What is elapsed time?
Elapsed time is the amount of time that passes from the start of an event to the end of that event. It is the difference between a starting time and an ending time. For example, if a movie starts at 4:15 p.m. and ends at 6:45 p.m., the elapsed time is 2 hours and 30 minutes.
- A soccer game starts at 10:00 a.m. and ends at 11:30 a.m. The elapsed time is 1 hour and 30 minutes.
- You begin reading at 7:20 p.m. and stop at 8:05 p.m. The elapsed time is 45 minutes.
- A train leaves at 2:50 p.m. and arrives at 5:10 p.m. The elapsed time is 2 hours and 20 minutes.
Elapsed time tells how much time passes between the start and end of an event. It can be zero or a positive amount of time.
Units of time you need to know
To find elapsed time, you must understand the relationships between units of time. The most important units for fourth grade elapsed time problems are hours and minutes. Sometimes you will also work with seconds or days, but most problems use hours and minutes.
- 1 minute = 60 seconds
- 1 hour = 60 minutes
- 1 day = 24 hours
- 1 week = 7 days
- 1 year = 12 months
- 1 year = 365 days (or 366 days in a leap year)
When you add or subtract time, you cannot simply add or subtract the numbers like whole numbers. You must regroup when you have 60 minutes or more. For example, 75 minutes equals 1 hour and 15 minutes.
Reading a clock and writing time
Before you can find elapsed time, you must be able to read both analog clocks (clocks with hands) and digital clocks. You also need to write time correctly using a.m. and p.m.
- An analog clock with the short hand between 3 and 4 and the long hand on 6 shows 3:30.
- A digital clock that shows 7:45 means 45 minutes after 7 o'clock.
- 8:15 a.m. is in the morning. 8:15 p.m. is in the evening.
- 12:00 p.m. is noon. 12:00 a.m. is midnight.
Always include a.m. or p.m. when writing a time unless the problem tells you not to. If a time is between midnight and noon, use a.m. If a time is between noon and midnight, use p.m.
Finding elapsed time by counting forward
One way to find elapsed time is to count forward from the starting time to the ending time. You can count by hours, then by minutes, or use a number line to help you keep track.
- Start: 9:15 a.m. End: 11:45 a.m. Count: 9:15 to 10:15 is 1 hour. 10:15 to 11:15 is another hour (2 hours total). 11:15 to 11:45 is 30 minutes. Elapsed time = 2 hours 30 minutes.
- Start: 1:40 p.m. End: 3:10 p.m. Count: 1:40 to 2:40 is 1 hour. 2:40 to 3:10 is 30 minutes. Elapsed time = 1 hour 30 minutes.
- Start: 6:50 a.m. End: 7:05 a.m. Count: 6:50 to 7:00 is 10 minutes. 7:00 to 7:05 is 5 minutes. Elapsed time = 15 minutes.
Counting forward is often the easiest method when the starting and ending times are on the same day and the elapsed time is not too large. You can jump to the next hour first, then count the remaining minutes.
Finding elapsed time by subtraction
Another way to find elapsed time is to subtract the starting time from the ending time. This works well when both times are in the same day and you can line up the hours and minutes carefully. Remember to regroup when needed.
- End: 4:30 p.m. Start: 2:15 p.m. Subtract: 4 hours 30 minutes − 2 hours 15 minutes = 2 hours 15 minutes.
- End: 10:10 a.m. Start: 8:45 a.m. Subtract: 10:10 − 8:45. Regroup 1 hour as 60 minutes: 9:70 − 8:45 = 1 hour 25 minutes.
- End: 6:00 p.m. Start: 3:20 p.m. Subtract: 6:00 − 3:20. Regroup: 5:60 − 3:20 = 2 hours 40 minutes.
When subtracting, always subtract the minutes first. If the ending minutes are less than the starting minutes, you must regroup 1 hour into 60 minutes. For example, 5:10 becomes 4:70 if you need more minutes.
Using a number line to find elapsed time
A number line is a helpful tool for finding elapsed time. You can draw a line and mark the starting time, then make jumps of hours and minutes until you reach the ending time. The total of your jumps is the elapsed time.
- Start: 1:30 p.m. End: 4:00 p.m. Draw a line. Mark 1:30. Jump 30 minutes to 2:00. Jump 1 hour to 3:00. Jump 1 hour to 4:00. Total jumps: 30 minutes + 1 hour + 1 hour = 2 hours 30 minutes.
- Start: 7:45 a.m. End: 9:15 a.m. Mark 7:45. Jump 15 minutes to 8:00. Jump 1 hour to 9:00. Jump 15 minutes to 9:15. Total: 15 + 60 + 15 = 90 minutes = 1 hour 30 minutes.
- Start: 11:20 a.m. End: 1:05 p.m. Mark 11:20. Jump 40 minutes to 12:00 p.m. Jump 1 hour to 1:00 p.m. Jump 5 minutes to 1:05. Total: 40 + 60 + 5 = 105 minutes = 1 hour 45 minutes.
Number lines are especially useful when the elapsed time crosses from a.m. to p.m. or when you need to make several jumps. Always label your jumps clearly so you do not lose track.
Elapsed time across a.m. and p.m.
Some elapsed time problems go from the morning to the afternoon or from the afternoon to the evening. When this happens, you must be careful with the 12:00 mark. Noon is 12:00 p.m., and midnight is 12:00 a.m.
- Start: 10:30 a.m. End: 1:45 p.m. Count: 10:30 to 11:30 is 1 hour. 11:30 to 12:30 p.m. is 1 hour (2 hours total). 12:30 to 1:30 is 1 hour (3 hours total). 1:30 to 1:45 is 15 minutes. Elapsed time = 3 hours 15 minutes.
- Start: 11:50 a.m. End: 12:20 p.m. Count: 11:50 to 12:00 p.m. is 10 minutes. 12:00 to 12:20 is 20 minutes. Elapsed time = 30 minutes.
- Start: 8:15 p.m. End: 11:00 p.m. Count: 8:15 to 9:15 is 1 hour. 9:15 to 10:15 is 1 hour (2 hours). 10:15 to 11:00 is 45 minutes. Elapsed time = 2 hours 45 minutes.
When counting across noon or midnight, it is often easiest to count to the next hour, then to 12:00, then continue. Do not skip the 12:00 mark.
Elapsed time with missing start or end time
Sometimes you are given the elapsed time and one of the times, and you must find the other time. You can use addition or subtraction to solve these problems.
- A movie lasts 2 hours 15 minutes. It ends at 5:30 p.m. What time did it start? Subtract: 5:30 − 2:15 = 3:15 p.m. The movie started at 3:15 p.m.
- A class starts at 9:40 a.m. and lasts 50 minutes. What time does it end? Add: 9:40 + 50 minutes = 10:30 a.m. The class ends at 10:30 a.m.
- A trip takes 3 hours 20 minutes. If you leave at 1:10 p.m., what time do you arrive? Add: 1:10 + 3 hours = 4:10 p.m. Then add 20 minutes = 4:30 p.m. You arrive at 4:30 p.m.
When adding minutes, if the total minutes are 60 or more, regroup to hours. For example, 9:50 + 20 minutes = 9:70, which is 10:10. Always check your answer by counting forward or backward.
Solving real-world elapsed time problems
Elapsed time is used in many real-life situations, such as scheduling, cooking, traveling, and planning your day. Read each problem carefully to find the start time, end time, or elapsed time.
- Maya starts her homework at 4:25 p.m. She finishes at 5:10 p.m. How long did she work? 4:25 to 5:00 is 35 minutes. 5:00 to 5:10 is 10 minutes. Total = 45 minutes.
- A cake bakes for 35 minutes. It is put in the oven at 2:50 p.m. When should it be taken out? 2:50 + 35 minutes = 3:25 p.m.
- A bus ride takes 1 hour 40 minutes. If the bus leaves at 8:15 a.m., when does it arrive? 8:15 + 1 hour = 9:15 a.m. 9:15 + 40 minutes = 9:55 a.m.
- Liam practices piano from 3:45 p.m. to 4:30 p.m. How long does he practice? 3:45 to 4:00 is 15 minutes. 4:00 to 4:30 is 30 minutes. Total = 45 minutes.
Always ask yourself: What am I trying to find? Am I finding how long something lasted, or am I finding a missing start or end time? Then choose the correct operation.
Common mistakes to avoid
Students often make small errors when finding elapsed time. Being aware of these common mistakes will help you get the correct answer every time.
- Forgetting to regroup when minutes are 60 or more. For example, 70 minutes = 1 hour 10 minutes, not 70 minutes.
- Mixing up a.m. and p.m. Always check whether the time is before or after noon.
- Subtracting minutes from hours without regrouping. For example, 5:10 − 2:45 requires regrouping: 4:70 − 2:45 = 2 hours 25 minutes.
- Counting the starting time as part of the elapsed time. Elapsed time is the time between the start and the end, not including the start itself.
- Forgetting that there are 60 minutes in an hour, not 100. Time does not work like decimal numbers.
Double-check your work by counting forward from the start time using your answer. If you end up at the correct end time, your answer is correct.
Practice tips for mastering elapsed time
Becoming good at elapsed time takes practice. Use these tips to improve your skills and become more confident.
- Draw a number line for every problem until you can solve them mentally.
- Practice reading analog clocks and digital clocks every day.
- Use real-life situations, such as timing your chores or favorite show, to practice elapsed time.
- Check your answers by working backward. If you found the end time, subtract the elapsed time to see if you get the start time.
- Remember the key fact: 1 hour = 60 minutes. This is the most important fact for elapsed time.
The more you practice, the easier elapsed time becomes. Soon you will be able to solve problems quickly and accurately.
Common Core alignment:
CCSS.MATH.CONTENT.4.MD.A.1 – Know relative sizes of measurement units within one system of units including km, m, cm; kg, g; lb, oz.; l, ml; hr, min, sec. Within a single system of measurement, express measurements in a larger unit in terms of a smaller unit. Record measurement equivalents in a two-column table.
CCSS.MATH.CONTENT.4.MD.A.2 – Use the four operations to solve word problems involving distances, intervals of time, liquid volumes, masses of objects, and money, including problems involving simple fractions or decimals, and problems that require expressing measurements given in a larger unit in terms of a smaller unit. Represent measurement quantities using diagrams such as number line diagrams that feature a measurement scale.
Notes for teachers
This resource is designed to provide clear explanations, examples, and practice for learners of different needs and levels of support. It may be used for whole-class instruction, guided practice, independent learning, review, homework, or additional reinforcement at home.