A while back Madasi posted about his inconsistent scale:
I get up one morning, weigh myself and see 319.9 lbs. "Wow!" I think, "I broke 320!". But, knowing my scale a little to well, I take another reading, and see 328 lbs. I take about 7 more readings before finally getting a rough consensus around 325.6 lbs. That's way too much variance between readings for my comfort.
Yesterday he posted about his new scale:
So, this is all really nice, except that my nice, shiny, accurate, new scale also tells me I don't weigh 277lbs any more, but actually something more like 310lbs. I've been posting my progress over at PhysicsDiet.com, and had actually started getting excited because I was only a pound or two away from making the biggest losers chart.
The way he handles this setback is inspirational and it is worth reading his entire post for that alone. But it also illustrates how important it is to get a good scale. Especially if you are over 300 pounds.
For several years I did not know how much I weighed simply because I did not have a scale that could handle more than 300 pounds. When I finally went to the doctor last summer for a severe sinus infection I learned the horrible truth. That simple act help snap me out of years of denial.
If you have any doubts about the accuracy of your scale go to the doctor, or a gym, and use their scale. Compare the results to your scale. If your scale is within a few pounds and it is fairly consistent you should be ok. But if there is a significant difference you owe it to yourself to invest in a decent scale. There are many on the market now that handle more than 300 pounds. Wattly recommends the Tanita HD372. I have a Health-o-Meter HAP200KD-41 Professional Dial Scale. It probably isn't as accurate or consistent as the Tanita. But it works well enough for me.