Keep one anchor for each direction
An anchor is a conversion you already know by heart. One anchor for each direction of a pair tells you roughly where an answer should land, so a slip in the arithmetic shows up.
Every line below is an anchor the practice checks answers against.
0 °C is 32 °F
37 °C is 98.6 °F
30 cm is about 11.8 in
12 in is about 30.5 cm
1 m is about 3.28 ft
10 ft is about 3.05 m
10 km is about 6.2 mi
10 mi is about 16.1 km
1 kg is about 2.2 lb
10 lb is about 4.54 kg
1 US gal is about 3.79 L
10 L is about 2.64 US gal
100 km/h is about 62 mph
60 mph is about 96.6 km/h
Anchors also settle the direction. Above −40 °C the Fahrenheit number is larger; at −40 both scales agree; below −40 °C the Fahrenheit number is smaller. Going from km to miles the number gets smaller. If your answer moved the wrong way, you ran the conversion backwards.
Convert 25 °C, then check it
Fahrenheit = Celsius × 9/5 + 32. Multiply first, then add the 32.
25 × 9/5 = 45
45 + 32 = 77
So 25 °C is 77 °F.
Now the anchor check. Body temperature is 37 °C, which is 98.6 °F. A room at 25 °C is cooler than a body. So the answer has to be below 98.6 °F.
A backwards conversion here would give (25 − 32) × 5/9, which is about −3.9. The anchor check catches that at once, because a warm room is not below freezing.
Try an oven temperature
A recipe says 180 °C. Work it out with the same formula. Then check the answer against an anchor before you choose.
Boiling water is 100 °C, which is 212 °F. An oven at 180 °C is far hotter than boiling water.
Try it
A recipe says 180 °C. What is that in °F?
- 356 °F
- 212 °F
Show the answer
356 °F
180 × 9/5 = 324, and 324 + 32 = 356 °F. The anchor agrees. 180 °C is above the boiling point of 100 °C. So the answer has to be above 212 °F.
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Practice Unit conversion