How to Calculate Greenwich Mean Time (GMT)

Have you ever registered on a forum or chat room and been asked to indicate your time zone as it relates to GMT, or Greenwich Mean Time? What the heck is that? And how do I figure out what mine is?

Greenwich Mean Time is calculated from the Prime Meridian, which runs north and south through Greenwich, England. It's also home of the Royal Observatory.

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Since the Earth is round, there needs to be a notational line where things begin and end, and the Prime Meridian is it for time. Your GMT is either plus or minus the time at the Prime Meridian.

It's not hard to calculate your time relative to the GMT. Follow the steps below and you'll know what to answer next time you're asked on a forum or chat room. This article uses Chicago, Illinois, which is Central Time, as an example.

Step 1

Find the number of longitude lines between you and the Prime Meridian in Greenwich, England. Remember when you were in school learning geography, and looking at the maps with those lines running north and south, from pole to pole? Those are longitude lines and they are used in calculating coordinates for travel and also for calculating time, relative to the Prime Meridian.

Count the number of north/south longitude lines between you and the Prime Meridian. Between the Prime Meridian in England, and Chicago, Illinois, there are six longitude, or meridian lines.

Step 2

Determine if you're east or west of the Prime Meridian. Look at the world map again. If you're west of the Prime Meridian, your GMT will be ahead of, or minus, the time at the Prime Meridian. If you are east, your time will be after, or plus, GMT. Put the minus or plus sign in front of the number you found from the previous step and that's your GMT. Chicago is west of the Prime Meridian, so Chicago will be - 6 GMT, or six hours less than the time at the Prime Meridian.

Step 3

Use a website to calculate your GMT. If all that is too much work, there are plenty of sites that can calculate your GMT. But sometimes learning the why behind the how is interesting.