Blood Sugar Numbers Explained: Targets, Testing and What to Log

A blood sugar reading only helps if you know what the number means and what to do next. Without that, a meter is a machine that produces worry four times a day.

I am not a doctor and this is not my story. I read numbers and logs for a living, and this is the explanation I would want before starting to log readings: the units, the ranges doctors use as a starting point, when a reading is worth taking, and how to record readings so patterns show up. Your doctor sets your targets. The ranges below are the general guidelines they start from, not a replacement for them.

Two units, one conversion

Blood glucose is measured in milligrams per deciliter (mg/dL) in the United States and in millimoles per liter (mmol/L) in most other countries, the Netherlands included. Divide mg/dL by 18 to get mmol/L. So 126 mg/dL is 7.0 mmol/L, and 180 mg/dL is 10.0.

HbA1c, the three-month average, has two units as well: a percentage in the US, mmol/mol in most of Europe, Dutch and British labs included. 7 percent is 53 mmol/mol; 6.5 percent is 48; 8 percent is 64.

Pick one unit for your log and never mix them. A 7.0 next to a 126 in the same column is the fastest way to misread your own data.

The ranges doctors start from

These are the American Diabetes Association's Standards of Care targets for most non-pregnant adults with diabetes; Dutch and other European guidelines land in the same neighborhood. Your own targets can be stricter or looser depending on age, medication, other conditions and how often you go low. That is exactly the conversation to have with your doctor.

• Before a meal: 80 to 130 mg/dL (4.4 to 7.2 mmol/L)
• One to two hours after the start of a meal: below 180 mg/dL (10.0 mmol/L)
• HbA1c: below 7 percent (53 mmol/mol) for many adults; below 8 percent (64) is a common target for older adults or people with complications

For people without a diagnosis, a fasting value below 100 mg/dL (5.6) is normal, 100 to 125 (5.6 to 6.9) is the prediabetes range, and 126 (7.0) or higher on two separate occasions is how diabetes is diagnosed.

The thresholds that need action rather than a logbook entry:

• Below 70 mg/dL (3.9 mmol/L): low. Take 15 grams of fast carbohydrate (glucose tablets, juice), recheck after 15 minutes, repeat if still low. Below 54 (3.0) is serious and worth telling your doctor about even if it resolved.
• Above 250 mg/dL (13.9) with type 1: check for ketones. Persistent highs with any type: call your doctor.
• Above 300 (16.7) with nausea, confusion or rapid breathing: urgent care.

When a reading is worth taking

How often you test is a treatment decision, so it comes from your doctor or diabetes nurse. What follows is the logic behind the usual advice, so the schedule makes sense instead of feeling arbitrary.

On insulin (type 1, or type 2 with insulin). Before meals, at bedtime, before driving, before and after exercise, and whenever you feel off. Those are the moments dose decisions get made. A CGM replaces nearly all of those finger pricks.

Type 2 without insulin. The evidence that testing at random moments every day improves anything is thin, and the ADA says as much. Paired readings tell you far more: test before a meal and again two hours after the start of it. The difference is that meal's effect on you, and after a couple of weeks you know which meals are fine and which are not. Ask your doctor what rise counts as acceptable for you; there is no single number.

Prediabetes. An occasional fasting reading to watch the trend, plus paired readings around the meals you suspect.

One dense day a week. Fasting, before and two hours after each meal, and bedtime: seven readings. Spot checks miss the shape of a day. One full day a week shows it, and it costs a lot less than seven strips every day.

What a CGM adds

A continuous glucose monitor is a small sensor under the skin that reads glucose in the fluid between cells every one to five minutes and sends it to a phone. It runs a few minutes behind blood, which matters when values are moving fast, and a sensor lasts 10 to 15 days depending on the brand.

The current names: Dexcom G7 (10-day sensor, with a 15-day version rolling out since 2025), Abbott FreeStyle Libre 3 and 3 Plus (14 and 15 days), and Medtronic's sensors, which are built to pair with its insulin pumps. In the United States, Dexcom Stelo and Abbott Lingo have been sold without a prescription since 2024, aimed at people who do not use insulin. Who gets a CGM reimbursed differs per country and per treatment, so ask before assuming.

A CGM changes what you look at. Instead of single readings you get:

• Time in range: the share of readings between 70 and 180 mg/dL (3.9 to 10.0). The usual goal is above 70 percent.
• Time below range: below 70. Goal under 4 percent, and under 1 percent below 54.
• Time above range: above 180. Goal under 25 percent.
• Coefficient of variation: how much the values swing. At or below 36 percent counts as stable.
• Glucose management indicator: an estimated HbA1c from the sensor data, useful between lab tests.

The manufacturers' apps (Dexcom Clarity, LibreView) calculate all of these and can share the report with your clinic directly.

Log it so patterns show up

A reading on its own is a data point. Two weeks of readings with context is information, and the context is the part people skip.

Whatever you log in, use these columns: date, time, the value, a tag (fasting, before meal, two hours after, bedtime, exercise), what you ate, and a notes field for stress, illness, alcohol, poor sleep and any change in medication. The tag column is what makes filtering possible later.

If you wear a CGM, its app does the pattern detection. For finger-prick readings, a logbook app such as mySugr or Glucose Buddy works, and so does a spreadsheet with one row per reading, which is what I would use myself because I can sort and filter it any way I like.

The patterns that show up most often, and what they usually mean:

• Fasting value higher than the bedtime value. The dawn phenomenon: hormones released in the early morning push glucose up. A medication timing question for your doctor, not a diet failure.
• A spike after the same meal every time. Portion or composition. A smaller carbohydrate portion, more fiber or protein alongside it, or a different food.
• Drops during or after exercise. Common on insulin. A small carbohydrate snack before, and glucose within reach during.
• Highs during a stressful week with no change in food. Cortisol raises glucose. Not everything is diet.
• Good control at some hours and not at others. Often medication timing. That one is for the doctor.

Bring the log to every appointment. Two weeks is the minimum before a pattern means anything.

Food, movement and stress: what moves the number

Carbohydrate moves it most. Sugar, white bread, juice and white rice raise glucose fast; beans, whole grains and vegetables raise it slowly. The glycemic index ranks foods on this, and it is a guide rather than a rule, because the response differs per person. That is why the paired readings above matter more than any table.

What tends to flatten the curve, for most people:

• Carbohydrate together with protein, fat or fiber, not on its own: an apple with peanut butter rather than an apple
• Vegetables and protein first, the carbohydrate last; small studies show a lower peak from the order alone
• A 10 to 15 minute walk after eating, which lowers the post-meal peak measurably
• Fiber, around 25 to 30 grams a day

Alcohol can push glucose down hours later, which is a real risk on insulin. Illness and stress push it up. A short, intense effort can raise it briefly, while steady aerobic exercise lowers it, sometimes for a day or more afterwards.

The foods that surprise people in a log: breakfast cereal, fruit juice, dried fruit, granola bars, white rice and restaurant sauces with sugar in them. Test your own response with a paired reading before you believe the label, or me.

If you count what you eat, the Calorie Calculator gives you a daily energy target to log intake against; a dietitian sets the carbohydrate share of it. In type 2, modest weight loss (around 5 percent of body weight) improves control for many people, and the BMI Calculator is a crude but honest starting point for that conversation.

FAQ

How accurate is a home meter?

Meters that meet the ISO 15197 standard must have 95 percent of readings within 15 mg/dL of the lab value below 100 mg/dL, and within 15 percent above it. So a true 100 can read anywhere from 85 to 115, and two meters disagreeing by 10 percent is normal. That is fine for daily decisions. Diagnosis and big medication changes are done on lab values.

Is tracking useful without diabetes?

People without diabetes increasingly wear a CGM to see which meals spike them. The medical benefit for healthy people is not established; the behavioral effect, mostly eating less of the things that spike you, is real for some. Treat it as curiosity, not treatment.

My readings are consistently above target. What now?

Do not change medication on your own. Bring the log, with the tags and the food column filled in, to your doctor. Meanwhile, look for the meals that spike you, add the walk after eating, and mind sleep and stress. If two to four weeks of that changes nothing, medication is the next conversation, and that one is for the doctor.

How does exercise change the numbers?

Steady aerobic exercise (walking, cycling, swimming) lowers glucose by making muscle take it up, and the effect can last a day or two. Short intense efforts can raise it briefly through the stress response. On insulin, test before and after until you know your own pattern.