Hello. I have a question. I weigh myself on my digital bathroom scale, and a scale at my local YMCA, this scale is a medical Mechanical scale (with the sliding bar’s and ruler to measure yourself looks like this http://www.healthchecksystems.com/d439.htm). The scale at the YMCA says I weight 9 lbs less than my bathroom scales. I’m not sure which one to go by? I don’t understand how the two can have such a different reading? Which one should I trust?
See if you can find a high-capacity postal scale to weigh yourself on - they are required to be accurate, and this will tell you which scale is correct (or how much both are wrong).
You could also weigh a heavy package on the postal scale, and then weigh that on both scales, but the error may not be linear, so this may not tell you which one is wrong at your weight.
Do you have any weight plates at home? Or something you can look up the weight on? Should be relatively easy to check your scale. Does it do 1/10ths of a pound?
Same as at the gym. If you don’t mind looking like a dork - drag over one of the weight plates.
Nine pounds is huge (assuming you are using the same clothes as at home).
The medical scale I assume is level at zero when nothin is on it?
You could also ask someone else at the gym. If it is off by 9 pounds - someone working there should know about it (oh yeah, that things been off since 1987) - or if you see someone else using it. A scale that is 9 pounds off is useless.
Those mechanical doctor’s scales are notoriously unreliable and that goes double for one in a public place. All thing equal, I’d have a little more faith in the digital bathroom scale
And absolutely do NOT trust the nominal weight figure on a dumbell or plate. I’ve seen them as much as 15% off.
I’d say the weights in the weight room are fairly accurate, if not find some known weight and take it in there. Take 100 lbs of weight into the locker room and place them on the scale and see how far off they are percentage wise. From there you can calculate your correct weight. And yes, gym scales are almost always screwed up.
Like the old saying “A man with two clocks doesn’t know what time it is” the same goes for two scales. As they read differently (and assuming that there isn’t 9 lbs of clothing difference) at the most only one scale can be right. It’s even more likely that both are somewhat wrong!
Mechanical scales can be calibrated. See if there’s a sticker on the scale that says when it was last calibrated. If it’s older than a year or you can’t find a calibration sticker, I wouldn’t trust it.
The double beam scale at the gym has a number of variables that will effect it’s accuracy. One, is it on carpet? Is the floor level? Is it stable, does it rock when you step on it? When the scale is at 00 is it balanced?
Can you check the accuracy of your home scale by weighing some water? It seems like the weight of water should be fairly constant and knowable, unless I’m missing something (entirely possible).
Also, you have to factor in kids messing with it and jumping up and down on it. It could balance out at zero fine but be way off towards the upper end. Calibration stickers are only good in controlled environments like quality and medical labs; not in public environments where god knows who is going to throw god knows what onto it. I’ve seen four kids get on a scale at once to see what their front line weighed (guess linemen can’t add) and they easily pushed it pass it’s rated load.:smack:
Like the weight of the vessel you are putting the water into.
A plastic milk jug weighs almost nothing compared to the 8 pounds of water it contains.
And while people are rightly pointing out how a balance can be out of whack, the same is true for a home scale. You’d need to calibrate either to know.
But the bottom line is it doesn’t really matter what your exact weight is. What matters is if the number is going up or down. Unless you are competing at events that have weight classes the actual number is fairly meaningless. Pick one scale and just use it over a period of time to identify your weight trend.
While a 1 gallon steel gas can may have significant weight percentage wise vs a gallon of water. It’s an unknown factor that is being thrown in; just properly anal-lizing about like a good little engineer. 
Personally I look at weight last when it comes to fitness, I care more about waist size and how my cloths fit rather than the weight on the scale. I also wear a ring on my finger to let me know when I am bloating up. Then comes my A1C1 number, daily blood sugar reading, cholesterol reading and blood pressure reading. Then comes age and lastly weight. If you look good you feel good; and I feel marvelous. 
[nitpick] One gal of water weighs 8.345404 lbs at sea level at room temp.[/nitpick]
Unless you are in the UK, where it would weigh 10 lbs
If we’re going to nitpick, I should point out that the thing at the Y is a balance.
They are terribly finicky (think about it—you’re balancing your entire weight against a dinky little counterweight on a short beam), and I suspect the one at the gym has been abused and unmaintained for a long time.
Oh, yeah, and where I wrote “weight” earlier, I meant “mass.” Sorry.
I was assuming people knew to weigh that first and then subtract, just like good ol chem lab.
[more nits!] Not on a scale (or balance) that’s accurate to to tenths of a pound - with that equipment, you’re looking at 8.3 ± 0.05 pounds. And just what exactly does “room temperature” mean, anyway? [/nitpick]
Weigh it on what? The scales we are trying to calibrate? :smack: