Say goodbye to smooth cheeks and get ready for beards, because it’s officially the time of year when, for better or worse, previously clean-shaven men everywhere decide to put their razors into hibernation and start growing facefuls of fur. It happens every fall like clockwork: the temperature drops—even just slightly—and, just as sweaters start to pop up everywhere you look, so do the beginnings of soon-to-be-formidable manes, coating the jawlines of guys who, just a few months ago, would not have been caught dead with even a fine coating of summer stubble.

Now, this isn’t the first time we’ve talked about the annual tradition of the seasonal face mane, or even when to start it. We’ve hit upon that exact topic at least once before, but what we didn’t account for that time was preference. You see, letting one’s facial bounty flourish is a highly personal endeavor—some guys go for Santa-style bush, while others strive for nothing more than a heftier-than-average five o’clock shadow—and so determining an appropriate start should indeed be an equally subjective matter, correlated to the exact length one wants his beard to be by the time the holidays roll around. Which is why we put our thinking beanies on and did our fair share of math, to come up with a formula you can use determine how long you need to hit your bearded end goal.

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Let us explain. It all starts with the body’s average rate of hair growth. Generally speaking, all the hair on your body grows at about the same rate: 0.5 inches/month. That goes for everything from the luscious locks on your head, to the thick underbrush of your chest hair, to the dark and curly strands squished between your ass cheeks. Inches per month isn’t exactly a helpful measurement, though, at least compared to, say, inches per day. Which, luckily is an easy conversion: all we need to do is divide 0.5 by 30.42, the average days in a months, and we can see that the length of hair growth is 0.016 inches/day. Let’s call that number the ideal growth constant.

The trouble is, that’s how long the hair would be if it was straight. And facial hair, as anyone who’s ever encountered any can tell you, is notoriously not straight. In fact, one might postulate that it takes about an inch and a half of kinky length for a beard to drape one inch from the jaw. And so, we actually have to divide the ideal growth constant by one and a half to get 0.011 inches/day, a value that we’ll dubb the apparent growth constant.

Once you know that number, it’s actually pretty easy to determine how long you need to reach peak beard, or your growth period. Simply divide the final length you want your final beard to be by the apparent growth constant, and that’s exactly how many days you need to perfect your beard in time for winter. Or, in mathematical terms:

Goal Length / Apparent Growth Constant = Growth Period

For example, say you want a solid half-inch beard beard by the first day of winter. By dividing 0.5 inches by 0.011 inches/day, you determine that it’ll take rough 45 days reach get that. Then all you need to do is work backwards from December 21st and boom, you’ve got a start date of November 6th.

See, wasn’t that easy?

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