I suggest getting a refractometer as well.
Super accurate for pre-boil and OG gravities. And you only need one drop to read the measurement. They aren't very expensive and can be bought on Amazon as well.
If you get one make sure you get a BRIX/SG refractometer as there are different ones as well.
Also some refractometers go up to an SG of 1.100, others 1.130, so depending on the beers you brew you might want the one that will show you high SG measurements as well.
Pretty cheap, very robust and way faster than a hydrometer.
Now a refractometer works by putting a drop of liquid (wort/beer) on the lens, which is then flattened by a visor. You hold it under a light is on the sun and the refracted light shows the amount of contents are in the sample of water. So how much sugar is in the sample.
When you use a refractometer to measure the beer during and after fermentation it will give you an incorrect reading. This is because alcohol is now present in the sample and has a different density than water. But there is an online calculator to correct for alcohol in the sample: