Guide · meters
Water meter rollover
Short answer
When a register rolls past its highest number back to zero, the water used is (rollover value − previous reading) + current reading. On a six-digit register that rolls over at 1,000,000: from 999,950 to 000,120 is 50 + 120 = 170. Multiply by the register's multiplier if it has one.
Whole-number wheels that turn; don't count fixed zeros.
How: a 6-wheel register goes from 999,999 back to 0. From 999,950 up to the rollover is 50 counts, plus 120 after it = 170 counts.
Only valid if the register rolled over. A reset or replaced meter needs a new baseline instead.Worked examples
| Register | Previous → current | Rolls over at | Used |
|---|---|---|---|
| 6 wheels, gallons | 999,950 → 000,120 | 1,000,000 | 50 + 120 = 170 gal |
| 6 wheels, gallons × 100 | 999,998 → 000,003 | 100,000,000 gal | (2 + 3) × 100 = 500 gal |
| 5 wheels + tenths, pump hours | 99,998.7 → 00,001.4 | 100,000 h | 1.3 + 1.4 = 2.7 h |
The rollover value is one more than the highest number the register can show, times its multiplier. A register marked “U.S. Gallons × 100” with six wheels shows up to 99,999,900 gallons and rolls over at 100,000,000, as in Washington Ecology's meter-reading guide. See how to read a water meter for multipliers and fixed zeros.
Rollover, reset or replacement?
- Rollover: the previous reading was close to all nines and the new one is small. No water is missing from the count; calculate it as above.
- Reset: an electronic register was zeroed. Water used between the last reading and the reset isn't counted anywhere.
- Replacement: a new meter starts from zero or its own number. Record the old meter's final reading and the new meter's first reading on the same day, with a note.
- Misread: a lower reading that's nowhere near the top of the register is usually a transposed digit. Re-read the meter before anything else.
Logging a rollover
Write the reading exactly as the register shows it, and note “rollover” in the notes column the day it happens. In the meter usage calculator, tick Reset on that reading to start a new baseline. Add the rollover interval from this page separately if you need it.
Questions
How often does a meter roll over?
It depends on the register size and how much water passes. A six-wheel gallon register rolls over every million gallons. At 5,000 gallons a day that's roughly every 200 days, so a busy source meter may roll over more than once a year.
Is a rollover the same as a meter reset?
No. A rollover is the register passing its highest number and continuing from zero, so no water is lost from the count. A reset or a replacement meter starts from a new number, and the water used between the last old reading and the new one isn't recorded by either reading.
Why doesn't the Water System Logbook app calculate across a rollover?
Because it needs the register size, and a wrong size gives a confident wrong number. The app treats a rollover like a reset: that reading starts a new baseline, the interval is marked, and usage resumes from the next reading. If you know the register size, this page's calculator works out the gap.
Sources
Last reviewed September 28, 2026. Rules change and states add their own; confirm with your state drinking water program.