Scales are not expensive. I have a digital one that was giving me inconsistent results. I found that it wasn't level. I moved it to a certain place in the floor and now it weighs with more precision. I've also found that clothes can add up to 5 pounds. For instance, I weighed myself right out of the shower, and then again fully dressed, and the difference was 5 pounds (2.3 kg). I've also heard that it's best to weigh first thing in the morning, and only once per week.
DISCLAIMER: I am NOT an expert in nutrition, fitness, or exercise. Any advice given is only meant to be helpful based on what information I've gathered, and should not replace any medical treatment or any other true advice you may get from another source. Take it with a grain of salt, and then log that in your Foods tab!