For a long time now, I’ve advocated “living in your calendar” in order to, among other reasons, understand your production capacity. Mapping out your work on a calendar helps prevent you from taking on more commitments than you have the time to handle.

I was wrong. (Sort of.)

I just finished reading Jim Benson and Tonianne DeMaria’s book, Personal Kanban, in which they point out that capacity is irrelevant. It’s about throughput. No one — not your boss, not your customers, not your family — cares about how much capacity (hours) you have each day to work. They care about how quickly that work gets done, whether it’s preparing next year’s budget or cleaning the garage.

What’s the lead time? What’s the cycle time? How long do I have to wait? These are the key questions they want answered. (Well, only engineers ask the first two questions. But everyone asks the last one.) And those are the key questions you should be asking yourself. Not, “How much time do I have to work this week?”, but “How can I get this work done most quickly?”

To shamelessly steal an analogy from Personal Kanban, no one cares what the capacity of a freeway is. In fact, it’s completely irrelevant to you how many cars can be packed into one stretch of asphalt. What’s really important is how long it takes to move down the road and whether you’ll make it home in time to watch reruns of “Webster.” And as any urban planner or operations manager will tell you, once your system exceeds 65-70% of maximum utilization, you’re guaranteed to reduce throughput and increase cycle time.

This is why living in the calendar can be dangerous. There’s a tendency to look at empty space on the calendar as something to be filled up with some ostensibly productive work. After all, if you’re not filling those minutes and hours, then clearly you’re either a lazy slacker or you’re just terribly inefficient. With unemployment at 9%, who wants to be accused of either?

But how fast would traffic move if every square foot of the freeway was occupied by cars? How fast will your work move if every moment of your day is occupied by some pre-planned task or meeting? It wouldn’t move at all. Just look at the cars around you at rush hour — or look at the crap that’s been piled up on your desk and your inbox for a few weeks. That tells you all you need to know about throughput.

So, by all means live in your calendar. Use it to assess your production capacity. But remember that 100% utilization of that capacity is ultimately self-defeating. You need slack in the system, because throughput is what counts. Not capacity.