Hourglass Puzzle 9 Minutes Answer [Solved]

The hourglass puzzle 9 minutes answer is: start both hourglasses together, flip the 4 minute glass when it runs out, flip the 7 minute glass when it runs out, and flip the 7 minute glass one more time at 8 minutes. It runs out at exactly 9 minutes.

You have two hourglasses. One measures 4 minutes and the other measures 7 minutes. Neither glass has markings, so you cannot judge a half glass by eye. Candidates have reported this question in Bank of America and Wayfair interview reports on Glassdoor, and it also turns up in general interview puzzle lists. The full method below uses only flips at the exact moment a glass runs out.

Hourglass Puzzle 9 Minutes: The Step-by-Step Solution

Call the small glass A and the large glass B. Glass A runs for 4 minutes. Glass B runs for 7 minutes. Start both glasses at the same moment, at time zero.

Time	What you do	What it gives you
0 minutes	Start glass A and glass B together.	Both glasses run at once.
4 minutes	Glass A runs out. Flip glass A at once.	Glass B still has 3 minutes left.
7 minutes	Glass B runs out. Flip glass B at once.	Glass A still has 1 minute left.
8 minutes	Glass A runs out for the second time. Flip glass B at once.	Glass B has run for 1 minute, so it holds 1 minute of sand.
9 minutes	Glass B runs out.	Exactly 9 minutes have passed.

Walking Through the Flips

Walk through each step slowly, because the last step is where most candidates lose the thread. At 4 minutes, glass A empties and you flip it, so it starts a fresh 4 minute run. Meanwhile glass B keeps running, and it has 3 minutes left at that point.

At 7 minutes, glass B empties. Flip it at once. Glass A has been running for 3 minutes since its flip at 4 minutes, so it has 1 minute left. That 1 minute is the whole secret of the puzzle.

Then, at 8 minutes, glass A empties for the second time. Glass B has now run for 1 minute since its flip at 7 minutes, so 1 minute of sand sits in the bottom of glass B and 6 minutes of sand sit on top. Flip glass B at once. The 1 minute of sand returns to the top and runs down in exactly 1 minute.

At 9 minutes, glass B empties. Stop there. You have measured exactly 9 minutes, and every flip happened at a moment you could see with certainty, when a glass ran out.

Hourglass Puzzle 9 Minutes: Why the 1 Minute Gap Matters

The puzzle works because 8 and 7 differ by 1. Run the 4 minute glass twice and you reach 8 minutes. Run the 7 minute glass once and you reach 7 minutes. The gap between those two landmarks is 1 minute, and the flip sequence captures that gap and reuses it.

Think of it as a difference, not as a guess. You never estimate a partial glass. You only act when a glass empties, which is the one event you can observe exactly.

The Traps That Catch Candidates

The first trap is the simple sum. Run glass A and then glass B in sequence, and you get 11 minutes, not 9. Run glass B and then glass A, and you still get 11 minutes. Adding the two timers never gives 9, so candidates who stay in sequential thinking stay stuck.

Next, watch for the half glass trap. Some candidates flip glass B after 7 minutes and then try to judge when glass A is half done, or they try to split a glass by eye. That fails because the glasses have no markings and sand does not fall at a visibly even rate. Interviewers reject any step that needs an unmarked measurement, so keep every step tied to a glass running out.

The third trap is the slow start. Some candidates wait for a glass to empty before starting the other glass. That wastes the overlap, and without overlap there is no gap to capture. Both glasses must start together at time zero, or the whole method falls apart.

Finally, the trap of stopping at 8 minutes. At 8 minutes you have done most of the work, but you have not reached the answer yet. The final flip at 8 minutes is easy to forget under pressure, so name it out loud as you go: flip at 4, flip at 7, flip at 8, done at 9.

What the Interviewer Is Actually Testing in the Hourglass Puzzle 9 Minutes Question

First, the interviewer tests whether you can use a difference instead of a sum. Strong candidates look for a way to create 1 minute from 4 and 7, because 1 minute plus 8 minutes gives the target. Weak candidates keep adding 4 and 7 and keep landing on 11.

Second, the interviewer tests discipline about observable events. Every flip in the correct method happens when a glass empties. That rule is the difference between a solution and a guess, and interviewers listen for it. If you say you will flip a glass halfway through, you have failed the puzzle even if your arithmetic looks right.

Third, the interviewer tests clear communication under pressure. The method has four flips and five timestamps, and it is easy to blur them into one long sentence. Candidates who state each timestamp, each flip, and each remaining amount in order make the logic easy to follow. Candidates who jump to the answer without the steps leave the interviewer unsure whether they reasoned it out or memorized it.

Fourth, the interviewer tests whether you check your own work. A strong finish restates the final step: glass B held 1 minute of sand at 8 minutes, it ran for 1 minute, so it emptied at 9 minutes.

How to Answer It Out Loud

Say the answer first, then the steps. For example, say: I will start both glasses together. At 4 minutes I flip the 4 minute glass. Then, at 7 minutes, I flip the 7 minute glass. Later, at 8 minutes, the 4 minute glass empties again, so I flip the 7 minute glass once more. It holds 1 minute of sand, so it empties at 9 minutes.

Keep your sentences short. If the interviewer interrupts with a follow up, answer from the table above, not from memory. Each row depends only on the row before it.

Variations Interviewers Use to Test You Again

Interviewers often change the target once you solve the first version. Two common follow ups use the same glasses and the same method.

Measure 10 minutes. Reach 9 minutes exactly as above. At 9 minutes, glass B has 1 minute of sand in the bottom. Flip it once more, and it empties at 10 minutes. The same 1 minute gap does the work again.

Measure 11 minutes. Run the glasses in sequence. Run glass B for 7 minutes, then run glass A for 4 minutes, and you reach 11 minutes.

If the interviewer changes the glass sizes, rebuild the method from scratch. Find two landmarks whose gap equals a useful small number, then capture that gap with a flip at the moment a glass empties.

Frequently Asked Questions

Can you measure 9 minutes with only the 4 minute glass?

No. The 4 minute glass alone gives only multiples of 4, such as 4, 8, or 12 minutes. You need the 7 minute glass to create the 1 minute gap that bridges 8 minutes to 9 minutes.

Why can you not just flip a glass halfway?

Because the glasses have no markings, so a halfway point is a guess, not a measurement. Interviewers accept only flips at the moment a glass runs out, since that moment is exact and visible.

What if you flip a glass a second late?

Then the measurement is off by that second, and the error carries into every later step. In a real interview, state the flips as exact events and assume instant flips, which is the standard rule for this puzzle.

Try Another Puzzle Next

If you liked the difference trick here, try the water jug puzzle next, where a 5 litre jug and a 3 litre jug measure exactly 4 litres. It rewards the same habit: use what you have, track every step, and never guess a measurement. For a second challenge in the same style, work through the burning rope puzzle, where two 60 minute ropes measure exactly 45 minutes.

Question source: candidates reported this hourglass question in a Bank of America interview report on Glassdoor. The solution and all explanation above are original to this site.