What is elapsed time?
Elapsed time is the amount of time that passes from the start of an activity to the end of that activity.
- If you start reading at 3:00 P.M. and finish at 4:30 P.M., the elapsed time is 1 hour and 30 minutes.
Elapsed time is like a stopwatch that measures how long something lasts. It answers the question, “How much time went by?”
How to find elapsed time on a clock
You can find elapsed time by counting forward from the start time to the end time.
- Start at 2:15 P.M.
- Count hours to 3:15 P.M. → 1 hour
- The minutes are the same (15), so no extra minutes are added.
- The elapsed time is 1 hour.
It often helps to first count the full hours, then count the extra minutes separately.
Using a number line to find elapsed time
A time number line is a helpful tool to count hours and minutes between two times.
- Draw a line from 8:20 to 10:45.
- From 8:20 to 10:20 is 2 hours.
- From 10:20 to 10:45 is 25 minutes.
- Add them together: 2 hours + 25 minutes = 2 hours 25 minutes.
When you "jump" on a number line, jump by hours first, then by minutes.
Finding elapsed time across A.M. and P.M.
When a start time is in the morning (A.M.) and the end time is in the afternoon (P.M.), you need to account for noon.
- First, find time from 11:10 A.M. to 12:00 P.M. → 50 minutes.
- Next, find time from 12:00 P.M. to 2:30 P.M. → 2 hours 30 minutes.
- Add: 50 minutes + 2 hours 30 minutes = 3 hours 20 minutes.
Noon (12:00 P.M.) is a helpful halfway point. Find the time to noon first, then the time from noon.
Common patterns in elapsed time
Some time problems have patterns that make them easier to solve.
- Same Minutes: If minutes are the same (3:15 to 5:15), just subtract the hours. (5 - 3 = 2 hours)
- To the Next Hour: From 4:40 to 5:00 is always 20 minutes (60 - 40 = 20).
- Half Hours: From 1:30 to 3:30 is exactly 2 hours.
Before you calculate, check the times for patterns. They can help you solve the problem faster and more accurately.