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Last Updated: September 10, 2020


Whether you’re new to Dexcom or experienced, review product instructions before using the Dexcom G6. Keep using your BG meter to make treatment decisions until you know how Dexcom works for you. Your meter gives you one number, if you test twice (using the same or a different meter) it gives you another number, and your Dexcom G6 gives you a third. What do you do with all those numbers?

The test

The test your doctor does is considered a more accurate glucose number than any products you use at home. Both meters and Dexcom G6 are compared to that doctor’s test to measure accuracy in clinical studies. They aren’t compared to each other. Because of this, the Dexcom G6 reading (G6 reading) and meter value are unlikely to be exactly the same number, but they should be close. Compare the meter and your Dexcom G6 to see how closely the numbers match each other: if your G6 reading and meter value are within what we call the 20 rule (also known as the %20/20 rule), they match closely.

The rule

To use the 20 rule follow these steps using the table below:

  1. Look up your meter value in the green middle column.
  2. The left G6 – column shows the low range for a G6 reading that’s a close match.
  3. The right G6 + column shows the high range for a G6 reading that’s a close match.

For example the orange highlighted row shows that if your meter value is 100 mg/dL, your G6 reading is a close match if it’s between 80 and 120 mg/dL.

G6 - Meter Value mg/dL G6 +
low 40 60
low 50 70
40 60 80
50 70 90
64 80 96
72 90 108
80 100 120
88 110 132
96 120 144
104 130 156
112 140 168
120 150 180
128 160 192
G6 - Meter Value mg/dL G6 +
136 170 204
144 180 216
152 190 228
160 200 240
168 210 252
176 220 264
184 230 276
192 240 288
200 250 300
208 260 312
216 270 324
224 280 336
G6 - Meter Value mg/dL G6 +
232 290 348
240 300 360
248 310 372
256 320 384
264 330 396
272 340 high
280 350 high
288 360 high
296 370 high
304 380 high
312 390 high
320 400 high

Good fingerstick practices

When using your meter, make sure:

  • Your test strips are stored as directed and not expired.
  • Your hands are clean for the fingerstick. Wash your hands thoroughly with soap and water (not hand gel) and dry them. Then test.

There are times when the numbers temporarily don’t match, but are likely to become closer over time. For example:

  • Sensor’s first day. With newly inserted sensors, the differences between your meter value and the G6 reading may be greater. Generally, the match gets closer over the first 24 hours.
  • Glucose changing quickly. When your glucose is changing quickly, it’s more difficult to compare numbers. The meter and Dexcom G6 measure glucose from different sources (blood and interstitial fluid), and blood glucose changes a little before interstitial fluid glucose. The match gets closer and easier to compare when your glucose stabilizes.
  • Pressure on sensor. When something is pressing on your sensor it can affect your G6 readings. The match should get closer after the pressure is relieved.

To determine what to do, watch your G6 readings over several hours. If the readings are always higher (or always lower) than your meter values and always outside the 20 rule, consider calibrating. Also consider calibrating if your Dexcom G6 and meter numbers don’t match and your expectations or symptoms fit the meter value, not the G6 reading.


Calibrating your Dexcom G6 with your meter is never required. It can make the Dexcom G6 more accurate or less accurate compared to the lab result, but it should bring the G6 readings closer to the meter values.

When calibrating, make sure:

  • You enter the calibration within 5 minutes of taking a fingerstick.
  • You don’t calibrate during Sensor Error.

If you’d like, you can calculate the 20 rule on your own. The Dexcom G6 reading must be within:

  • 20% of the meter value when the meter value is 80 mg/dL or higher
  • 20 mg/dL of the meter value when the meter value is under 80 mg/dL

Please note: the information listed here is applicable to Dexcom CGM users within the US only.

LBL019054 Rev001

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Request Sensor Overpatches

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Available Monday - Sunday, 7AM - 9PM (PDT)