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Celsius, Fahrenheit and Everyday Time Calculations

Goal Convert Celsius and Fahrenheit, and answer everyday time questions like how many weeks are in a year or seconds in a day.

WHY THIS MATTERS

Weather apps, oven dials, thermometers and recipes may use Celsius or Fahrenheit depending on where they come from. And questions like "how many weeks are in a year?" are among the most searched maths questions on the internet. Knowing how to work them out means you never have to look them up again.

KEY WORDS

Celsius (°C): water freezes at 0 °C and boils at 100 °C. Used in most countries.
Fahrenheit (°F): water freezes at 32 °F and boils at 212 °F. Used mainly in the United States.
Leap year: a year with 366 days instead of 365. February gets an extra day, the 29th.

CELSIUS TO FAHRENHEIT

Formula: °F = (°C × 9 ÷ 5) + 32
You can use 1.8 instead of 9 ÷ 5.
Example: 25 °C → 25 × 1.8 = 45, then 45 + 32 = 77 °F.

FAHRENHEIT TO CELSIUS

Formula: °C = (°F − 32) × 5 ÷ 9
Example: 86 °F → 86 − 32 = 54, then 54 × 5 ÷ 9 = 30 °C.
Order matters: always subtract 32 first.

QUICK ESTIMATE DOUBLE AND ADD 30

Celsius to Fahrenheit: double it, then add 30. 20 °C → 40 + 30 = 70 °F (exact: 68 °F).
Fahrenheit to Celsius: take away 30, then halve it. 80 °F → 50 ÷ 2 = 25 °C (exact: 26.7 °C).

This is close for everyday weather, but not accurate enough for cooking or a fever.

TEMPERATURES WORTH KNOWING

0 °C = 32 °F: water freezes
20 °C = 68 °F: a comfortable room
30 °C = 86 °F: a hot day
37 °C = 98.6 °F: normal body temperature
38 °C = 100.4 °F: a fever, generally
100 °C = 212 °F: water boils
180 °C = 356 °F: a moderate oven (recipes usually round this to 350 °F)
−40 °C = −40 °F: the one temperature where both scales match

Celsius and Fahrenheit side by side. Water freezes at 0 Celsius, 32 Fahrenheit. A room is 20 Celsius, 68 Fahrenheit. Body temperature is 37 Celsius, 98.6 Fahrenheit. Water boils at 100 Celsius, 212 Fahrenheit.

TIME FACTS PEOPLE SEARCH FOR

60 seconds = 1 minute, and 60 minutes = 1 hour
1 day = 24 hours = 1,440 minutes = 86,400 seconds
1 week = 7 days = 168 hours
1 year = 365 days (366 in a leap year)
Weeks in a year: 365 ÷ 7 = 52 weeks and 1 day (52 weeks and 2 days in a leap year)
Weeks in a month: on average 52 ÷ 12, which is about 4.3 weeks. A month is never exactly 4 weeks, except February in a normal year.
Hours in a year: 365 × 24 = 8,760
Seconds in a year: 365 × 86,400 = 31,536,000

LEAP YEARS

A year is a leap year if it can be divided by 4, for example 2024 and 2028. The exception: years ending in 00 are only leap years if they can be divided by 400. So 2000 was a leap year, but 1900 was not.

USING THIS AT WORK WEEKLY, FORTNIGHTLY AND MONTHLY PAY

Weekly to yearly: multiply by 52.
Fortnightly to yearly: multiply by 26.
Monthly to yearly: multiply by 12.
Yearly to weekly: divide by 52.

Example: a rent of $400 a week is 400 × 52 = $20,800 a year. Divided by 12, that's about $1,733 a month, not $1,600. Using "4 weeks in a month" makes monthly costs look lower than they really are.

REAL-LIFE CASE STUDY MARIA CHECKS HER CHILD'S TEMPERATURE

Maria's thermometer was bought in the United States and shows 101.3 °F. Her doctor's advice sheet says to call if a child's temperature is above 38.5 °C.

101.3 − 32 = 69.3
69.3 × 5 ÷ 9 = 38.5 °C

Her child's temperature is right at the level on the sheet, so she calls the doctor for advice. The quick "take away 30 and halve" estimate gives 35.7 °C, which would have been badly wrong here. For health and cooking, always use the exact formula.

PRACTICE

Complete the sentences.

Drag a word into each gap, or tap a word and then tap the gap.

Water freezes at 0 °C, which is °F.
Water boils at 100 °C, which is °F.
One day has hours.
A normal year has 52 weeks and day.
A leap year has days.

✓ Check Your Understanding

Pick one answer for each question. You'll see the explanation straight away.

1. The weather forecast says 35 °C. What is that in Fahrenheit?

35 × 1.8 = 63, then 63 + 32 = 95 °F. (63 °F is what you get if you forget to add 32, and 67 °F comes from adding 32 without multiplying.)

2. An American recipe says to bake at 400 °F. What is that in Celsius, roughly?

400 − 32 = 368, then 368 × 5 ÷ 9 = 204 °C, so set the oven to about 200 °C. (222 °C comes from multiplying by 5 ÷ 9 without subtracting 32 first, and 368 °C from subtracting 32 but not multiplying. Always subtract first, then multiply.)

3. How many weeks are in a normal (non-leap) year?

365 ÷ 7 = 52 remainder 1, so a year is 52 weeks and 1 day. That extra day is why your birthday moves one day of the week later each year (two days after a 29 February).

4. How many seconds are in one day?

24 hours × 60 minutes × 60 seconds = 86,400 seconds. (3,600 is the number of seconds in an hour, 1,440 is minutes in a day, and 8,760 is hours in a year.)

5. Your rent is $350 a week. What is it per month on average?

First find the yearly rent: $350 × 52 = $18,200. Then divide by 12 months: $18,200 ÷ 12 ≈ $1,517. Using 4 weeks a month gives $1,400, which is too low, because most months are longer than 4 weeks.

You got out of 5. Excellent work! Read the explanations above, then try again.

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