Back Of The Napkin
Where Fermi meets your napkin.
Basic Concepts
Back Of The Napkin helps you make rough calculations with uncertain values. Instead of exact numbers, you can use ranges and get statistical results. It's perfect for market sizing, forecasting, and numerical estimations.
Fermi Problems is named after the Physicist Enrico Fermi, who famously estimated the power of the first atomic bomb using nothing but intuition and a few clever back-of-the-envelope calculations. This app helps you channel that same mindset — one sketchy napkin at a time.
Input Formats
- Simple Numbers: Just type the number (e.g.,
42)
- Ranges: Use
~ between numbers (e.g., 40~50)
- Percentages: Add
% symbol (e.g., 20% or 15%~25%)
- Large Numbers: Use K, M, B, T suffixes
- K = thousands (e.g.,
50K = 50,000)
- M = millions (e.g.,
1.5M = 1,500,000)
- B = billions (e.g.,
2B = 2,000,000,000)
- T = trillions (e.g.,
1T = 1,000,000,000,000)
Examples
Example 1: Business Revenue Estimation
Estimating annual revenue for a business:
- customers:
1000~1200 (customers per month)
- spend:
20~30 (dollars per customer)
- Formula:
customers * spend * 12
- This will calculate the range of annual revenue
Example 2: Project Cost with Uncertainty
Estimating project cost with contingency:
- base_cost:
100K~120K
- contingency:
15%~25%
- Formula:
base_cost * (1 + contingency)
Example 3: Market Size Calculation
Calculating total addressable market:
- population:
5M~6M
- adoption_rate:
10%~15%
- price:
50~75
- Formula:
population * adoption_rate * price
Tips
- Use descriptive variable names to make your calculations clearer
- Start with wider ranges if you're more uncertain
- You can use basic mathematical operators:
+ - * / ( ) ^
- The calculator will show you different confidence intervals for your results
- Use the share button to save and share your calculations
Why “Back Of The Napkin”?
I have always liked guesstimating and working with ballpark numbers. As a consultant/FP&A and a business owner, I have to make a lot of estimates, which aren't accurate but directionally right.
But I travel quite a bit and I can't quickly share what I am thinking. So I wanted to make this tool with these features:
- Ability to name the Estimation. Even though the URL was shareable, but you would have to then explain what the calculation was showing
- Ability to name the input variables, so the person receiving the url, knows what your ranges meant
- Also an easy way to share questions with your friends or on social media and track other people's solutions to the same questions
So I built this tool to have all the features I wanted and more. I hope you enjoy using it as much as I enjoyed building it.
What You Can Do Here
- Make your own questions and try solving them
- Can't think of something to estimate? Explore guesstimation questions by clicking the Try a Random Question button
- Adjust variables, tweak assumptions, and see how the estimate changes
- Learn structured thinking through real-world, often absurd, questions
- Share your napkin logic with others
- Build intuition for orders of magnitude — fast
- Create questions and share with your friends to see how they are solving it
This tool helps you break complex questions into clear, logical steps, Fermi-style — all on a virtual napkin.