Name the number.
Say exactly what you are counting or measuring, and in which unit. Minutes or hours? Kilograms or grams? Many bad guesses are really answers to a different question.
How to estimate anything
You can make a good guess at almost any number, even one nobody has told you. You only need a few things you already know, some round numbers and a quick check at the end.
This way of thinking is often called a Fermi estimate, after the physicist Enrico Fermi, who was known for quick estimates made with very little data. The aim is not the exact answer. The aim is to be close, and to know why.
Break it into parts.
Split the big unknown into smaller numbers you can picture. Then multiply or divide them. Each small number is much easier to guess than the whole.
Round, then check.
Use round numbers so the arithmetic stays easy. Then ask: is this too big or too small? Compare it with something you already know.
Worked example: seconds in a year
Break it into parts: seconds in a minute, minutes in an hour, hours in a day and days in a year.
- There are 60 × 60 = 3,600 seconds in an hour.
- 3,600 × 24 hours is about 90,000 seconds in a day. (The exact number is 86,400. We rounded up.)
- Round 365 days down to 350, to balance the rounding up in step 2.
- 90,000 × 350 = 31,500,000. So a year has about 30 million seconds.
The exact answer is 31,536,000. The estimate is within 1%. When you round one number up, try to round another one down. The errors then cancel out instead of piling up.
Worked example: grains of rice in a bag
Some numbers are too big to count, but you can count a small part and scale up.
- Count out 100 grains of rice and weigh them on a kitchen scale. Suppose they weigh 2 grams. (Your rice may be different. That is why you weigh it.)
- A 1 kg bag holds 1,000 grams. That is 500 lots of 2 grams.
- 500 × 100 grains = 50,000 grains in the bag.
This “sample and scale” method works for sand, beads, leaves on a branch or people in a crowd. Measure a small piece you can count, then multiply.
Guess in the middle of your range
Sometimes you can only say that the answer is more than one number and less than another. Suppose you are sure it is more than 100 and less than 10,000. Your first thought might be to guess halfway, about 5,000. That is risky.
A better guess is the middle in ratio terms. Multiply the two limits and take the square root: √(100 × 10,000) = 1,000. That guess is at most 10 times too high or 10 times too low. A guess of 5,000 could be 50 times too high.
How Oddly scores an estimate
Oddly compares your guess with the answer as a ratio, not as a difference. Being twice too high costs the same as being twice too low. A perfect guess earns 1,000 points. Here is what different guesses would earn if the answer were 100:
| Your guess | Points |
|---|---|
| 110 | 909 |
| 200 | 500 |
| 50 | 500 |
| 1,000 | 100 |
So a thoughtful guess of the right size scores well, even if it is not exact. Some quantities vary naturally from case to case. For those, Oddly tells you before you guess, accepts a documented range, and any guess inside it earns full accuracy points.
Tips for playing Oddly
Estimate asks you to type your best guess. This or That asks you to pick the right one of two options. Both games have a new five-question round every day. Play your first round of each game free, no account needed. A free account unlocks every round.
- Read the assumptions before you guess. They tell you the size, time or conditions to use.
- Type big numbers the easy way: 80k or 40 million both work.
- Use the built-in calculator. It has powers, square roots and a notepad for your working.
- A hint shows a way to reason, but it costs 20% of that question’s points.
- After each answer, read the explanation. The idea behind one question often helps with the next.
Practise on classic questions
Ready to try the method? Work through 16 Fermi questions with answers, from how many breaths you take in a day to how many golf balls fit in a school bus.