure that reflects your general pattern. A single good or bad day tells you very little. If your report covers less than that, treat the number as a hint rather than a verdict.

Where the boundaries 3.9 and 10.0 come from

The range is not a round number someone picked. The lower bound, 3.9 mmol/L (70 mg/dL), is the level at which counter-regulatory hormone responses begin in most people without diabetes — the point the body itself treats as low. The upper bound, 10.0 mmol/L (180 mg/dL), approximates the renal threshold, above which glucose starts spilling into urine, and is close to the post-meal ceiling seen in people without diabetes. The tighter secondary thresholds, 3.0 and 13.9 mmol/L, mark clinically significant hypoglycaemia and the level above which ketone risk climbs. Every band on this page traces back to a physiological event rather than a preference.

GMI is not your A1c, and the difference is normal

Most CGM reports show a Glucose Management Indicator alongside Time in Range. It is calculated straight from your mean sensor glucose: GMI% = 3.31 + 0.02392 × mean glucose in mg/dL, or 3.31 + 0.431 × mean glucose in mmol/L. It is an estimate of what a lab A1c would probably show, derived from the same population data as any A1c conversion — not a measurement of your haemoglobin. Roughly a fifth of people show a gap of half a percentage point or more between GMI and their actual lab result, in either direction, and that gap tends to be consistent for a given person. A stable personal difference between the two is a known pattern, not a fault in either number.

Coefficient of variation — the number under the number

Time in Range tells you how much of the period you spent inside the band. It says nothing about how violently you moved within it. Coefficient of variation does: it is your glucose standard deviation divided by your mean, expressed as a percentage, and the consensus threshold is 36% or below. Below that, days tend to resemble each other and readings cluster. Above it, the same average is being produced by larger swings, and unpredictable lows become more likely. Two reports showing 70% Time in Range with a CV of 30% and 45% describe genuinely different weeks. If your report shows CV, it is worth as much attention as the headline figure.

A percentage hides when things happened

Time in Range flattens a whole period into one figure, and in doing so it discards timing entirely. Seventy percent in range with the missing thirty percent scattered evenly across the day is a different situation from seventy percent where every low lands between two and five in the morning, or every high follows the same meal. The percentage is identical; what is happening is not. This is why standard reports pair the number with a daily profile curve — the curve is where the pattern lives, and the percentage is only its summary.

What the number cannot tell you

A CGM measures interstitial fluid, not blood, so readings lag capillary glucose by several minutes and lag furthest when levels are moving fastest. Sensors also carry their own error margin, and pressure on a sensor during sleep can produce compression lows that were never real. Time in Range inherits all of it. None of this makes the figure untrustworthy — it makes it a summary of sensor data over a period, which is exactly what it claims to be, and nothing more.