Medically reviewed guide · Last updated: 23 June 2026 · Reading time: approx. 13 min

Measuring blood sugar correctly and understanding it

Measuring one's own blood sugar is for many people with diabetes part of everyday life, and those who want to keep an eye on their values out of curiosity or for prevention too benefit from the right approach. A correct measurement and the understanding of the most important values are the basis for assessing one's own metabolism better. Even small mistakes with the measurement can distort the result, and wrongly placed values quickly lead to unnecessary worry or deceptive security. This guide shows step by step how the classic finger prick works, which target values are considered normal, what the HbA1c says as a long-term value and how modern sensors for continuous measurement work. Important beforehand: this guide serves information but does not replace a medical diagnosis and no individual advice. The target values fitting for you are always determined by the medical practice. The aim is to explain the technique and the most important figures to you so understandably that you can read off your values securely and go well prepared into the conversation with the practice.

At a glance

  • Fasting, the blood sugar with metabolically healthy people lies under 100 mg/dl, about 70 to 99 mg/dl.
  • Two hours after eating, the value should lie again under 140 mg/dl.
  • The HbA1c shows as blood sugar memory the average of the last eight to twelve weeks.
  • With the finger prick: wash hands, use the side of the fingertip, do not squeeze too strongly.
  • CGM sensors measure the tissue sugar around the clock, with about ten minutes delay.

Step by step: how to measure the blood sugar correctly

The most frequent method for self-measurement is the finger prick, with which a small drop of capillary blood is obtained from the fingertip and analysed with a meter. This spot measurement delivers a snapshot of the current value and is the most widespread form of self-check in everyday life. So that the result is reliable, a clean approach is worthwhile. First wash the hands thoroughly with warm water and soap and dry them well. That is more important than it sounds, because sugar residues on the fingers, for example from fruit or a sweet drink, can distort the value clearly upwards. Residues of disinfectants too can change the result, which is why warm water and soap are more reliable than pure disinfecting. Warm water has moreover the practical advantage that it promotes the circulation of the fingers, so that the drop of blood can be obtained more easily.

Then prick with the lancing device at the side into the fingertip, ideally on the middle or ring finger, since thumb and index finger are more sensitive and are more strongly used in everyday life. The side spot is moreover less sensitive to pain than the middle of the fingertip, and it helps to change the puncture sites regularly, so that the skin does not harden. Do not press the finger too firmly, because strong squeezing mixes tissue fluid under the blood and distorts the value. Put the drop onto the test strip, read off the result after a few seconds and note value, time and unit. A blood sugar diary, whether on paper or in an app, helps to recognise patterns over the course and to discuss the values later with the medical practice or the diabetes team. Many modern meters store the values automatically, nevertheless it is worth additionally noting particularities such as meals, sport or stress, since they influence the blood sugar.

Capillary at home, venous in the laboratory

For the self-check at home, capillary blood from the fingertip is used. For the actual diagnosis of prediabetes or diabetes, on the other hand, mostly venous blood is determined in the laboratory, since it is more accurate, above all after eating. A single increased value from the home meter is therefore not yet a diagnosis but an occasion to have the values medically clarified. For a secure diagnosis, as a rule several measurements on different days are necessary, since individual values can be distorted by stress, illness or measuring errors. Pay attention moreover to which unit your device measures in, in mg/dl or mmol/l, and always document it, in order to avoid mix-ups.

Understanding target values and HbA1c

In order to be able to place one's own values, a look at the most important orientation figures helps. With metabolically healthy people, the fasting blood sugar, that is the value after at least eight hours without food, lies under 100 mg/dl, normal are about 70 to 99 mg/dl. After a meal the blood sugar rises but should at the latest after two hours lie again under 140 mg/dl. Over the course of the day, a range of about 70 to 140 mg/dl is considered optimal. A healthy sugar metabolism manages to bring the value back into this range again and again, in that the hormone insulin makes the glucose available for the cells according to the key-and-lock principle. A repeated fasting value of 126 mg/dl or higher or an occasional value of 200 mg/dl or higher speaks for a diabetes, a fasting value between 100 and 125 mg/dl for a preliminary stage, the prediabetes. Also a value between 140 and 200 mg/dl two hours after eating points to a disturbed glucose tolerance, that is a preliminary stage with which doctors become alert and often attach an oral glucose tolerance test.

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The HbA1c as a long-term value

Besides the spot measurements, the HbA1c value plays a central role. It is often called long-term blood sugar or blood sugar memory and shows in percent how much of the red blood pigment haemoglobin was sugared in the past about eight to twelve weeks. The higher the blood sugar was on average and the longer it remained increased, the higher the HbA1c turns out. Its great advantage is that it does not deliver a single snapshot but the trend over a longer period. With this it complements the daily measurements ideally, because it shows whether the therapy takes effect overall over weeks. With metabolically healthy people it lies under 5.7 percent. It is determined in the laboratory from venous blood and with diabetes mostly checked every three months. Unlike the self-measurement on the finger, the HbA1c is thus not a value that one determines oneself at home but a laboratory value that assesses the therapy over longer periods.

With an existing diabetes, the German Diabetes Society names an HbA1c target corridor of about 6.5 to 7.5 percent, which is determined individually. Thereby the personal situation plays a big role: with younger people without accompanying illnesses a stricter target can be sensible, while with older or multiply ill people somewhat higher values are justifiable, in order to avoid low blood sugar. An HbA1c under 7.0 percent corresponds approximately to an average blood sugar of around 154 mg/dl. Which target value applies for you is not a question of rules of thumb but is determined together with the medical or diabetological practice. Thereby complaints, accompanying illnesses, the age and the lifestyle are taken into account, so that in the end a target value stands that fits the personal situation and avoids over- as well as low blood sugar as far as possible.

Value Metabolically healthy Sign of diabetes
Fasting under 100 mg/dl repeatedly from 126 mg/dl
2 hours after eating under 140 mg/dl from 200 mg/dl
Occasional value about 70 to 140 mg/dl from 200 mg/dl
HbA1c (long-term) under 5.7 percent target corridor 6.5 to 7.5 percent

CGM and FGM: continuous instead of spot

An alternative to the classic finger prick are modern sensor systems. With the continuous glucose measurement, CGM for short, as well as with the related flash glucose monitoring, FGM for short, one wears a small sensor mostly on the upper arm or belly, whose fine filament sits in the subcutaneous fatty tissue. The sensor measures the tissue sugar around the clock in short intervals, often every five minutes, and transmits the values to a reader or smartphone. That saves the daily finger prick and shows not only a single value but the course over 24 hours including trend arrows that indicate whether the sugar is currently rising or falling. Precisely this constant up and down of the blood sugar, for example after meals or with movement, can be represented clearly better with a sensor than with individual spot measurements.

The most important difference between both systems lies in the transmission: a CGM sends the values automatically and can give an alarm with too high or too low values, while an FGM often shows the value only with active scanning and more rarely has an alarm function. Important to know is that the tissue sugar follows the blood sugar with about ten minutes delay. Precisely with fast changes, for example after eating or during sport, the sensor value can therefore deviate from the actual blood sugar. With conspicuous or implausible sensor values, the classic measurement on the finger continues to serve as control. The continuous glucose measurement is as a rule used under certain conditions, for example with an intensified insulin therapy, and increasingly reimbursed by the health insurers. Whether a sensor is sensible for you is clarified by the treating practice.

Take warning signs seriously

Very low or very high values are a warning signal. With a low blood sugar, sweating, racing heart, trembling, dizziness, ravenous hunger and confusion can occur, more frequently with people who use insulin or certain tablets. A high blood sugar shows itself often through strong thirst, frequent urination and tiredness. Such signs, persistent complaints or strongly deviating values belong promptly medically clarified, since only a medical assessment can reliably place the cause. With a severe low blood sugar with disturbance of consciousness, it is an emergency: call the emergency number 112. Never change a prescribed diabetes therapy on your own or independently.

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In sum it applies: the correct measuring is the basis for understanding the blood sugar. Those who, with the finger prick, pay attention to clean hands, the right spot and the unit and note the values obtain reliable reference points. Important is to place the figures, from the fasting value over the value after eating to the HbA1c as a long-term value. Modern sensors as with the continuous glucose measurement additionally facilitate the course control. Which method and which measuring rhythm are sensible for you depends on your personal situation and should be decided together with the practice, instead of orienting oneself on blanket recommendations from the internet. With suspicion of diabetes, with symptoms such as excessive thirst or with questions about the therapy with insulin, the medical practice is the right point of contact, which also determines the individual target values.

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Record in brite your values, questions and medicines. This way you can in the medical or diabetological practice specifically speak about your blood sugar and the fitting therapy.

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Frequently asked questions about measuring blood sugar

First wash the hands with warm water and soap and dry them well, since sugar residues or disinfectant distort the value. Prick at the side into the fingertip of the middle or ring finger, not into thumb or index finger, and do not squeeze the finger too strongly. Put the drop onto the test strip and read off the result. Note value, time and unit, best in a diary or an app.
With metabolically healthy people the fasting blood sugar lies under 100 mg/dl, normal are about 70 to 99 mg/dl, that is around 3.9 to 5.5 mmol/l. Two hours after eating the value should lie again under 140 mg/dl, over the course of the day about 70 to 140 mg/dl is considered optimal. Slight fluctuations are normal. The target values fitting for you are, in the case of an illness, determined by the medical practice.
The HbA1c, often called long-term blood sugar or blood sugar memory, shows in percent how much haemoglobin was sugared in the last about eight to twelve weeks. It captures the trend instead of individual outliers. With metabolically healthy people it lies under 5.7 percent, with diabetes the German Diabetes Society names an individual target corridor of about 6.5 to 7.5 percent. It is determined in the laboratory, mostly every three months.
Remember the number 18: a value in mg/dl you divide by 18 to get mmol/l, and a value in mmol/l you multiply by 18 to get mg/dl. So about 100 mg/dl correspond to around 5.6 mmol/l. In Germany and the USA, mg/dl is widespread, internationally mmol/l. Document the used unit with every note, in order to avoid mix-ups.
CGM stands for continuous glucose measurement. A small sensor on the upper arm or belly measures the sugar in the subcutaneous fatty tissue around the clock in short intervals and transmits the values to a reader or smartphone. This way course and trends over 24 hours can be represented, and many systems give an alarm with over- or undersugar. The sensor is changed depending on the model every one to two weeks.
Both measure the glucose value via a sensor in the tissue fluid without a daily finger prick. A CGM sends the values automatically and can give an alarm with over- or undersugar. An FGM often shows the value only with active scanning and more rarely has an alarm function. Which system is suitable is discussed medically. With conspicuous sensor values, the classic measurement serves as control.
That depends on the form of diabetes and the therapy and is determined medically. With type 1 diabetes or an intensified insulin therapy, measuring is often done several times daily, with type 2 diabetes without insulin a less frequent check often suffices. Metabolically healthy people as a rule do not have to measure themselves. Sensible are fixed times such as in the morning fasting and two hours after eating. The measuring plan is determined by the practice.
A low blood sugar shows itself often through sweating, racing heart, trembling, dizziness, ravenous hunger and confusion, more frequently with insulin or certain tablets. A high blood sugar becomes noticeable through strong thirst, frequent urination and tiredness. Both are to be taken seriously and belong, with clear signs, medically clarified. With a severe low blood sugar with disturbance of consciousness it applies: call the emergency number 112.
Yes. Insulin and other blood-sugar-lowering medicines are prescription only and available only after medical diagnosis, since choice, dosage and adjustment require a medical, often diabetological assessment. Meters and test strips are freely available, with diabetes the insurers take over the costs under conditions. This guide gives no instruction for insulin dosing, the placement happens in the practice.

Related topics

Sources

  • diabinfo.de (German Diabetes Information Portal): target values, time in range, mg/dl and mmol/l, CGM and tissue sugar delay
  • German Diabetes Society and Helios Gesundheit: HbA1c target corridor, key-and-lock principle of insulin, low and high blood sugar
  • DKV and pflegecampus: finger prick guide, hand washing, side of the fingertip, sources of error, CGM and FGM in comparison
  • gesundheitsinformation.de and professional guides: fasting and postprandial values, prediabetes, oral glucose tolerance test
  • National care guidelines and diabetological sources: diagnosis limits, venous versus capillary blood, measuring frequency
  • Professional overviews on HbA1c: average blood sugar at under 7.0 percent, individual age-dependent target values

This guide serves general, neutral information and does not replace medical advice, diagnosis or treatment. It contains no dosage recommendation, in particular no instruction for insulin dosing. The mentioned target values are general orientation, the individual target values are determined medically. A self-measurement at home does not replace a medical diagnosis, which as a rule is based on venous blood in the laboratory. A prescribed diabetes therapy must not be changed on one's own. With complaints, strongly deviating values or suspicion of diabetes, turn to your medical or diabetological practice. With a severe low blood sugar with disturbance of consciousness or another acute emergency, call the emergency number 112.