Press ± on "People to serve" at the top left and all ten ingredients on the right move at once. The arithmetic is the same as doing ten multiplications by hand — but the multiplication happens once.
How to read this board
Only three cards hold a number: people to serve, what the recipe is written for (2), and the scale factor = people ÷ 2. Every ingredient card has the same shape — "amount for 2 × scale factor" — and they all look at that one scale-factor card.
Which means there is only one place to fix. Copying a recipe that was written for four? Change "Recipe is written for" to 4 and the factor and all ten ingredients correct themselves. Ten multiplications on a calculator has a failure mode this shape does not: reusing the previous number in one row and not noticing.
Proportion means multiply one side by n and the other side is multiplied by n too. The lines on the board draw where that n travels.
Where it goes wrong
The things that catch people out when doubling a recipe, next to how to check each one on the board. The subject of this table is that some quantities come out better when you do not scale them.
| What catches people out | Why it happens | How to check it on the board |
|---|---|---|
| Doubled salt tastes too salty | Seasoning is judged by concentration, not amount. Scale the salt with everything else and a longer reduction still moves the concentration. | Delete the formula on the salt card and type a number straight in. That card alone drops out of the scaling. |
| Cooking time does not double | Heat comes in through the area of the pan, so twice the food is nowhere near twice the time. Burning arrives sooner than that. | There is no cooking-time card. Leaving non-proportional quantities off the board is part of the design. |
| Three servings gives you fractions | ×1.5 turns one clove of garlic into 1.5, one egg into 1.5. Some ingredients do not halve. | Set people to 3. Garlic reads 1.5; in the kitchen it is 1 or 2. The board does not round for you — that call stays with a person. |
| Skipping the recipe's own serving count | Written for 2 or written for 4 changes the factor by a factor of two. The number with the largest effect, and the one most often skipped. | Change "Recipe is written for". It is the one card on the board that changes what every ingredient means. |
| Scaled water is too much water | Liquid for a braise is set by the depth that covers things, not by an amount. A different pan needs a different volume, and evaporates differently. | There is no water card either. What is on the board is only the ingredients that may be scaled. |
| One serving is unmeasurable | ×0.5 turns 0.2 g of pepper into 0.1 g. What a kitchen scale can weigh becomes the floor. | Set people to 1. Whichever ingredient goes too small tells you how far down that recipe can be scaled. |
One factor driving everything works well beyond cooking: a scale model (one ratio fixes every dimension), enlarging a drawing, print sizes for a photo. The shape is the same — a row of "original × factor" cards with the factor pulled out on its own — and whether the number you want to change sits in one place decides how easy the thing is to correct later.
Try moving it
- Set people to 1 — the factor becomes 0.5 and pepper reads 0.1 g. Where a number stops being weighable is that recipe's floor.
- Set people to 8 — 720 g of spaghetti. The number is right; whether it fits in the pan is a separate question the board knows nothing about.
- Set "Recipe is written for" to 4 — every ingredient halves. That is what misreading the serving count costs, and it is one card to put right.
Read next — Per-unit quantities. That one goes the other way, from "×n" to "per one", which is proportion's other doorway.