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Blood glucose 21 — mmol/L or mg/dL?

378mg/dL21.0 mmol/L

Most meters outside the US show mmol/L. A reading of 21 is far more likely to be 21 mmol/L than 21 mg/dL.

If it is 21 mmol/L

= 378 mg/dL. Fasting without diabetes: Diabetes range · With diabetes before a meal: Very high

Full explanation for 378 mg/dL →

If it really is 21 mg/dL

That is 1.2 mmol/L — severely low. Most meters show "LO" below 20 mg/dL. If the person is shaky, sweaty, confused or drowsy, treat it as a severe low: 15–20 g of fast sugar if they can swallow safely; if not, glucagon if available, recovery position and call 911 (US) or your local emergency number.

Get medical advice now

  • Check ketones if you use insulin. Moderate or high ketones, vomiting, abdominal pain, deep fast breathing or fruity-smelling breath can mean diabetic ketoacidosis — call 911 (US) or your local emergency number or go to the emergency room.
  • Very high readings (especially above 600 mg/dL, often shown as "HI") with extreme thirst, confusion or drowsiness can mean hyperosmolar hyperglycemic state — an emergency.
  • Otherwise contact your care team today and re-check every 2–3 hours.

If you don't have diabetes — reading 21 mmol/L (378 mg/dL)

378 mg/dL is in the diabetes range whether fasting or after a meal. A random reading of 200 mg/dL or more together with typical symptoms (thirst, frequent urination, weight loss) is enough for a diagnosis — see a doctor soon, today.

Fasting (8+ hours without food)

Diabetes rangeScale with 378 mg/dL marked55100126378 mg/dL

2 hours after eating (or glucose tolerance test)

Diabetes rangeScale with 378 mg/dL marked55140200378 mg/dL

If you have diabetes — reading 21 mmol/L (378 mg/dL)

378 mg/dL is very high. Check ketones if you use insulin, and contact your care team if it stays above 300 mg/dL or you feel unwell.

Before a meal / fasting

Very highScale with 378 mg/dL marked5480131181241300378 mg/dL

1–2 hours after a meal

Very highScale with 378 mg/dL marked54180241300378 mg/dL

Targets for most non-pregnant adults (American Diabetes Association (ADA) Standards of Care). Your own targets may be different — for example in pregnancy, for children, older adults or people prone to lows. See pregnancy targets.

378 mg/dL as an average: HbA1c 14.8%

If 378 mg/dL were your average glucose over two to three months, it would match an HbA1c of about 14.8% (above the common treatment goal of below 7%). A single reading says little about your HbA1c — it is the long-term average that counts. Learn more in how to measure blood sugar.

Make sure the number is right

  • Wash and dry your hands before a finger-stick: fruit juice or sugar on the skin can raise the result a lot.
  • Home meters may differ from a lab value by up to about 15%. At 378 mg/dL that means roughly 321–435 mg/dL.
  • CGM sensors measure tissue fluid and lag 5–15 minutes behind blood when glucose changes quickly — confirm lows with a finger-stick.

Check another reading

Nearby values

This page explains one reading in general terms. It is not a diagnosis or a treatment instruction. Follow the plan agreed with your doctor or diabetes team.