Time conversions
Time is the funny one. Length, mass and volume all use a ladder of tens. Time does not. It uses 60 for seconds and minutes, and 24 for hours. That looks strange — and it has a long history going back to the ancient Babylonians — but the rules are short and easy to learn.
The time ladder
| Step | Equality |
|---|---|
| seconds → minutes | 60 s = 1 min |
| minutes → hours | 60 min = 1 h |
| hours → days | 24 h = 1 day |
| days → week | 7 days = 1 week |
So 1 hour has 60 minutes and 3 600 seconds (60 × 60). One day has 1 440 minutes.
Going down — multiply
4 minutes = ? seconds
4 × 60 = 240 s.
2 hours = ? minutes
2 × 60 = 120 min.
Half a day = ? hours
½ × 24 = 12 h.
Going up — divide
180 s = ? min
180 ÷ 60 = 3 min.
240 min = ? h
240 ÷ 60 = 4 h.
The 24-hour clock
The clock face goes from 1 to 12, but a whole day has 24 hours. To avoid the words a.m. and p.m., we often use the 24-hour clock: after 12:59 it continues 13:00, 14:00, … up to 23:59, then back to 00:00.
- 1 p.m. → 13:00
- 5 p.m. → 17:00
- 8:30 p.m. → 20:30
- midnight → 00:00
Elapsed time
Working out how long something lasted means subtracting two times. With 60-minute hours it is easier to count in two jumps: first up to the next whole hour, then up to the target time.
A film starts at 17:45 and ends at 20:15. How long is it?
- 17:45 → 18:00 is 15 minutes.
- 18:00 → 20:00 is 2 hours.
- 20:00 → 20:15 is 15 minutes.
Total: 2 h + 15 min + 15 min = 2 h 30 min.
Lessons start at 8:10 and break is at 8:55. How long is the lesson?
- 8:10 → 8:55 is 45 minutes (count up from 10 to 55).
A worked conversion problem
A train journey took 2 h 20 min. How many minutes is that?
Convert the hours: 2 × 60 = 120. Add the leftover minutes: 120 + 20 = 140 min.
200 minutes — how many hours and minutes is that?
200 ÷ 60 = 3 remainder 20, so 200 min = 3 h 20 min.
What's next
- Length conversions — millimetres to kilometres
- Mass and volume — grams, kilograms, litres
- Back to the introduction