How to use common sense to solve any problem

Take your guesstimates to the next level

Have you ever had to guess the amount of something?

“How many bags of soil do I need to fill my garden?”

“When can I expect to break even on my investment?“

“How many people will sign up for the Four Minute University next month?“

We do this more often than we think.

Yet, many of us are bad at guesstimates because it feels overwhelming.

A what?

You may know guesstimates, an estimate based on a mixture of guesswork and calculation, by other names:

  • Ballpark figure

  • Back-of-the-envelope calculation

  • Approximation

  • Estimation

You use them to quantify something so you can better understand it.

You go from abstract to concrete.

The technical term is a Fermi problem.

Fermi problem

Italian-American physicist Enrico Fermi was known for his ability to make good approximate calculations with little or no actual data.

In 1945, Fermi was trying to determine the strength of the atomic bomb.

During a test blast, he dropped pieces of paper from his hand to see how far they traveled.

He measured the distance the paper moved to estimate the bomb’s power.

His estimate (10 kilotons of TNT) was well within the range of today’s accepted result (21 kilotons).

A good estimate is within 10x the actual value, also known as an order of magnitude.

With the atomic bomb, a good value would’ve been between 10-100 kilotons.

Fermi made a justified guess using little more than common sense.

A true story

It’s my first day of graduate school at Carnegie Mellon.

I’m sitting in a room with my classmates waiting for the welcome reception to begin.

I’m excited to get started.

The director of the program walks in.

The first thing he says is:

“How many slices of pizza will be eaten today in NYC?”

No greetings. No warm welcome message.

Just an impossible problem to solve.

I looked around the room and my classmates had the same puzzled look as me.

My excitement quickly stopped.

Perhaps I made a mistake coming to this school.

I didn’t come here to solve riddles.

After the shock factor wore off, we formed groups and talked about how to solve the problem.

Jumbo NYC pizza slice

How to make good guesstimates

The most important thing is to focus on what you know.

It sounds basic, because it is.

Too often, we obsess about what we don’t know.

These what-if questions stop us from solving a problem.

Focus on facts, not hypotheticals.

Make reasonable assumptions.

Pizza estimate

Let’s take the NYC pizza example and make a guesstimate.

Here’s the question again:

How many slices of pizza will be eaten today in NYC?

First, start by breaking down the problem into things you need to know about:

  • Food: pizza

  • Time: today

  • Location: NYC

Then, ask yourself questions about things you know:

Food: Pizza

  • How many slices are in a typical pizza?

    • Estimate: 8 slices per pizza

  • How many slices would a person eat in one sitting?

    • Estimate: 1-2 slices per person

  • How many different food options can you choose from besides pizza?

    • Estimate: 10-15 food options

Time: Today

  • Is today a weekday or the weekend?

    • If it’s a weekday, there may be a company lunch or an event with pizza

    • Estimate: 2x more likely to have pizza on a weekday than a weekend

  • When do people eat a slice of pizza?

    • Lunch

    • Quick meal or snack

    • Dinner

    • Late night

    • Probably not breakfast (apologies to the cold pizza leftover crowd)

    • Estimate: 1.5x more likely to have pizza than other food

Location: NYC

  • What’s the population of NYC?

    • It’s the biggest city by population in the US

    • Estimate: 10 million

  • How often does a person in NYC eat pizza?

    • Estimate: 5% of the time

  • How many pizzerias are there in NYC?

    • Estimate: 1,000

And so on. You get the idea.

Start with a very broad problem.

Use some common sense.

Break the impossible problem into smaller questions that are easier to answer (see: chunking).

Get an estimate.

Journey, not destination

What’s the answer to the pizza riddle?

It doesn’t matter.

What’s most important is the process you use to get to the answer.

You can guesstimate anything

There are many practical ways you can use common sense to solve any problem.

Some examples:

  • Estimate the Total Addressable Market (TAM) for your product or service

  • Estimate your carbon footprint

While Fermi problems typically deal with very large numbers, you can use the same concept for any problem.


  • “How many 2×4s were used to build my house?”

Bottom line

No problem is too big or complex to solve.

Break down problems into smaller questions that you can answer.

Make reasonable assumptions based on facts.

Use common sense.

Thanks for reading.

See you next week.