Numerical Estimation Techniques
Estimation is a powerful tool in numerical reasoning tests. When exact calculation is slow or unnecessary, a good estimate can give you the answer—or at least eliminate wrong options. Here's how to use it effectively.
When to Estimate
- "Approximately" or "closest to" – The question invites estimation. Round and calculate quickly.
- Options are far apart – If answers are 100, 500, 1000, 5000, a rough estimate can rule out several.
- Time pressure – When you're running low on time, estimation may be the only way to attempt more questions.
- Sanity check – After calculating, estimate to verify. If your answer is 847 and you estimated ~800, it's plausible. If you got 8470, something may be wrong.
Techniques
Round to friendly numbers – 47 × 23 ≈ 50 × 20 = 1000. 156 ÷ 7 ≈ 140 ÷ 7 = 20. Choose round numbers that make mental math easy.
Round consistently – If you round 47 up to 50, round 23 down to 20 to partially cancel error.
Percentage shortcuts – 10% of X = X ÷ 10. 5% of X = X ÷ 20. 15% of X = 10% + 5%.
Order of magnitude – 47 × 23 is in the hundreds. Use this to eliminate options like 100 or 10000.
Practice
- Round first, then calculate – Get used to rounding before you compute.
- Know your limits – Estimation works when options are spread out. When they're close, calculate more precisely.
- Use elimination – Even a rough estimate can rule out 2–3 options.
Pitfalls
- Over-rounding – For close options, you need more precision.
- Wrong direction – Rounding both numbers up gives a high estimate; both down gives low. Adjust accordingly.
- Units – Don't forget to apply units (e.g. thousands, millions).
Practice with numerical reasoning questions and the numerical reasoning test.
Frequently Asked Questions
How accurate does my estimate need to be?
It depends on the options. If they're far apart, "around 1000" may suffice. If they're 980, 1020, 1080, you need more precision.
Can I use estimation for percentage change?
Yes. Round New and Old to make (New − Old) ÷ Old easy. For "approximately" or spread-out options, it often suffices.
What if the numbers have many digits?
Focus on leading digits and order of magnitude. 47,382 × 0.23 ≈ 47,000 × 0.2 = 9,400.