Where do the “120” and “240” come from?

In September 2022, Guido Baena Wehrmann and I published an article on how to calculate the (approximate) ROI of a design system in Smashing Magazine and The UX Collective. At the heart of the article is a ready-to-use formula based on only three variables: X, the percentage of time invested in building the design system; Y, the percentage of time invested in ongoing maintenance after the ramp-up phase; and Z, the amount of time saved by using the design system in percent. It’s specifically intended to support anyone needing a tangible, concrete argument for investing effort into a design system.

After reading our article, quite some readers reached out to us, slightly (or not so slightly) confused about where exactly the seemingly magic numbers “120” and “240” come from. In fact, it’s happened often enough by now that we’ve decided to write this separate note as an explanation. We can assure you there’s nothing magic about these two numbers.

Before continuing, please make sure to refresh your memory and have the original article present.

We gotta admit, the “120” and “240” aren’t perfectly intuitive, and we’re gonna revisit that for future editions/revisions of our formula. But they’re relatively easy to reverse-engineer. The “240” stems from the assumption that, in the cost part of the formula, a 20% investment leads to a 12-month ramp-up phase: 240/20 = 12.

With this, a lower investment results in more months of ramp-up, and vice versa. Correspondingly, we have only “120” in the gain part of the formula because we assume gains only for half the ramp-up phase.

Talking about units of measurement, strictly speaking it must be 240 months and the 20 is dimensionless. If you’d prefer to express the 20 as 20%, it would be 2.4 months / 20% = 12 months. In the end, we only have one unit of measurement, and that is “months”, which we omitted in the formula for matters of readability.