Overview
Continuous glucose monitoring (CGM) is a genuine game-changer. It shows where your glucose is now, where it’s been, and where it’s heading. The result: faster learning, fewer surprises, and a better percentage of time in range (TIR; 3.9–10.0 mmol/L or 70–180 mg/dL).
A CGM turns glucose from an occasional spot-check into a continuous signal you can respond to in real time. If you don’t have one yet, it’s worth talking with your diabetes team about access.
If you’re already well-versed in CGM, you may want to jump ahead:
- How to choose a CGM — CGM series hub
- Companion podcasts — CGM series podcasts
The detail
CGM glucose vs finger-prick glucose
This diagram shows why CGM readings (interstitial fluid) and finger-prick readings (capillary blood) won’t match exactly.
Key things:
- The CGM reading is usually ~5–10 minutes behind blood glucose, so they will never match perfectly.
- If in doubt, get the finger-prick blood glucose meter out.
- Always keep an ISO-standard meter available, with in-date strips, and use good testing technique.
- CGM updates every ~5 minutes, creating a continuous feedback loop.
- Trend arrows show where glucose is heading. Each CGM has its own arrow system, but they all communicate the same idea: where glucose is likely to be soon (roughly the next 10–20 minutes).
If you use mg/dL, keep this conversion in mind: 1.0 mmol/L ≈ 18 mg/dL. Quick mental maths:
- 2.0 mmol/L ≈ 36 mg/dL
- 1.5 mmol/L ≈ 27 mg/dL
- 1.0 mmol/L ≈ 18 mg/dL
How accurate are CGM devices today?
If you choose wisely, around 90% of readings will be highly accurate, 9% will be accurate enough, and about 1% will be wrong.
That 1% is why you should always have a high-quality, ISO-standard finger-prick meter available.
If the number on the screen doesn’t match how you feel — if something seems off or doubt creeps in — don’t argue with your body. Confirm it.
When in doubt, get the blood glucose meter out.
Why is CGM so much better than finger pricks 99% of the time?
Three main reasons:
- More feedback = faster learning. More data points means quicker cause-and-effect learning.
- Alerts reduce surprises. High and low alarms act like barriers at a bowling alley: they stop you falling off the edge.
- Trend arrows let you act early. They help you anticipate what’s next and use Dynamic Glucose Management to stay on target.
How much time in range should I aim for?
Time in range (TIR) is usually defined as 3.9–10.0 mmol/L (70–180 mg/dL). This chart links higher TIR with better day-to-day energy and future health. But there’s a trade-off: the effort and mental load of diabetes management can rise as TIR targets tighten.
The international target for time-in-range is 70%, but the Measuring success Foundations Guide explains what that number means in real life.
What matters is finding the sweet spot between time-in-range and quality of life.
That balance will change over time as new challenges and priorities come your way — and that’s normal.
The goal isn’t rigidity; it’s adaptation. Stay responsive, not perfectionist, and keep moving forward.
Practical foundations
Don’t chase day-to-day perfection. Use CGM to learn your patterns, adjust one lever at a time, and aim for a sustainable TIR that fits your current life constraints.
What’s next
Next up: Basal insulin.
Foundations / related content
The order shown below is recommended, but navigate as you see fit.
- What is Diabetes?
- Continuous glucose monitoring (CGM)
- Basal insulin
- Bolus insulin
- Correction insulin
- Carbohydrate counting
- Three balanced meals
- Hypoglycaemia
- Measuring success
