This article is by our resident fitness expert, Melanie Reeves Wicklow.
If you’re fitness savvy you know you should be gaining muscle as you lose fat. So isn’t focusing on body weight silly, since muscle is denser than fat and ultimately we all want to be svelte and strong and lean, like a jungle cat? Maybe you have a fancy scale that tells you what really matters: percent body fat.
Unfortunately, estimating body fat from those electrical impedance devices is hardly better than a random number generator. They tell you something plausible only by factoring in your weight, height, sex, and age. Change any of these values and your body fat percentage on the scale magically changes. The Mayo Clinic backs us up on this.
“Beemind down to an upper bound on your ideal weight”
Minding your scale weight is actually reasonable even though it’s not the metric you truly care about. This is definitely true if you’re well outside your ideal weight range. Say you’re over a third body fat. Then you surely want your scale weight to go down as a side effect of losing the body fat, unless you also want to add bodybuilder levels of new muscle.  As long as your scale weight is right now definitely more than what it’s going to be when you’re at your ideal body composition then it’s a good metric to track. In other words, beemind down to an upper bound on your ideal weight. At that point you can switch to a truer but noisier or less convenient metric. 
Of course scale weight itself is a noisy metric, but not so noisy that it isn’t overcome by getting more data, which is the point of the rest of this article. For a nerdier treatment, check out the Signal and Noise chapter of The Hacker’s Diet. (The Hacker’s Diet helped inspire Beeminder and Beeminder incorporates its data-smoothing recommendations.) Non-nerds, read on.
Why Weigh Daily?
A single measurement of our weight is like a single photograph of our looks. You’re more photogenic when you’re laughing, for example, or sometimes the lighting or photographer is just better or worse. How would you react if you saw 149 on the scale and then a week later 150, even though you went to the gym more than usual? Would you throw your hands up, eat all the junk food in the house, and gear up for a better plan starting Monday? No! Maybe dinner the night before was a little larger than normal, or you drank a lot of water, or maybe you just haven’t emptied your bowels recently enough! In fact, one of our beeminders (UPDATE: it was Rob Felty, who assures us he doesn’t mind being quoted even when it’s about poop) recently commented:
Weighing daily has been crucial for me. I realized that I sometimes fluctuated 1-2 pounds in one day, maybe just because I hadn’t pooped. If I only weighed myself once a week, I could easily attribute that fluctuation to real weight gain or loss.
These normal fluctuations in weight have nothing to do with whether you may have made progress that week.  Since weight isn’t a fixed number, sampling it only once a week doesn’t give you good information on which direction it’s going or how quickly, and you don’t learn much about your natural weight range. On shows like The Biggest Loser they weigh weekly, but that’s a game — they’re just trying to make good TV.
“More information is good”
There’s an established correlation between weighing more frequently and losing more weight / maintaining a healthy weight.  More information is good! For example, you can more quickly reverse a trend in the wrong direction. If you are scared to look at your weight for fear that you won’t like what you see, it becomes easier to not feel accountable for your actions. It’s a slippery slope once what you do right now doesn’t matter. You’ll be “good” when you start a new plan, soon, so it’s okay to be “bad” right now. Sound familiar? It’s a diet mentality and it doesn’t lend itself to achieving a balanced life, or maintaining a healthy weight and habits. People who achieve and maintain a healthy weight miss workouts sometimes and eat dessert or too many chips sometimes and it’s not bad, it’s part of life. Another beeminder put it this way:
I like that it motivates me to think in the short term about a long-term goal. Previously I avoided weighing myself frequently because I was too scared to see those day-to-day fluctuations, but I’m coming to the realization that those small fluctuations are what combine to make a difference — one way or the other.
I personally made this same shift once I started using Beeminder. It puts meaning to the daily fluctuating numbers.
Daily weighing can completely shift your focus. Note that I’m not advocating a focus on daily weight, but simply weighing daily in order to focus on what’s really happening. It takes a lot of trees to make a forest, and a Beeminder graph shows this very nicely. You can stop treating your weight loss as a Big Thing to deal with from date X to date Y while gearing up to follow a strict plan where deviating from it will throw you off the wagon, into the kitchen, and back to your Sunday night willpower-mustering. Instead, you begin to focus on each day for what it is: life, part of a journey, and a process. Will you sometimes miss workouts or overeat during that process? Of course! Your simple, attainable goal of being on track tomorrow, even with mistakes, becomes much more action oriented, short term, and therefore doable. Do something every day without letting a mistake derail you and pretty soon you’ve gotten somewhere!
“What you do today matters because you have a yellow brick road to be on tomorrow”
If you’ve spent a lifetime in the diet mentality, it’s not an easy thing to shift from. Weighing today at 199 and tomorrow at 201 messes with your head  even though you logically know you couldn’t possibly have gained two pounds in a day (you’d have to eat an excess of 7,000 calories  to accomplish that!). Weighing daily with the feedback of a Beeminder graph is a great way to shift out of that mentality. You’ll find you still have to fight to change your thinking in the beginning. With the actual number staring you in the face from the scale, you have a choice to make. You can let yourself believe that you’ve gained weight and therefore failed and need to start over with a new and better plan. Or, you can go look at your weight graph, see that you’re still on the road and take on the new day with the motivation to still be on track tomorrow. Work out a little harder, be a little more conscious of your food intake, get plenty of sleep (a great metabolism boost), and remind yourself repeatedly that you’re on track to your goal. You’re in the process of losing weight and what you do today matters because you have a yellow brick road to be on tomorrow. Drilling those positive thoughts into your head will keep you from dissolving into past habits, or dwelling on frustration or defeat. The next day, when your weight is (likely) back down a bit, you can enjoy the easy-to-come-by motivation, fostering yet another good day. The positive reinforcement from being a little more diligent the day before makes you want to do it again! If your weight then bounces up a bit again on another day, drill those positive thoughts into your head. Look at your graph to see that you are indeed moving in the right direction overall, and pretty soon the mental shift will happen and you will get over the hurdle that has held you back for so long.
 Don’t get me wrong, I’m very much in favor of muscle building and recommend weight training as part of any fitness plan. Besides, muscle burns calories just sitting there, so it’s a huge help for weight loss, i.e., fat loss. Losing weight by just eating less can often be a death spiral where you have to keep eating less and less as your metabolism adapts to the fewer calories. The human body is impressively adept at maintaining its weight.
Speaking of which, another thing we recommend is having occasional flat spots on your yellow brick road. If it’s hard to keep from gaining, let alone losing, then you’re doing it wrong.
 Beeminder is agnostic about what metric you use to measure your progress, though the simpler it is to measure the better. And we’ve put thought into the case of scale weight as a metric, figuring out how to adjust for random fluctuations, and how to avoid losing weight the wrong way. But for most people that still means a gradually decreasing scale weight so we stand by it as a useful metric.
 Pro tip: As recommended by PhysicsDiet.com [UPDATE: The link to physicsdiet.com/WhenShouldIWeighMyself.aspx died. Sad face.], another site inspired by The Hacker’s Diet, you can further reduce the noise in your weight measurements by consistently weighing first thing in the morning, after using the bathroom but before eating or drinking anything.
 Another pro tip: If you’re really struggling, try switching the units on your scale (kilograms, pounds, stone). Whomp! Suddenly your scale is speaking gibberish (unless you’re really good at mental math), and you can just compare the numbers to previous days, instead of to your entire mental history of your weight and body issues.
 That’s like 10 loaves of bread, or 100 large eggs.