You Need a Kitchen Slide Rule
15th January 2026
itchen work is all about proportions: sometimes the recipe is for four servings but you need six; maybe the recipe calls for 80 g of butter but you only have 57 g, so you have to adjust the other ingredients to match.
We could use an electronic calculator to figure out the rescaled amounts, but a slide rule makes it so much easier. The picture above was taken while following a recipe that called for 2 tsp of baking powder, and I wanted to make as large a batch as I could given the remaining 3.3 tsp of baking powder I had – a proportion of 2:3.3. You can see the slide rule is set to a proportion of 2:3.3 because – if you open the image in a new tab to make it larger – the number 2 on the C scale (on the bottom of the sliding middle part) is above 3.3 on the D scale just below.
But wait, the number 1 on the C scale is also just above 1.65 on the D scale. And 14 (or, if you will, 1.4, since with slide rules we ignore decimal points) on the C scale is above 23.1 on the D scale. Indeed. It’s set to those proportions too, because they are the same proportion. This is what makes the slide rule so powerful.
You can also do this in Excel, which is probably easier to find than a slide rule.