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Remember the slide rule?

I sure do. Back in high school chemistry, years ago, we all had to learn how to use a slide rule to quickly multiply and divide large numbers. Not everybody succeeded; the gizmo looked so complicated that a lot of kids just gave up. But if you strip a slide rule down to just two scales, it’s very easy to figure out.

Browsing through an equally ancient book of woodworking projects the other day, I spotted something called a “Kitchen Slide Rule.” The idea was that if you had a recipe that made three servings, but wanted to feed seven—or whatever combination of numbers—you could use this device to figure out how many teaspoons of this or tablespoons of that you would actually need. The Slide Rule itself was nothing more than a long sliding dovetail with a simple whole-number scale printed on both pieces. To work the thing, you just slid the two pieces past each other to the correct position and Presto! All the calculations are at your fingertips.

Even in a digital era, it seems like a neat idea, doesn’t it? Well, we sized the pieces in this article to exactly correspond to the original Kitchen Slide Rule project. If your joint was a success, you can go on and turn the two pieces into a fascinating gift for anyone who cooks.

I’ve made one major change to the original design, however: There are two scales, not just one. You use the first scale to match up the number of servings in the recipe with the number of servings you actually need. Then you turn the device around, without moving the slide, and read off the calculations from the second scale. Both of these scales were combined into one in the original design, and boy, was that confusing. I felt like I had been transported back to that chemistry class! The two-scale system is much easier to grasp, even for those who’ve only seen a slide rule in an antique shop. Enjoy!

To increase or decrease the number of servings in a recipe, slide the Calculator to align the appropriate numbers.

Turn the Calculator around. Now, you can easily convert the amount of each ingredient. This one is easy—everything is doubled.

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