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“I don’t know”: how to handle a guesstimate in an interview
What to do with a guesstimate, or any interview question you can’t answer straight away.
What interviewers are really checking: structure, logic and composure.
The five-step method (two to four minutes)
Clarify. Scope before numbers: “One canteen or all of them? A normal weekday?”
Split it. Break it into three or four parts you can estimate. Parts, not a guess.
Assume. A number plus a reason: “About half drink chai, because it’s cheap and classes start early.”
Calculate. Round numbers: 3,000 × 50% × 1.5, not 2,870 × 47%.
Check it. Does it feel right? “That’s about 5 cups a minute. A busy counter can do that.”
Short on time? Clarify, break it down, assume with a reason, calculate, sanity-check.
7 weak moves, and what to do instead
The wrong way | Do this instead |
Freezing in silence. | “Let me think out loud for a moment.” Silence after five seconds looks like panic. |
“I don’t know,” and stopping there. | “I don’t know the exact figure, but here’s how I’d estimate it.” |
Jumping straight into numbers. | Ask one or two scoping questions first. They earn marks. |
A number with no reason: “Say 40%.” | “About 40%, because…” The reason is what gets scored, not the number. |
Messy maths: 2,870 × 37%. | Round it: 3,000 × 40%. They’re checking your logic, not your arithmetic. |
Folding the moment they push back. | Defend the assumption with a reason, or change it with a reason. |
Ending on the number. | Sanity-check it out loud. Almost nobody does, so it stands out. |
Not a guesstimate, just a question you can’t answer? Say so, say how you’d find out, and bridge to what you do know: “I haven’t used Tableau, but I’ve built dashboards in Power BI. The logic is the same, and I’d pick it up quickly.”
Worked example: the campus classic
“How many cups of chai does your college canteen sell in a day?”
Step | What to say |
Clarify | “One canteen, a normal weekday?” Okay. |
Break it down | Students × share who drink chai × cups each, plus staff. |
Assume | 3,000 students · 50% drink chai (cheap, early classes) · 1 or 2 cups, so 1.5 · about 200 staff cups. |
Answer | 3,000 × 50% × 1.5 ≈ 2,200, plus 200 ≈ 2,500 cups a day. |
Sanity-check | 2,500 ÷ 8 hours ≈ 5 cups a minute. For a busy canteen, that sounds right. |
Worked example: market size
“How many two-wheelers are sold in India in a year?”
Step | What to say |
Clarify | “New ones only, scooters and bikes, all of India?” |
Break it down | Households × share that own one ÷ years before replacing, plus first-time buyers. |
Assume | About 30 crore households · half own a two-wheeler · replaced about every 10 years · plus about 30 lakh first-time buyers a year. |
Answer | 30 crore × 50% ÷ 10 ≈ 1.5 crore, plus 0.3 crore ≈ about 1.8 crore a year. |
Sanity-check | That’s about 50,000 a day across the country. For India’s roads, that feels plausible. Say “I’d verify this with industry data.” |
These are illustrative samples. Use the same pattern with your own numbers.
Phrases that buy time without sounding stuck
“Let me structure this first.”
“I’ll make an assumption here; tell me if it’s off.”
“Can I take ten seconds to think?”
Practise one a day, out loud
Autos in your city · phones sold in India in a month · petrol pumps in your state · samosas sold at your station. Five minutes each.
Before you walk in
Clarify first, every time
Say every assumption with a reason
Use round numbers
End with a sanity check
If they push back, defend or adjust. Never just fold.
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