How many bullets does it take to cut down a tree?

According to Guesstimation 2.0, it would take about 10,000 bullets to cut down a tree with a 20 cm radius. The book doesn’t just announce the result but shows how you might come to this conclusion by computing how much energy it would take to fell the tree and how much energy bullets deliver. After going through a rough calculation, the book reports that Mythbusters actually cut down a tree, a smaller one, with about 2,000 bullets.

Guesstimation 2.0 explores about 100 questions with back-of-the-envelope solutions. (I started to add up the number of problems by looking at the table of contents, but it would be more in line with the spirit of the book and say that since it has on the order to 10 questions and on the order of 10 problems per chapter, it has about 100 questions.)

Another question I found interesting was estimating the amount of fuel needed to transport food across the United States. If you lived in New York and all of your food came from California, it would take about 10 gallons of fuel to bring you your groceries. It may take more fuel to bring your groceries home from the grocery store than it takes to bring the food from around the country to the grocery store.

7 thoughts on “How many bullets does it take to cut down a tree?

  1. I read the original Guesstimation several years ago, doing several problems at lunch each day. It was a real eye-opener – I highly recommend it to any numerically-oriented geek. Among the points it makes (consistent with John’s estimate of how many problems are in the new book) is that in many cases, a rough estimate is much better than an accurate value if it takes too long to obtain the value.

  2. Rough estimates also have the advantage that you know they’re rough estimates. Sometimes detailed estimates are no better, but they have an aura of precision.

  3. I couldn’t agree more with that last point, John. I teach cost estimation now and then, and it’s very hard to get across to people that the extremely detailed bottom-up estimate based on identifying every work package in the project and its material and labor requirements is seldom as accurate as a rough extrapolation from the actual costs of 3 or 4 similar projects in the past.

  4. So if you could use a 20cm tree as a club could you kill 2000 people with it or would you be better off using the bullets directly?

    Admittedly far from the size of a tree, but I remember working the back of the shooting range when I was in the army if people were a decent shot by the time they got through the qualification test (90 rounds I believe) they would have a pretty good chance of chopping through the 2X2 center picket holding up the target. Not bad from various ranges 100-400m with a bunch of those in full auto and standing sitting etc. You could also slightly feel the bullet impact if it hit the wood (holding target above a bunker, not standing in the middle of a field down range) (like a light tap) which kind of blows away the whole Dirty Harry style flying backwards through a window when shot myth.

  5. When I was in college, I went christmas tree hunting with some friends. We did not have a saw, and I cut a small tree down with a bread knife. My friend used his .22 rifle. My recollection is that it took less than a dozen shots with the barrel just a few inches from the trunk. The trunks were about 10 cm in diameter. 2000 )(or 1000) shots seems excessive for a 40 cm diameter trunk. A larger caliber bullet would be needed to travel all the way through, but if you line up bullets side by side, you can cover the width with fewer than 100. Even with some sloppiness in aiming, these larger numbers should not be needed. Are they including aiming errors from farther away?

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