Glucose Variability Explained: How SD and CV% Measure Glycemic Swings
Two glucose logs can have the exact same average of 140 mg/dL while feeling completely different day to day. Learn how Standard Deviation (SD) and Coefficient of Variation (CV%) quantify glucose swings.
Why Average Glucose Alone Can Hide Swings
Consider two sets of readings: Set A is [115, 120, 125] mg/dL (mean = 120 mg/dL), while Set B is [65, 120, 175] mg/dL (also mean = 120 mg/dL). Both sets have the exact same average, but Set B spans both low and high excursions.
Standard Deviation (SD) measures absolute spread around the mean in mg/dL or mmol/L. Coefficient of Variation (CV%) divides SD by the mean (CV% = 100 × SD / mean), producing a normalized percentage that is identical whether calculated in mg/dL or mmol/L.
What the 36% CV Consensus Reference Means
In international CGM consensus guidelines (Monnier et al., Diabetes Care 2017), a CV% threshold of 36% was proposed for continuous glucose monitoring profiles to distinguish relatively stable glucose profiles (<36%) from higher variability (>=36%).
Important caveat: When calculating SD and CV% from fingerstick logs rather than continuous 5-minute CGM streams, the result depends heavily on when fingersticks were taken (for example, testing mostly when symptomatic or after meals) and how many readings were entered.
Limitations and When to Seek Medical Care
- Frequent wide swings—especially readings below 70 mg/dL (3.9 mmol/L) or <54 mg/dL (3.0 mmol/L)—warrant prompt review of medication timing and meal patterns with your healthcare team.
Related Calculators
Glucose Variability Calculator (CV% & SD)
Paste a list of glucose readings (separated by commas, spaces, or newlines) to compute arithmetic mean, population standard deviation (SD), coefficient of variation (CV%), min/max, and count of entered readings within your selected target range.
GMI Calculator (Glucose Management Indicator)
Calculate your Glucose Management Indicator (GMI) in both NGSP-aligned % and IFCC-equivalent mmol/mol from your continuous glucose monitor (CGM) mean glucose using the published Bergenstal et al. (2018) formula.
Blood Glucose Unit Converter (mg/dL ↔ mmol/L)
Convert blood glucose values between milligrams per deciliter (mg/dL) and millimoles per liter (mmol/L) using the exact molar mass factor of glucose (18.0182).
Frequently Asked Questions
Is "Readings in Range" on a fingerstick log the same as CGM Time in Range (TIR)?
No. CGM Time in Range measures the percentage of time spent in range across ~288 automatic readings per day, whereas fingerstick logs only count the discrete moments you chose to test.
Sources and Review Information
- [S22] Monnier L, Colette C, Wojtusciszyn A, et al. Toward Defining the Threshold Between Low and High Glucose Variability in Diabetes (CV 36% consensus threshold). Diabetes Care. 2017;40(7):832-838. Reference Source
- [S11] Bergenstal RM, Beck RW, Close KL, et al. Glucose Management Indicator (GMI): A New Term for Estimating A1C From Continuous Glucose Monitoring. Diabetes Care. 2018;41(11):2275-2280. Reference Source
Prepared by: GlucoBP Editorial & Engineering Team · Verification status: Formula verified against published reference; clinical review pending · Last updated: 2026-11-15