BYTETOOLS

GPS Week Number Calculator

Convert UTC to GPS week, day of week and time of week, or back again, with configurable leap seconds and both 10-bit and 13-bit rollover values.

2400
GPS week
1 — Monday
Day of week
129618
Time of week (s)
352
Week mod 1024

GPS time breakdown

Week 2400, day 1, 12:00:18 into the day

UTC instant
2026-01-05T12:00:00Z
GPS time (UTC + 18 s)
2026-01-05T12:00:18Z
Calendar date
Monday, January 5, 2026
Continuous week number
2400
10-bit week (mod 1024)
352 — GPS epoch 2
13-bit week (mod 8192, CNAV)
2400
Time of week
129618 s (12:00:18 on day 1)
GPS seconds since epoch
1451649618
Start of this GPS week
2026-01-03T23:59:42Z UTC

Rollover reference

RolloverContinuous weekUTC instant
GPS epoch (week 0)01980-01-06T00:00:00Z
First rollover10241999-08-21T23:59:47Z
Second rollover20482019-04-06T23:59:42Z
Third rollover (projected)30722038-11-20T23:59:42Z

The legacy navigation message carries the week number in 10 bits, so it wraps every 1024 weeks — about 19.6 years. The first wrap happened at 1999-08-21 23:59:47 UTC and the second at 2019-04-06 23:59:42 UTC, both of which this table reproduces. Modern CNAV messages use 13 bits, pushing the next wrap out to 2137.

GPS time is a continuous atomic timescale with no leap seconds, so it has drifted ahead of UTC by 18 seconds. Choose the offset that was in force at your instant — 13 seconds through the late 1990s, 15 from 2009, 16 from 2012, 17 from 2015 and 18 since 1 January 2017 — or leave it at 0 to treat the input as GPS time directly. Everything is computed in your browser; nothing is sent anywhere.

What is the GPS Week Number Calculator?

GPS time is counted as a week number plus seconds into that week, starting from midnight UTC on 6 January 1980.

  • Both directions: UTC to GPS week, and week plus time-of-week back to UTC
  • Continuous week number, 10-bit rollover, 13-bit CNAV rollover and epoch number
  • Configurable GPS minus UTC leap seconds from 0 to 60
  • Day of week, seconds of day, time of week and total GPS seconds since the epoch
  • Reference table of the 1999 and 2019 rollovers and the projected 2137 one
  • Shows the exact UTC instant at which the current GPS week began

How to use the GPS Week Number Calculator

  1. 1

    Choose a direction: UTC to GPS week, or GPS week and time-of-week back to UTC.

  2. 2

    Enter the UTC date and time, or the week number and time-of-week in seconds.

  3. 3

    Set the GPS minus UTC leap-second offset for your epoch — 18 seconds for anything since January 2017.

  4. 4

    Read the week number, day of week, time of week and both rollover values.

  5. 5

    Copy the whole breakdown, including the start of the GPS week and the total GPS seconds since the epoch.

About the GPS Week Number Calculator

GPS time is counted as a week number plus seconds into that week, starting from midnight UTC on 6 January 1980. It is a continuous atomic timescale that never inserts a leap second, so it has drifted 18 seconds ahead of UTC and keeps drifting. This calculator converts between a UTC timestamp and the GPS week, day of week and time of week.

It reports the continuous week number that modern receivers use plus both rolled-over values: the 10-bit week that wraps every 1024 weeks in the legacy navigation message, and the 13-bit CNAV week that wraps every 8192. The rollover reference table reproduces the two real rollovers, in August 1999 and April 2019.

The leap-second offset is an input rather than a constant, because the right value depends on when your timestamp is: 13 seconds through the late 1990s, 18 since January 2017. Everything is computed in your browser — useful when you are checking RINEX headers or receiver logs offline.

Frequently asked questions

What is the current GPS week number?

Press Use now to convert the current UTC time. The GPS week is a continuous count from the epoch of 6 January 1980, so it passed 2300 in early 2024 and rises by one every Sunday at 00:00:00 GPS time.

What is the GPS epoch?

Midnight UTC at the start of Sunday 6 January 1980. That instant is GPS week 0, day 0, time-of-week 0. Every GPS timestamp is measured forward from there with no leap seconds inserted along the way.

What is the GPS week rollover?

The legacy navigation message carries the week number in only 10 bits, so it wraps back to zero every 1024 weeks — about 19.6 years. It happened on 21 August 1999 and again on 6 April 2019, and old receivers that ignore it report dates 19.6 years in the past.

Why is GPS time ahead of UTC?

UTC inserts leap seconds to stay aligned with Earth's rotation; GPS time does not. The gap was zero at the 1980 epoch and has grown to 18 seconds since 1 January 2017. Receivers apply the offset broadcast in the navigation message to display UTC.

What is time-of-week in GPS?

The number of seconds elapsed since the start of the current GPS week, which begins at midnight between Saturday and Sunday in GPS time. It runs from 0 to 604799 and appears in RINEX files, receiver logs and the navigation message itself.

Which leap-second value should I use?

Use the one in force at your timestamp: 13 seconds from 1996 to 1998, 14 from 1999, 15 from 2009, 16 from 2012, 17 from 2015 and 18 since 1 January 2017. Setting it to 0 treats your input as GPS time directly.

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