Sometimes it seems like your bathroom scale is lying to you, and not just out of a sense of denial about just how much of that Thanksgiving pie stuck around. Stepping on the scale can show you one weight, and then stepping off and back on can settle at another, plus or minus as much as 5-10 pounds!
When you're trying to lose weight, chart your progress, and keep an eye on how you're doing, the last thing you need is for your measurement tools to be reliant on the alignment of the stars, the flapping of butterfly wings in Asia, and your mood when you step on it. You want a scientific device, not a device that spits out something in the right ballpark.
So how can you test the accuracy of your scale?
There are typically two kinds of scales you'll find in a bathroom; mechanical and digital. You can generally tell which one you have by looking at the readout. Does it have a dial with a needle that points to your weight as a number, or does it have a digital display?
How Mechanical Bathroom Scales Work (And Fail)
Mechanical scales use a cross arrangement of levers to support your weight and spread it out evenly across the scale. Your weight then presses down on these levers, which pushes down on a central spring. The spring uses a slightly complex mechanical arrangement to turn the dial with weights on it. You can see an example here.
There are a number of different ways this can cause issues. For one thing, in order to be small enough to fit in a bathroom, a scale like this uses a particular kind of mathematical ratio between the point at which your weight presses on the levers and where the fulcrum of the lever is. This effectively reduces your weight by a factor, usually 10 or 12. The reason is space; the spring that works as resistance and the pull that turns the dial doesn't need to be very large that way. Without that ratio, you'd be looking at a much larger scale.
The trouble here is that reducing your weight via ratio means small fluctuations can be missed. There's a margin of error in the display, since it's just a physical radial dial. If the spring isn't quite as strong as the average, or if the levers are positioned slightly out of true, or if the scale is just old and wearing out, it might now show your weight appropriately.
With any mechanical arrangement, proper calibration is essential for an accurate reading. That's why industrial scales have tight tolerances, careful overview on their manufacturing processes, and plenty of testing to ensure their accuracy.
If you think a $20 bathroom scale from WalMart is going to go through anything like those processes, you're quite wrong. Whatever Chinese manufacturer is producing the scale is likely relying on a design and a form who knows how far removed from the actual manufacturing process. Assembly can vary, part tolerances are broader, and the whole thing just doesn't have the accuracy you would want.
Mechanical scales also have an upper limit to the weight they can accurately read. If you're on the upper bound of that limit – typically around 250 pounds, though it can vary depending on the ratios inside – it will be less accurate.
How Digital Bathroom Scales Work (And Fail)
Digital bathroom scales work in an entirely different way, and are generally considered more accurate and more reliable than a comparable mechanical scale. Of course, a truly high-tolerance mechanical scale can be extremely accurate, while a low quality digital scale can be just as variable as a mechanical box. 
So how does a digital scale work? They start with a small electronic item called a strain gauge. A strain gauge is a device that has been around since the late 1930s, and is really not a whole lot more than a carefully arrayed layer of metallic foil. This gauge is positioned on what is called a load cell, which also includes a sensor and a transducer.
When you step on the scale, your weight is evenly distributed across the surface, and you press down on the strain gauge. This converts your weight into an electrical signal. That signal is measured, and the circuits in the device convert that measurement into an accurate reading of the weight on the scale.
This allows a scale to be much more compact than a mechanical scale, while still maintaining reliability. It's also the same system used in postage scales, which can measure in mere ounces reliably.
Electronic scales are accurate and reliable, assuming they were created and calibrated properly. Most electronic scales have a method in the instructions (possibly as easy as just stepping on and off) to "zero out" the scale. Zeroing the scale tells the scale what level of signal, or lack thereof, should be considered "zero". After all, the scale's surface has weight of its own that needs to be accounted for.
In general, the circuitry in a digital scale will be reliable, though in ultra-cheap scales that might not be the case. The more common points of failure are in the parts themselves. Over time, circuits degrade, the load gauge can crack and fail, and parts can wear out.
Testing a Scale
So how can you test a scale?
Before you begin, you want to make sure you've set the stage properly. That is, make sure your bathroom floor (or whatever other surface you're using) is hard and flat. Using a scale on carpet or an uneven surface will throw off the sensors, both mechanical and digital. Pick up the scale and clean beneath it; even small bits of debris can have an impact.
You also may need to consider environmental factors. Scales may be subject to something called "drift". Drift is when your scale works fine, but the measurements aren't quite the same day to day. Why not? Things like temperature can affect the resilience of the materials used to construct the scale. If it's colder, springs and load gauges might not flex the same amount, and can give different readings.
As far as testing your scale goes, here are some steps you can take.
First, make sure you zero out the scale properly. A digital scale will have some means of zeroing it out, whether it's an action like stepping on and off, or a button you can press to reset it. Refer to the instructions on the scale, or if you don't have the instructions, look for them online. You can usually find some instructions if you can Google search the model number of the scale, and possibly even a PDF of the original instruction booklet.
For mechanical scales, there may be something simple like a dial you can turn to make sure it reads zero before you step on it. For other scales, you may need to open it up and turn something internally to make the same adjustment. Be careful if you do this; the last thing you want to do is break your scale.
Incidentally, you want to calibrate or zero out your scale every time you use it. With proper calibration, home scales can be perfectly accurate for weight monitoring purposes. If you move the scale, though, it needs to be recalibrated. In general, you want to calibrate it once every month or two as well, just in case. When in doubt, zero out!
For electronic scales, it might also be worthwhile to replace the batteries. When batteries get low, the device will still work, but might not be as accurate.
Second, test for consistency. Step on your scale, get a reading of your weight, and step off. Repeat this process four or five times, and note if there's any variance in your weight. Make sure you're stepping on it the same way, and that you're not bouncing, shifting, or otherwise adjusting your weight distribution as you test it.
If your scale is consistent, great! You can move on to the next step. If it's not consistent, you may have an issue with the internal elements of the scale. This is usually a symptom of something breaking, whether it's the load gauge in the digital scale breaking down, or the pinion sticking inside the mechanical scale.
In these cases, you can potentially open up and repair the scale, but it's generally easier to just go buy a new one. Old scales break down, and it's usually more trouble than it's worth to repair them.
If you've determined that your scale is consistent, then you can try to test it with something with a known weight. We like to use small hand weights, something like these, which have a known weight written on the side of them. You want to use something around 10lbs or so. Some scales don't "activate" for lower amounts of weight, because of how they're calibrated internally.
If you put a known 10lb weight on your scale and it shows something other than 10 pounds, you know it's not necessarily accurate. You can try to recalibrate it again, but chances are it's just a margin of error for how the scale works.
You can also take more than one known weight – like two of those dumbbells – and weight them at the same time. If 1x10lb weight measures in at 9 lbs, and 2x10lb weights measure in at 19lbs, you know it's within 1 lb of accurate.
That's one of the more difficult things to accept. The fact is, most bathroom scales aren't going to be too terribly accurate, at least not to a sub-pound level. They generally only have a couple of sensors, and their tolerances aren't great, so they will be accurate within about 2-4 pounds. Unfortunately, when you're trying to chart your weight loss over time, being "off" by a few pounds isn't great.
What to Do if Your Scale is Inaccurate
If your scale has been tested and you've determined it's not accurate, there are a few things you can try.
First, you can try taking it to a different location. The surface of your bathroom might be slightly slanted, for example, but a different room could be more accurate. Make sure to calibrate the scale before you use it in the new location.
Another option is to try to repair the scale. Sometimes a mechanical scale might need a little cleaning to be accurate again, though that requires disassembling and reassembling the scale, which can be difficult if you don't know what you're doing. Unfortunately, it's harder to repair a digital scale.
You can also just live with it. Weight loss is all about the journey, not each individual stop along the way. If you're 185 and the scale is showing 188, but you do some dieting and working out and come back at 175 with it showing 178, well. You know you're trending downward, and that's what matters, right? If the scale is wrong, but it's wrong in the same way every time, you can adjust your measurements to account for it. Losing weight is losing weight, even if it's not always pound-for-pound accurate.
Finally, you can just buy a new, better scale. Modern digital scales are pretty good in the $30-$50 range, though they can still be slightly inaccurate, especially over time. Alternatively, you can spend as much more as you like; high-precision scales exist and are very reliable, but they can cost hundreds or even thousands of dollars. For simple weight checks, rather than something like laboratory work, that's probably unnecessary.
