Estimates and Estimating
- id: 1716471688
- Date: May 15, 2025, 7:14 p.m.
- Author: Donald F. Elger
Goals
- Describe estimates and estimating.
- Make estimates of X that are good enough for your context (surrounding circumstance), where X = anything that can be estimated.
Estimates and Estimating
An estimate is a reasoned approximation of a value that is not exactly known but is good enough to support useful decisions in a given context.
Estimating is the process of making such an approximation.
Let X = anything that can be estimated.
X can be time, cost, effort (in hours), number of pages, force, velocity, temperature, weight, acceleration, and so on.
Good estimates are both simple (easy and quick to make) and accurate enough for the decisions at hand—balancing what is possible to achieve with what the situation requires.
Examples of Estimates
- Estimating the time and cost needed to install a new inverter and battery system in a camper van.
- Estimating the amount of water and food two people need for a two-day trip in a desert area.
- Estimating the price to charge for a month-long construction job.
- Estimating the cost of items before checkout, so you can verify if the cashier is charging correctly.
- Estimating a result to check if a detailed math calculation is roughly correct.
Rationale for Estimates
Here are some reasons why it’s worth learning how to estimate skillfully:
- Estimates are sometimes the only option when exact values are unknown due to uncertainty.
- Estimates are often much faster and easier than detailed calculations.
- Estimates are excellent tools for checking and validating results.
- Estimating helps reveal which factors matter most in a situation.
- On multiple-choice exams, estimates can help students quickly eliminate wrong answers.
- Most engineering tasks involve estimation, so an estimation mindset is extremely useful.
- Estimation is essential for some professions such as engineering, construction, and architecture.
How to Skillfully Estimate
Principles
- Anything can be estimated.
- Nearly everything is an estimate—except for textbook problems, which often have a “right answer.”
Framework (How To)
1. Clarify What You’re Estimating
- Define the quantity (e.g., time to complete a task, energy used
during a hike).
- Understand the unit (e.g., minutes, dollars, Newtons, Joules).
- Decide how accurate the estimate needs to be (order-of-magnitude vs. precise approximation).
2. Break It Into Parts (Fermi Technique)
- Decompose the problem into smaller, estimable chunks.
- Example: To estimate time to write a report:
→ estimate time per section × number of sections.
3. Use Reference Points
- Anchor your estimate to something familiar.
Example: Walking 1 mile takes about 20 minutes → 3 miles = ~60 minutes.
4. Use Ranges, Not Single Numbers
- Give a low, likely, and high estimate.
- This builds in realism and prepares you for variation.
5. Round to Useful Levels
- Use round numbers or powers of 10 early on (e.g., 10, 100,
1000).
- Avoid false precision. “About 30 minutes” is better than “32.4 minutes.”
6. Check Units and Orders of Magnitude
- Make sure units are correct and conversions make sense.
- Ask: “Am I off by a factor of 10, 100, or 1000?”
7. Use Estimation Shortcuts
- Rule of 72 for doubling time
- 1 horsepower ≈ 750 watts
- 1 gallon ≈ 4 liters
Develop your own estimation toolkit over time.
8. Sense-Check the Result
- Does the answer seem reasonable compared to similar cases?
- What would surprise you if the estimate were too high or too low?
9. Revise Based on Feedback
- Whenever possible, compare your estimate to actual outcomes.
- Use this feedback to improve your accuracy over time.
10. Communicate Clearly
- Be explicit: “This is a rough estimate.”
- State your assumptions: “Assuming no interruptions, this should take 2–3 hours.”