Highlighted rows fall within 09:00–18:00 in both places: 3 shared working hours.
How many hours is IST ahead of GMT?
As abbreviations, GMT is always 5.5 hours behind of IST: IST is UTC+5:30 and GMT is UTC+0. To convert, subtract 5.5 hours.
Does the IST to GMT time difference change?
No. Gmt is 5.5 hours behind of ist all year, because neither place changes its clocks.
When IST and GMT are both at work
When GMT is 5.5 hours behind of IST: 2 shared office hours, 15:00 to 17:00 in IST (09:30 to 11:30 in GMT).
IST to GMT conversion
IST is GMT+5:30. This is fixed. India does not observe daylight saving time, so its offset from UTC never changes.
GMT is the time zone used by the United Kingdom between late October and late March. The rest of the year the UK uses British Summer Time (BST), which is UTC+1.
So the conversion depends on the date:
Winter (late October to late March): IST is 5 hours 30 minutes ahead of GMT. Subtract 5.5 hours from IST to get GMT.
Summer (late March to late October): The UK is on BST, not GMT. IST is 4 hours 30 minutes ahead of BST. Subtract 4.5 hours from IST to get BST.
Example: 14:00 IST in January → 08:30 GMT. The same 14:00 IST in July → 09:30 BST.
GMT vs BST: UK seasonal change
The UK moves clocks forward on the last Sunday of March and back on the last Sunday of October. For 2026:
BST starts: 28 March 2026
BST ends: 25 October 2026
Any meeting scheduled across these transition dates needs a double check. A 10:00 IST call on 27 March is 04:30 GMT. On 29 March it is 05:30 BST, one hour later in London.
IST to GMT time chart
IST
GMT (winter)
BST (summer)
06:00
00:30
01:30
09:00
03:30
04:30
12:00
06:30
07:30
15:00
09:30
10:30
18:00
12:30
13:30
21:00
15:30
16:30
What hours do IST and GMT overlap for work?
India and the UK share a good working overlap. Standard office hours in India run 09:30-18:00 IST. London's standard workday is 09:00-17:30 GMT/BST.
Winter overlap: 14:00-17:30 IST (08:30-12:00 GMT). You have about 3.5 hours.
Summer overlap: 14:00-17:30 IST (09:30-13:00 BST). You have about 3.5 hours, but the London start is one hour later in local time.
For calls that need both teams fresh, schedule between 14:00 and 16:00 IST. That avoids the early London morning and the late India afternoon.
Anyone scheduling a cross-team call in July should ignore the "IST is GMT+5:30" rule and use the BST offset instead. The 30-minute difference between winter and summer conversions is the most common source of missed meetings between India and the UK.
Frequently asked questions about IST and GMT time
What is the easiest way to convert IST to GMT in my head?
Subtract 5 hours, then subtract 30 minutes. For 15:00 IST, that is 10:00 minus 30 minutes = 09:30. This works only when the UK is on GMT (winter). In summer, subtract 4 hours then 30 minutes.
Does India use the same time zone as Sri Lanka?
Yes, both use UTC+5:30. Sri Lanka Standard Time uses the same offset as IST, but it is a legally separate time zone under the IANA zone Asia/Colombo.
Why is IST 30 minutes off the full hour?
IST is based on the 82.5°E meridian, which passes through Mirzapur, Uttar Pradesh. The 5.5-hour offset was chosen as a compromise between Bombay Time (UTC+4:51) and Calcutta Time (UTC+5:53:20) when IST was adopted on 1 January 1906.
Is there any time of year when India and London are 5 hours apart?
No. The offset is either 5.5 hours (UK on GMT) or 4.5 hours (UK on BST). It never rounds to a full 5-hour difference.
What about Ireland and Israel? They also use "IST".
Yes. Irish Standard Time (UTC+1, summer only) and Israel Standard Time (UTC+2, winter only) share the same three-letter abbreviation. Always check the context. For India Standard Time, the IANA zone is Asia/Kolkata.
How many hours ahead is GMT of IST?
GMT (UTC+0) is 5.5 hours behind of IST (UTC+5:30).
Does the time difference between IST and GMT change during the year?
No, it stays 5.5 hours behind all year because neither place changes its clocks.