Elapsed time: how to find how long something took

"The film starts at 3:45 and ends at 5:20. How long is it?" Questions like this are called elapsed time problems. Many children find them hard because time does not work in tens: there are 60 minutes in an hour, not 100.

The counting-on method

Count on in three steps. Each step is easy on its own.

1. From the start time, count on to the next whole hour.
2. Count on in whole hours to the hour before the end time.
3. Count on the remaining minutes, then add the three parts.

Worked examples

1. From 3:45 to 5:20. 3:45 to 4:00 is 15 minutes. 4:00 to 5:00 is 60 minutes. 5:00 to 5:20 is 20 minutes. Total: 15 + 60 + 20 = 95 minutes (1 hour 35 minutes).

2. From 7:50 to 9:25. 7:50 to 8:00 is 10 minutes. 8:00 to 9:00 is 60 minutes. 9:00 to 9:25 is 25 minutes. Total: 10 + 60 + 25 = 95 minutes.

3. Adding time to a start time. A class starts at 10:15 and lasts 1 hour 40 minutes. Add the hour first: 11:15. Then add 40 minutes: 11:15 + 40 minutes is 11:55. The class ends at 11:55.

4. Minutes to hours and minutes. How many hours and minutes are in 135 minutes? 135 = 60 + 60 + 15, so 2 hours 15 minutes.

5. Hours and minutes to minutes. 2 hours 15 minutes = 2 × 60 + 15 = 135 minutes.

Mistakes to avoid

• Subtracting like ordinary numbers. 5:20 − 3:45 is not "1:75". Use the counting-on method, or convert both times to minutes.
• Using 100 instead of 60. After 59 minutes comes the next hour.
• Forgetting am and pm. If a time crosses noon or midnight, think in the 24-hour clock: 11:30 am to 2:15 pm is 2 hours 45 minutes.
• Counting the start minute. The time between two times is the gap, so do not count both ends.

Make it concrete

Draw a time line with the start time on the left and the end time on the right, and mark the whole hours between them. Children who can see the line find the three jumps easy. A real clock with moving hands helps too: try the Clock Game.

Practise it

• Elapsed time practice online with instant feedback and a mastery score.
• Hours and minutes practice for changing between the two.
• Elapsed time worksheets with an answer key.

Common questions

What is elapsed time?

It is the amount of time that passes between a start and an end: how long something took.

How do I find the end time?

Add the length of time to the start time, hours first and then minutes. If the minutes go past 60, add one to the hour and take away 60.

How many minutes are there in 2 hours 15 minutes?

2 × 60 + 15 = 135 minutes.

Which grade learns elapsed time?

It is usually taught around Grade 3 to Grade 4 (Class 3 to 4), though schools differ.

This guide is general information. For exams, follow your school, teacher and official exam board, and use your own judgement about what suits your child.